diff --git a/src/base64.cr b/src/base64.cr
index 951684afc7ef..d0caf05c21bb 100644
--- a/src/base64.cr
+++ b/src/base64.cr
@@ -41,7 +41,7 @@ module Base64
   # Tm93IGlzIHRoZSB0aW1lIGZvciBhbGwgZ29vZCBjb2RlcnMKdG8gbGVhcm4g
   # Q3J5c3RhbA==
   # ```
-  def encode(data) : String
+  def encode(data : String | Slice(UInt8) | StaticArray(UInt8, 5)) : String
     slice = data.to_slice
     String.new(encode_size(slice.size, new_lines: true)) do |buf|
       appender = buf.appender
@@ -58,7 +58,7 @@ module Base64
   # ```
   # Base64.encode("Now is the time for all good coders\nto learn Crystal", STDOUT)
   # ```
-  def encode(data, io : IO)
+  def encode(data : String | Slice(UInt8) | StaticArray(UInt8, 5), io : IO) : Int32
     count = 0
     encode_with_new_lines(data.to_slice) do |byte|
       io << byte.unsafe_chr
@@ -95,11 +95,11 @@ module Base64
   # ```text
   # Tm93IGlzIHRoZSB0aW1lIGZvciBhbGwgZ29vZCBjb2RlcnMKdG8gbGVhcm4gQ3J5c3RhbA==
   # ```
-  def strict_encode(data) : String
+  def strict_encode(data : Slice(UInt8) | StaticArray(UInt8, 5) | String | StaticArray(UInt8, 20) | StaticArray(UInt8, 16)) : String
     strict_encode data, CHARS_STD, pad: true
   end
 
-  private def strict_encode(data, alphabet, pad = false)
+  private def strict_encode(data : Slice(UInt8) | StaticArray(UInt8, 5) | String | StaticArray(UInt8, 20) | StaticArray(UInt8, 16), alphabet : String, pad : Bool = false) : String
     slice = data.to_slice
     String.new(encode_size(slice.size)) do |buf|
       appender = buf.appender
@@ -115,11 +115,11 @@ module Base64
   # ```
   # Base64.strict_encode("Now is the time for all good coders\nto learn Crystal", STDOUT)
   # ```
-  def strict_encode(data, io : IO)
+  def strict_encode(data : Slice(UInt8) | StaticArray(UInt8, 5) | String, io : IO) : Int32
     strict_encode_to_io_internal(data, io, CHARS_STD, pad: true)
   end
 
-  private def strict_encode_to_io_internal(data, io, alphabet, pad)
+  private def strict_encode_to_io_internal(data : Slice(UInt8) | StaticArray(UInt8, 5) | String, io : String::Builder | IO::Memory, alphabet : String, pad : Bool) : Int32
     count = 0
     to_base64(data.to_slice, alphabet, pad: pad) do |byte|
       count += 1
@@ -137,7 +137,7 @@ module Base64
   #
   # The *padding* parameter defaults to `true`. When `false`, enough `=` characters
   # are not added to make the output divisible by 4.
-  def urlsafe_encode(data, padding = true) : String
+  def urlsafe_encode(data : String | Slice(UInt8), padding : Bool = true) : String
     slice = data.to_slice
     String.new(encode_size(slice.size)) do |buf|
       appender = buf.appender
@@ -152,13 +152,13 @@ module Base64
   # Alphabet" in [RFC 4648](https://tools.ietf.org/html/rfc4648).
   #
   # The alphabet uses `'-'` instead of `'+'` and `'_'` instead of `'/'`.
-  def urlsafe_encode(data, io : IO)
+  def urlsafe_encode(data : String, io : IO) : Int32
     strict_encode_to_io_internal(data, io, CHARS_SAFE, pad: true)
   end
 
   # Returns the base64-decoded version of *data* as a `Bytes`.
   # This will decode either the normal or urlsafe alphabets.
-  def decode(data) : Bytes
+  def decode(data : String) : Bytes
     slice = data.to_slice
     buf = Pointer(UInt8).malloc(decode_size(slice.size))
     appender = buf.appender
@@ -168,7 +168,7 @@ module Base64
 
   # Writes the base64-decoded version of *data* to *io*.
   # This will decode either the normal or urlsafe alphabets.
-  def decode(data, io : IO)
+  def decode(data : String, io : IO) : Int32
     count = 0
     from_base64(data.to_slice) do |byte|
       io.write_byte byte
@@ -180,7 +180,7 @@ module Base64
 
   # Returns the base64-decoded version of *data* as a string.
   # This will decode either the normal or urlsafe alphabets.
-  def decode_string(data) : String
+  def decode_string(data : String) : String
     slice = data.to_slice
     String.new(decode_size(slice.size)) do |buf|
       appender = buf.appender
@@ -189,13 +189,13 @@ module Base64
     end
   end
 
-  private def encode_size(str_size, new_lines = false)
+  private def encode_size(str_size : Int32, new_lines : Bool = false) : Int32
     size = (str_size * 4 / 3.0).to_i + 4
     size += size // LINE_SIZE if new_lines
     size
   end
 
-  private def decode_size(str_size)
+  private def decode_size(str_size : Int32) : Int32
     (str_size * 3 / 4.0).to_i + 4
   end
 
diff --git a/src/benchmark/bm.cr b/src/benchmark/bm.cr
index 4b94e4ca6eee..f9844b5b4307 100644
--- a/src/benchmark/bm.cr
+++ b/src/benchmark/bm.cr
@@ -44,13 +44,13 @@ module Benchmark
       end
 
       # Reports a single benchmark unit.
-      def report(label = " ", &block)
+      def report(label : String = " ", &block) : Array(Tuple(String, Proc(Nil)))
         @label_width = label.size if label.size > @label_width
         @reports << {label, block}
       end
 
       # :nodoc:
-      def execute
+      def execute : Nil
         if @label_width > 0
           print " " * @label_width
         end
diff --git a/src/benchmark/ips.cr b/src/benchmark/ips.cr
index def5b09c7c66..2f0f856ae0c2 100644
--- a/src/benchmark/ips.cr
+++ b/src/benchmark/ips.cr
@@ -31,7 +31,7 @@ module Benchmark
       end
 
       # Adds code to be benchmarked
-      def report(label = "", &action) : Benchmark::IPS::Entry
+      def report(label : String = "", &action) : Benchmark::IPS::Entry
         item = Entry.new(label, action)
         @items << item
         item
@@ -61,7 +61,7 @@ module Benchmark
 
       # The warmup stage gathers information about the items that is later used
       # in the calculation stage
-      private def run_warmup
+      private def run_warmup : Nil
         @items.each do |item|
           GC.collect
 
@@ -79,7 +79,7 @@ module Benchmark
         end
       end
 
-      private def run_calculation
+      private def run_calculation : Nil
         @items.each do |item|
           GC.collect
 
@@ -113,11 +113,11 @@ module Benchmark
         end
       end
 
-      private def ran_items
+      private def ran_items : Array(Benchmark::IPS::Entry)
         @items.select(&.ran?)
       end
 
-      private def run_comparison
+      private def run_comparison : Nil
         fastest = ran_items.max_by(&.mean)
         ran_items.each do |item|
           item.slower = (fastest.mean / item.mean).to_f
@@ -175,12 +175,12 @@ module Benchmark
         cycles.times { action.call }
       end
 
-      def set_cycles(duration, iterations) : Nil
+      def set_cycles(duration : Time::Span, iterations : Int32) : Nil
         @cycles = (iterations / duration.total_milliseconds * 100).to_i
         @cycles = 1 if cycles <= 0
       end
 
-      def calculate_stats(samples) : Nil
+      def calculate_stats(samples : Array(Float64) | Array(Int32)) : Nil
         @ran = true
         @size = samples.size
         @mean = samples.sum.to_f / size.to_f
diff --git a/src/bit_array.cr b/src/bit_array.cr
index 4f4eb5726055..d3ce01f0f405 100644
--- a/src/bit_array.cr
+++ b/src/bit_array.cr
@@ -51,7 +51,7 @@ struct BitArray
     arr
   end
 
-  def ==(other : BitArray)
+  def ==(other : BitArray) : Bool
     return false if size != other.size
     # NOTE: If BitArray implements resizing, there may be more than 1 binary
     # representation and their hashes for equivalent BitArrays after a downsize as the
@@ -366,7 +366,7 @@ struct BitArray
   end
 
   @[AlwaysInline]
-  private def set_bits(bytes : Slice(UInt8), value, index, mask)
+  private def set_bits(bytes : Slice(UInt8), value : Bool, index : Int32, mask : UInt8) : UInt8
     if value
       bytes[index] |= mask
     else
@@ -376,7 +376,7 @@ struct BitArray
 
   # returns (1 << from) | (1 << (from + 1)) | ... | (1 << to)
   @[AlwaysInline]
-  private def uint8_mask(from, to)
+  private def uint8_mask(from : Int32, to : Int32) : UInt8
     (Int8::MIN >> (to - from)).to_u8! >> (7 - to)
   end
 
@@ -396,7 +396,7 @@ struct BitArray
   # ba.toggle(3)
   # ba[3] # => true
   # ```
-  def toggle(index) : Nil
+  def toggle(index : Int32) : Nil
     bit_index, sub_index = bit_index_and_sub_index(index)
     @bits[bit_index] ^= 1 << sub_index
   end
@@ -417,7 +417,7 @@ struct BitArray
   # ba.toggle(1..-2)
   # ba.to_s # => "BitArray[01110]"
   # ```
-  def toggle(range : Range)
+  def toggle(range : Range) : UInt32?
     toggle(*Indexable.range_to_index_and_count(range, size) || raise IndexError.new)
   end
 
@@ -438,7 +438,7 @@ struct BitArray
   # ba.toggle(1, 3)
   # ba.to_s # => "BitArray[01110]"
   # ```
-  def toggle(start : Int, count : Int)
+  def toggle(start : Int, count : Int) : UInt32?
     start, count = normalize_start_and_count(start, count)
     return if count == 0
 
@@ -459,7 +459,7 @@ struct BitArray
 
   # returns (1 << from) | (1 << (from + 1)) | ... | (1 << to)
   @[AlwaysInline]
-  private def uint32_mask(from, to)
+  private def uint32_mask(from : Int32, to : Int32) : UInt32
     (Int32::MIN >> (to - from)).to_u32! >> (31 - to)
   end
 
@@ -641,7 +641,7 @@ struct BitArray
     bit_array
   end
 
-  private def bit_index_and_sub_index(index)
+  private def bit_index_and_sub_index(index : Int32) : Tuple(Int32, Int32)
     bit_index_and_sub_index(index) { raise IndexError.new }
   end
 
@@ -652,17 +652,17 @@ struct BitArray
     index.divmod(32)
   end
 
-  protected def clear_unused_bits
+  protected def clear_unused_bits : UInt32?
     # There are no unused bits if `size` is a multiple of 32.
     bit_index, sub_index = @size.divmod(32)
     @bits[bit_index] &= ~(UInt32::MAX << sub_index) unless sub_index == 0
   end
 
-  private def bytesize
+  private def bytesize : Int32
     (@size - 1) // 8 + 1
   end
 
-  private def malloc_size
+  private def malloc_size : Int32
     (@size - 1) // 32 + 1
   end
 end
diff --git a/src/channel.cr b/src/channel.cr
index 4e23f8bb9b09..933eccf7e232 100644
--- a/src/channel.cr
+++ b/src/channel.cr
@@ -31,7 +31,7 @@ class Channel(T)
   record UseDefault
 
   class ClosedError < Exception
-    def initialize(msg = "Channel is closed")
+    def initialize(msg : String = "Channel is closed")
       super(msg)
     end
   end
@@ -290,20 +290,20 @@ class Channel(T)
     pp.text inspect
   end
 
-  def self.receive_first(*channels)
+  def self.receive_first(*channels) : Int32
     receive_first channels
   end
 
-  def self.receive_first(channels : Enumerable(Channel))
+  def self.receive_first(channels : Enumerable(Channel)) : Int32
     _, value = self.select(channels.map(&.receive_select_action))
     value
   end
 
-  def self.send_first(value, *channels) : Nil
+  def self.send_first(value : Int32, *channels) : Nil
     send_first value, channels
   end
 
-  def self.send_first(value, channels : Enumerable(Channel)) : Nil
+  def self.send_first(value : Int32, channels : Enumerable(Channel)) : Nil
     self.select(channels.map(&.send_select_action(value)))
     nil
   end
diff --git a/src/channel/select.cr b/src/channel/select.cr
index 05db47a79a4c..db05f7cb5d3d 100644
--- a/src/channel/select.cr
+++ b/src/channel/select.cr
@@ -48,28 +48,28 @@ class Channel(T)
   end
 
   # :nodoc:
-  def self.select(*ops : SelectAction)
+  def self.select(*ops : SelectAction) : Tuple(Int32, String) | Tuple(Int32, Nil) | Tuple(Int32, Bool | String) | Tuple(Int32, String | Nil) | Tuple(Int32, Bool | String | Nil) | Tuple(Int32, Int32)
     self.select ops
   end
 
   # :nodoc:
-  def self.select(ops : Indexable(SelectAction))
+  def self.select(ops : Indexable(SelectAction)) : Tuple(Int32, Int32) | Tuple(Int32, Nil) | Tuple(Int32, String) | Tuple(Int32, Bool | String) | Tuple(Int32, String | Nil) | Tuple(Int32, Bool | String | Nil) | Tuple(Int32, Bool | Nil) | Tuple(Int32, Int32 | Nil)
     i, m = select_impl(ops, false)
     raise "BUG: Blocking select returned not ready status" if m.is_a?(NotReady)
     return i, m
   end
 
   # :nodoc:
-  def self.non_blocking_select(*ops : SelectAction)
+  def self.non_blocking_select(*ops : SelectAction) : Tuple(Int32, Channel::NotReady | String) | Tuple(Int32, Bool | Channel::NotReady | String) | Tuple(Int32, Channel::NotReady | String | Nil) | Tuple(Int32, Channel::NotReady | Nil)
     self.non_blocking_select ops
   end
 
   # :nodoc:
-  def self.non_blocking_select(ops : Indexable(SelectAction))
+  def self.non_blocking_select(ops : Indexable(SelectAction)) : Tuple(Int32, Channel::NotReady | String) | Tuple(Int32, Bool | Channel::NotReady | String) | Tuple(Int32, Channel::NotReady | String | Nil) | Tuple(Int32, Channel::NotReady | Nil) | Tuple(Int32, Bool | Channel::NotReady) | Tuple(Int32, Bool | Channel::NotReady | String | Nil) | Tuple(Int32, Channel::NotReady | Exception | Nil)
     select_impl(ops, true)
   end
 
-  private def self.select_impl(ops : Indexable(SelectAction), non_blocking)
+  private def self.select_impl(ops : Indexable(SelectAction), non_blocking : Bool) : Tuple(Int32, Channel::NotReady | Int32) | Tuple(Int32, Channel::NotReady | Nil) | Tuple(Int32, Channel::NotReady | String) | Tuple(Int32, Bool | Channel::NotReady | String) | Tuple(Int32, Channel::NotReady | String | Nil) | Tuple(Int32, Bool | Channel::NotReady | String | Nil) | Tuple(Int32, Bool | Channel::NotReady) | Tuple(Int32, Bool | Channel::NotReady | Nil) | Tuple(Int32, Channel::NotReady | Int32 | Nil) | Tuple(Int32, Channel::NotReady | Exception | Nil)
     # ops_locks is a duplicate of ops that can be sorted without disturbing the
     # index positions of ops
     if ops.responds_to?(:unstable_sort_by!)
diff --git a/src/char.cr b/src/char.cr
index 62189a0d8ceb..74365edb7731 100644
--- a/src/char.cr
+++ b/src/char.cr
@@ -140,7 +140,7 @@ struct Char
   end
 
   # :ditto:
-  def step(*, to limit = nil, exclusive : Bool = false)
+  def step(*, to limit : Char? = nil, exclusive : Bool = false) : Steppable::StepIterator(Char, Char, Int32)
     if limit
       direction = limit <=> self
     end
@@ -681,7 +681,7 @@ struct Char
   #
   # This means characters which are `control?` or `whitespace?` (except for ` `)
   # are non-printable.
-  def printable?
+  def printable? : Bool
     !control? && (!whitespace? || self == ' ')
   end
 
@@ -746,7 +746,7 @@ struct Char
   end
 
   # :ditto:
-  def dump(io)
+  def dump(io : String::Builder | IO::Memory) : String::Builder | IO::Memory
     io << dump
   end
 
@@ -1050,11 +1050,11 @@ struct Char
   # 'c' === 99    # => true
   # 'z' === 99    # => false
   # ```
-  def ===(byte : Int)
+  def ===(byte : Int) : Bool
     ord === byte
   end
 
-  def clone
+  def clone : Char
     self
   end
 end
diff --git a/src/char/reader.cr b/src/char/reader.cr
index bde6de53b4c3..b9523c8b6e95 100644
--- a/src/char/reader.cr
+++ b/src/char/reader.cr
@@ -80,7 +80,7 @@ struct Char
 
     # Creates a reader with the specified *string* positioned at
     # byte index *pos*.
-    def initialize(@string : String, pos = 0)
+    def initialize(@string : String, pos : Int32 | UInt64 | Int64 | UInt8 | UInt32 = 0)
       @pos = pos.to_i
       @current_char = '\0'
       @current_char_width = 0
@@ -239,7 +239,7 @@ struct Char
     # reader.pos = 0
     # reader.current_char # => 'a'
     # ```
-    def pos=(pos)
+    def pos=(pos : Int32) : Int32
       if pos > @string.bytesize
         raise IndexError.new
       end
@@ -335,7 +335,7 @@ struct Char
     end
 
     @[AlwaysInline]
-    private def decode_current_char
+    private def decode_current_char : Char
       decode_char_at(@pos) do |code_point, width, error|
         @current_char_width = width
         @error = error
@@ -423,7 +423,7 @@ struct Char
     end
 
     @[AlwaysInline]
-    private def decode_previous_char
+    private def decode_previous_char : Char?
       return nil if @pos == 0
 
       decode_char_before(@pos) do |code_point, width, error|
@@ -434,7 +434,7 @@ struct Char
       end
     end
 
-    private def byte_at(i)
+    private def byte_at(i : Int32) : UInt32
       @string.to_unsafe[i].to_u32
     end
   end
diff --git a/src/colorize.cr b/src/colorize.cr
index 60ab17a28718..ff0d4d4a46d6 100644
--- a/src/colorize.cr
+++ b/src/colorize.cr
@@ -137,7 +137,7 @@ module Colorize
   # This is determined by the environment variable called `TERM`.
   # If `TERM=dumb`, color won't be enabled.
   # If `NO_COLOR` contains any value color won't be enabled conforming to https://no-color.org
-  def self.on_tty_only!
+  def self.on_tty_only! : Bool
     self.enabled = STDOUT.tty? && STDERR.tty? && ENV["TERM"]? != "dumb" && !ENV.has_key?("NO_COLOR")
   end
 
@@ -177,13 +177,13 @@ module Colorize::ObjectExtensions
 
   # Wraps `self` in a `Colorize::Object` and colors it with the given `Color256`
   # made up from the single *fore* byte.
-  def colorize(fore : UInt8)
+  def colorize(fore : UInt8) : Colorize::Object(String)
     Colorize::Object.new(self).fore(fore)
   end
 
   # Wraps `self` in a `Colorize::Object` and colors it with the given `ColorRGB` made
   # up from the given *r*ed, *g*reen and *b*lue values.
-  def colorize(r : UInt8, g : UInt8, b : UInt8)
+  def colorize(r : UInt8, g : UInt8, b : UInt8) : Colorize::Object(String)
     Colorize::Object.new(self).fore(r, g, b)
   end
 
@@ -193,7 +193,7 @@ module Colorize::ObjectExtensions
   end
 
   # Wraps `self` in a `Colorize::Object` and colors it with the given *fore* color.
-  def colorize(fore : Symbol)
+  def colorize(fore : Symbol) : Colorize::Object(Char) | Colorize::Object(String)
     Colorize::Object.new(self).fore(fore)
   end
 end
@@ -513,7 +513,7 @@ struct Colorize::Object(T)
     end
   end
 
-  private def self.append_start(io, color)
+  private def self.append_start(io : IO::FileDescriptor | String::Builder | IO | IO::Memory, color : NamedTuple(fore: Colorize::Color256 | Colorize::ColorANSI | Colorize::ColorRGB, back: Colorize::Color256 | Colorize::ColorANSI | Colorize::ColorRGB, mode: Colorize::Mode)) : Bool
     last_color_is_default =
       @@last_color[:fore] == ColorANSI::Default &&
         @@last_color[:back] == ColorANSI::Default &&
diff --git a/src/complex.cr b/src/complex.cr
index e2a5830b395a..e1d92c5904e6 100644
--- a/src/complex.cr
+++ b/src/complex.cr
@@ -26,17 +26,17 @@ struct Complex
     @imag = imag.to_f
   end
 
-  def self.new(c : Complex)
+  def self.new(c : Complex) : Complex
     c
   end
 
   # Determines whether `self` equals *other* or not.
-  def ==(other : Complex)
+  def ==(other : Complex) : Bool
     @real == other.real && @imag == other.imag
   end
 
   # :ditto:
-  def ==(other : Number)
+  def ==(other : Number) : Bool
     @real == other && @imag.zero?
   end
 
@@ -267,7 +267,7 @@ struct Complex
     Complex.new(@real / other, @imag / other)
   end
 
-  def clone
+  def clone : Complex
     self
   end
 
@@ -306,7 +306,7 @@ struct Complex
   end
 
   # Rounds to the nearest *digits*.
-  def round(digits = 0) : Complex
+  def round(digits : Int32 = 0) : Complex
     Complex.new(@real.round(digits), @imag.round(digits))
   end
 end
@@ -322,7 +322,7 @@ struct Number
     Complex.new(0, self)
   end
 
-  def ==(other : Complex)
+  def ==(other : Complex) : Bool
     other == self
   end
 
diff --git a/src/compress/deflate/deflate.cr b/src/compress/deflate/deflate.cr
index 58479f503fbf..af19e0774817 100644
--- a/src/compress/deflate/deflate.cr
+++ b/src/compress/deflate/deflate.cr
@@ -23,7 +23,7 @@ module Compress::Deflate
   end
 
   class Error < Exception
-    def initialize(ret, stream)
+    def initialize(ret : LibZ::Error, stream : LibZ::ZStream)
       msg = stream.msg
       msg = LibZ.zError(ret) if msg.null?
 
diff --git a/src/compress/deflate/writer.cr b/src/compress/deflate/writer.cr
index f9a5adeb366f..7d9cf8c56467 100644
--- a/src/compress/deflate/writer.cr
+++ b/src/compress/deflate/writer.cr
@@ -85,7 +85,7 @@ class Compress::Deflate::Writer < IO
     to_s(io)
   end
 
-  private def consume_output(flush)
+  private def consume_output(flush : LibZ::Flush) : Nil
     loop do
       @stream.next_out = @buf.to_unsafe
       @stream.avail_out = @buf.size.to_u32
diff --git a/src/compress/gzip/writer.cr b/src/compress/gzip/writer.cr
index f02a320805f6..130197852733 100644
--- a/src/compress/gzip/writer.cr
+++ b/src/compress/gzip/writer.cr
@@ -109,7 +109,7 @@ class Compress::Gzip::Writer < IO
     @io.close if @sync_close
   end
 
-  private def write_header
+  private def write_header : Compress::Deflate::Writer
     flate_io = @flate_io
     unless flate_io
       flate_io = @flate_io = Compress::Deflate::Writer.new(@io, level: @level)
diff --git a/src/compress/zip/checksum_writer.cr b/src/compress/zip/checksum_writer.cr
index da5a33351c97..317ba6795fd6 100644
--- a/src/compress/zip/checksum_writer.cr
+++ b/src/compress/zip/checksum_writer.cr
@@ -21,7 +21,7 @@ module Compress::Zip
       io.write(slice)
     end
 
-    def io=(@io)
+    def io=(@io : IO | Compress::Deflate::Writer) : UInt32
       @count = 0_u32
       @crc32 = ::Digest::CRC32.initial
     end
diff --git a/src/compress/zip/file.cr b/src/compress/zip/file.cr
index cea1101c9f30..df2434415832 100644
--- a/src/compress/zip/file.cr
+++ b/src/compress/zip/file.cr
@@ -44,7 +44,7 @@ class Compress::Zip::File
   end
 
   # Opens a `Zip::File` for reading from the given *filename*.
-  def self.new(filename : Path | String)
+  def self.new(filename : Path | String) : Compress::Zip::File
     new(::File.new(filename), sync_close: true)
   end
 
@@ -85,14 +85,14 @@ class Compress::Zip::File
 
   # Tries to find the directory end offset (by searching its signature)
   # in the last 64, 1024 and 65K bytes (in that order)
-  private def find_directory_end_offset
+  private def find_directory_end_offset : Int32 | Int64
     find_directory_end_offset(64) ||
       find_directory_end_offset(1024) ||
       find_directory_end_offset(65 * 1024) ||
       raise Compress::Zip::Error.new("Couldn't find directory end signature in the last 65KB")
   end
 
-  private def find_directory_end_offset(buf_size)
+  private def find_directory_end_offset(buf_size : Int32) : Int32 | Int64 | Nil
     @io.seek(0, IO::Seek::End)
     size = @io.pos
 
@@ -113,7 +113,7 @@ class Compress::Zip::File
     i == -1 ? nil : size - buf_size + i
   end
 
-  private def read_directory_end(directory_end_offset)
+  private def read_directory_end(directory_end_offset : Int32 | Int64) : Tuple(UInt16, UInt32)
     @io.pos = directory_end_offset
 
     signature = read UInt32
@@ -134,7 +134,7 @@ class Compress::Zip::File
     {entries_size, directory_offset}
   end
 
-  private def read_entries(directory_offset, entries_size)
+  private def read_entries(directory_offset : UInt32, entries_size : UInt16) : Nil
     @io.pos = directory_offset
 
     entries_size.times do
@@ -149,7 +149,7 @@ class Compress::Zip::File
     end
   end
 
-  private def read(type)
+  private def read(type : UInt32.class | UInt16.class) : UInt32 | UInt16
     @io.read_bytes(type, IO::ByteFormat::LittleEndian)
   end
 
diff --git a/src/compress/zip/file_info.cr b/src/compress/zip/file_info.cr
index db503ea6a048..0998c05c2985 100644
--- a/src/compress/zip/file_info.cr
+++ b/src/compress/zip/file_info.cr
@@ -51,7 +51,7 @@ module Compress::Zip::FileInfo
   end
 
   # :nodoc:
-  def initialize_meta(io : IO)
+  def initialize_meta(io : IO) : Tuple(UInt16, UInt16, Time)
     @general_purpose_bit_flag = read(io, UInt16)
     @compression_method = CompressionMethod.new(read(io, UInt16))
     time = read(io, UInt16)
@@ -65,7 +65,7 @@ module Compress::Zip::FileInfo
     {file_name_length, extra_field_length, time}
   end
 
-  def initialize(@filename : String, @time = Time.utc, @comment = "", @extra = Bytes.empty)
+  def initialize(@filename : String, @time : Time = Time.utc, @comment : String = "", @extra : Slice(UInt8) = Bytes.empty)
   end
 
   # Returns `true` if this entry is a directory.
@@ -78,7 +78,7 @@ module Compress::Zip::FileInfo
     !dir?
   end
 
-  protected def to_io(io : IO)
+  protected def to_io(io : IO) : Int32
     write io, SIGNATURE # 4
     write io, @version  # 2
     meta_count = meta_to_io(io)
@@ -87,7 +87,7 @@ module Compress::Zip::FileInfo
     meta_count + 6 + @filename.bytesize + extra.size
   end
 
-  protected def meta_to_io(io : IO)
+  protected def meta_to_io(io : IO) : Int32
     write io, @general_purpose_bit_flag # 2
     write io, @compression_method.value # 2
     date, time = to_dos
@@ -101,7 +101,7 @@ module Compress::Zip::FileInfo
     24                                  # the 24 bytes we just wrote
   end
 
-  protected def write_data_descriptor(io : IO)
+  protected def write_data_descriptor(io : IO) : Int32
     io.write FileInfo::DEFLATE_END_SIGNATURE # 4
     write io, @crc32                         # 4
     write io, @compressed_size               # 4
@@ -109,7 +109,7 @@ module Compress::Zip::FileInfo
     16                                       # the 16 bytes we just wrote
   end
 
-  protected def decompressor_for(io, is_sized = false)
+  protected def decompressor_for(io : IO | IO::Memory, is_sized : Bool = false) : IO
     case compression_method
     when .stored?
       io = IO::Sized.new(io, compressed_size) unless is_sized
@@ -129,7 +129,7 @@ module Compress::Zip::FileInfo
     io
   end
 
-  protected def bit_3_set?
+  protected def bit_3_set? : Bool
     (general_purpose_bit_flag & 0b1000) != 0
   end
 
@@ -142,7 +142,7 @@ module Compress::Zip::FileInfo
   # - 6 bits for minute
   # - 5 bits for seconds (0..29), precision of two seconds
 
-  private def from_dos(date, time)
+  private def from_dos(date : UInt16, time : UInt16) : Time
     year = 1980 + (date >> 9)
     month = (date >> 5) & 0b1111
     day = date & 0b11111
@@ -154,7 +154,7 @@ module Compress::Zip::FileInfo
     Time.utc(year.to_i, month.to_i, day.to_i, hour.to_i, minute.to_i, second.to_i)
   end
 
-  private def to_dos
+  private def to_dos : Tuple(UInt16, UInt16)
     date = ((@time.year - 1980) << 9) |
            (@time.month << 5) |
            @time.day
@@ -166,11 +166,11 @@ module Compress::Zip::FileInfo
     {date.to_u16, time.to_u16}
   end
 
-  private def read(io, type)
+  private def read(io : IO | IO::Memory, type : UInt16.class | UInt32.class) : UInt16 | UInt32
     io.read_bytes(type, IO::ByteFormat::LittleEndian)
   end
 
-  private def write(io, value)
+  private def write(io : IO, value : Int32 | UInt16 | UInt32) : Nil
     io.write_bytes(value, IO::ByteFormat::LittleEndian)
   end
 end
diff --git a/src/compress/zip/writer.cr b/src/compress/zip/writer.cr
index 52ddee7400a5..5a88b900f015 100644
--- a/src/compress/zip/writer.cr
+++ b/src/compress/zip/writer.cr
@@ -45,7 +45,7 @@ class Compress::Zip::Writer
   end
 
   # Creates a new writer to the given *filename*.
-  def self.new(filename : Path | String)
+  def self.new(filename : Path | String) : Compress::Zip::Writer
     new(::File.new(filename, "w"), sync_close: true)
   end
 
@@ -175,7 +175,7 @@ class Compress::Zip::Writer
   end
 
   # Adds a directory entry that will have the given *name*.
-  def add_dir(name) : Nil
+  def add_dir(name : String) : Nil
     name = name + '/' unless name.ends_with?('/')
     add(Entry.new(name)) { }
   end
@@ -192,7 +192,7 @@ class Compress::Zip::Writer
     @io.close if @sync_close
   end
 
-  private def write_central_directory
+  private def write_central_directory : Nil
     @entries.each do |entry|
       write Zip::CENTRAL_DIRECTORY_HEADER_SIGNATURE # 4
       write Zip::VERSION                            # version made by (2)
@@ -219,7 +219,7 @@ class Compress::Zip::Writer
     end
   end
 
-  private def write_end_of_central_directory(offset, size)
+  private def write_end_of_central_directory(offset : UInt32, size : UInt32) : IO
     write Zip::END_OF_CENTRAL_DIRECTORY_HEADER_SIGNATURE
     write 0_u16                    # number of this disk
     write 0_u16                    # disk start
@@ -231,7 +231,7 @@ class Compress::Zip::Writer
     @io << @comment                # comment
   end
 
-  private def write(value)
+  private def write(value : Int32 | UInt16 | UInt32) : Nil
     @io.write_bytes(value, IO::ByteFormat::LittleEndian)
   end
 
diff --git a/src/compress/zlib/reader.cr b/src/compress/zlib/reader.cr
index 36c65ca25716..b1b1bdfcfe6c 100644
--- a/src/compress/zlib/reader.cr
+++ b/src/compress/zlib/reader.cr
@@ -27,7 +27,7 @@ class Compress::Zlib::Reader < IO
     yield reader ensure reader.close
   end
 
-  protected def self.read_header(io, dict)
+  protected def self.read_header(io : IO | IO::Memory, dict : Slice(UInt8)?) : Nil
     cmf = io.read_byte || invalid_header
 
     cm = cmf & 0xF
@@ -105,7 +105,7 @@ class Compress::Zlib::Reader < IO
     initialize(@io, @sync_close, @flate_io.dict)
   end
 
-  protected def self.invalid_header
+  protected def self.invalid_header : Nil
     raise Compress::Zlib::Error.new("Invalid header")
   end
 end
diff --git a/src/compress/zlib/writer.cr b/src/compress/zlib/writer.cr
index 909123a13d90..795634bf76b8 100644
--- a/src/compress/zlib/writer.cr
+++ b/src/compress/zlib/writer.cr
@@ -80,7 +80,7 @@ class Compress::Zlib::Writer < IO
     @io.close if @sync_close
   end
 
-  private def write_header
+  private def write_header : Nil
     @wrote_header = true
 
     # CMF byte: 7 for window size, 8 for compression method (deflate)
diff --git a/src/crypto/bcrypt.cr b/src/crypto/bcrypt.cr
index 92d90495c1df..40cb6edea55f 100644
--- a/src/crypto/bcrypt.cr
+++ b/src/crypto/bcrypt.cr
@@ -56,7 +56,7 @@ class Crypto::Bcrypt
   #
   # Crypto::Bcrypt.hash_secret "secret"
   # ```
-  def self.hash_secret(password, cost = DEFAULT_COST) : String
+  def self.hash_secret(password : String, cost : Int32 = DEFAULT_COST) : String
     # We make a clone here to we don't keep a mutable reference to the original string
     passwordb = password.to_unsafe.to_slice(password.bytesize + 1).clone # include leading 0
     saltb = Random::Secure.random_bytes(SALT_SIZE)
@@ -71,7 +71,7 @@ class Crypto::Bcrypt
   # password = Crypto::Bcrypt.new "secret", "salt_of_16_chars"
   # password.digest
   # ```
-  def self.new(password : String, salt : String, cost = DEFAULT_COST)
+  def self.new(password : String, salt : String, cost : Int32 = DEFAULT_COST) : Crypto::Bcrypt
     # We make a clone here to we don't keep a mutable reference to the original string
     passwordb = password.to_unsafe.to_slice(password.bytesize + 1).clone # include leading 0
     saltb = Base64.decode(salt, SALT_SIZE)
@@ -122,7 +122,7 @@ class Crypto::Bcrypt
 
   delegate to_slice, to: to_s
 
-  private def hash_password
+  private def hash_password : Slice(UInt8)
     blowfish = Blowfish.new(BLOWFISH_ROUNDS)
     blowfish.enhance_key_schedule(salt, password, cost)
 
diff --git a/src/crypto/bcrypt/base64.cr b/src/crypto/bcrypt/base64.cr
index 01c59f237d62..bbf26bb19fc9 100644
--- a/src/crypto/bcrypt/base64.cr
+++ b/src/crypto/bcrypt/base64.cr
@@ -20,7 +20,7 @@ module Crypto::Bcrypt::Base64
     51, 52, 53, -1, -1, -1, -1, -1,
   ]
 
-  def self.encode(d, len) : String
+  def self.encode(d : Slice(UInt8) | Array(UInt8), len : Int32) : String
     off = 0
 
     String.build do |str|
@@ -57,7 +57,7 @@ module Crypto::Bcrypt::Base64
     end
   end
 
-  def self.decode(string, maxolen) : Bytes
+  def self.decode(string : String, maxolen : Int32) : Bytes
     off, slen, olen = 0, string.size, 0
 
     i = -1
@@ -87,7 +87,7 @@ module Crypto::Bcrypt::Base64
     str[0, olen]
   end
 
-  private def self.char64(x)
+  private def self.char64(x : Char) : Int32 | Int8
     TABLE[x.ord]? || -1
   end
 end
diff --git a/src/crypto/bcrypt/blowfish.cr b/src/crypto/bcrypt/blowfish.cr
index 4a608c502811..6a09d4eab243 100644
--- a/src/crypto/bcrypt/blowfish.cr
+++ b/src/crypto/bcrypt/blowfish.cr
@@ -3,7 +3,7 @@ require "../blowfish"
 
 # :nodoc:
 class Crypto::Bcrypt::Blowfish < Crypto::Blowfish
-  def enhance_key_schedule(data, key, cost) : Nil
+  def enhance_key_schedule(data : Slice(UInt8), key : Slice(UInt8), cost : Int32) : Nil
     enhance_key_schedule(data, key)
 
     (1_u32 << cost).times do
@@ -12,7 +12,7 @@ class Crypto::Bcrypt::Blowfish < Crypto::Blowfish
     end
   end
 
-  def enhance_key_schedule(data, key) : Nil
+  def enhance_key_schedule(data : Slice(UInt8), key : Slice(UInt8)) : Nil
     pos = 0
 
     0.upto(17) do |i|
diff --git a/src/crypto/bcrypt/password.cr b/src/crypto/bcrypt/password.cr
index b98658c9e22f..e1526c369bfb 100644
--- a/src/crypto/bcrypt/password.cr
+++ b/src/crypto/bcrypt/password.cr
@@ -27,7 +27,7 @@ class Crypto::Bcrypt::Password
   # password = Crypto::Bcrypt::Password.create("super secret", cost: 10)
   # # => $2a$10$rI4xRiuAN2fyiKwynO6PPuorfuoM4L2PVv6hlnVJEmNLjqcibAfHq
   # ```
-  def self.create(password, cost = DEFAULT_COST) : self
+  def self.create(password : String, cost : Int32 = DEFAULT_COST) : self
     new(Bcrypt.hash_secret(password, cost).to_s)
   end
 
diff --git a/src/crypto/blowfish.cr b/src/crypto/blowfish.cr
index fc9734ad28aa..60e1c06ff262 100644
--- a/src/crypto/blowfish.cr
+++ b/src/crypto/blowfish.cr
@@ -15,7 +15,7 @@ class Crypto::Blowfish
     @s3 = @s.to_unsafe + 768
   end
 
-  def expand_key(key) : Nil
+  def expand_key(key : Array(UInt8) | Slice(UInt8)) : Nil
     pos = 0
 
     0.upto(17) do |i|
@@ -37,7 +37,7 @@ class Crypto::Blowfish
     end
   end
 
-  def encrypt_pair(l : UInt32, r : UInt32)
+  def encrypt_pair(l : UInt32, r : UInt32) : Tuple(UInt32, UInt32)
     0.upto(@rounds - 1) do |i|
       l ^= @p.to_unsafe[i]
       r ^= f(l)
@@ -68,7 +68,7 @@ class Crypto::Blowfish
   end
 
   @[AlwaysInline]
-  private def f(x : UInt32)
+  private def f(x : UInt32) : UInt32
     d = x.to_u8!
     c = (x >> 8).to_u8!
     b = (x >> 16).to_u8!
@@ -76,7 +76,7 @@ class Crypto::Blowfish
     ((@s.to_unsafe[a] &+ @s1[b]) ^ @s2[c]) &+ @s3[d]
   end
 
-  private def next_word(data, pos)
+  private def next_word(data : Array(UInt8) | Slice(UInt8), pos : Pointer(Int32)) : UInt32
     word = 0_u32
 
     4.times do
diff --git a/src/crypto/subtle.cr b/src/crypto/subtle.cr
index 41b572e5e4c4..e6c94779a93b 100644
--- a/src/crypto/subtle.cr
+++ b/src/crypto/subtle.cr
@@ -9,7 +9,7 @@ module Crypto::Subtle
   # ```
   #
   # NOTE: *x* and *y* must be able to respond to `to_slice`.
-  def self.constant_time_compare(x, y) : Bool
+  def self.constant_time_compare(x : Slice(Int32) | String | Slice(UInt8), y : Slice(Int32) | String | Slice(UInt8)) : Bool
     x = x.to_slice
     y = y.to_slice
     return false if x.size != y.size
@@ -23,7 +23,7 @@ module Crypto::Subtle
     constant_time_byte_eq(v, 0) == 1
   end
 
-  def self.constant_time_byte_eq(x, y)
+  def self.constant_time_byte_eq(x : UInt8 | Int32, y : Int32 | UInt8) : UInt8 | Int32
     z = ~(x ^ y)
     z &= z >> 4
     z &= z >> 2
diff --git a/src/csv/error.cr b/src/csv/error.cr
index 866cb6e7be7e..4a7a44880114 100644
--- a/src/csv/error.cr
+++ b/src/csv/error.cr
@@ -10,7 +10,7 @@ class CSV
     getter line_number : Int32
     getter column_number : Int32
 
-    def initialize(message, @line_number, @column_number)
+    def initialize(message : String, @line_number : Int32, @column_number : Int32)
       super("#{message} at line #{@line_number}, column #{@column_number}")
     end
   end
diff --git a/src/csv/lexer.cr b/src/csv/lexer.cr
index 9d3d04c68c0f..36da66d263c1 100644
--- a/src/csv/lexer.cr
+++ b/src/csv/lexer.cr
@@ -15,12 +15,12 @@ require "csv"
 # ```
 abstract class CSV::Lexer
   # Creates a CSV lexer from a `String`.
-  def self.new(string : String, separator = DEFAULT_SEPARATOR, quote_char = DEFAULT_QUOTE_CHAR)
+  def self.new(string : String, separator : Char = DEFAULT_SEPARATOR, quote_char : Char = DEFAULT_QUOTE_CHAR) : CSV::Lexer::StringBased
     StringBased.new(string, separator, quote_char)
   end
 
   # Creates a CSV lexer from an `IO`.
-  def self.new(io : IO, separator = DEFAULT_SEPARATOR, quote_char = DEFAULT_QUOTE_CHAR)
+  def self.new(io : IO, separator : Char = DEFAULT_SEPARATOR, quote_char : Char = DEFAULT_QUOTE_CHAR) : CSV::Lexer::IOBased
     IOBased.new(io, separator, quote_char)
   end
 
@@ -101,7 +101,7 @@ abstract class CSV::Lexer
     @token
   end
 
-  private def consume_quoted_cell
+  private def consume_quoted_cell : String
     @buffer.clear
     while true
       case char = next_char
@@ -126,7 +126,7 @@ abstract class CSV::Lexer
     @buffer.to_s
   end
 
-  private def check_last_empty_column
+  private def check_last_empty_column : Bool?
     case next_char
     when '\r', '\n', '\0'
       @last_empty_column = true
@@ -135,7 +135,7 @@ abstract class CSV::Lexer
     end
   end
 
-  private def next_char
+  private def next_char : Char
     @column_number += 1
     char = next_char_no_column_increment
     if char.in?('\n', '\r')
@@ -145,7 +145,7 @@ abstract class CSV::Lexer
     char
   end
 
-  private def raise(msg)
+  private def raise(msg : String) : Nil
     ::raise CSV::MalformedCSVError.new(msg, @line_number, @column_number)
   end
 end
diff --git a/src/csv/lexer/io_based.cr b/src/csv/lexer/io_based.cr
index f33e83d99d9e..ab68f85056b3 100644
--- a/src/csv/lexer/io_based.cr
+++ b/src/csv/lexer/io_based.cr
@@ -2,7 +2,7 @@ require "csv"
 
 # :nodoc:
 class CSV::Lexer::IOBased < CSV::Lexer
-  def initialize(@io : IO, separator = DEFAULT_SEPARATOR, quote_char = DEFAULT_QUOTE_CHAR)
+  def initialize(@io : IO, separator : Char = DEFAULT_SEPARATOR, quote_char : Char = DEFAULT_QUOTE_CHAR)
     super(separator, quote_char)
     @current_char = @io.read_char || '\0'
   end
diff --git a/src/csv/lexer/string_based.cr b/src/csv/lexer/string_based.cr
index c9d1193adc34..b1ab6521b0f2 100644
--- a/src/csv/lexer/string_based.cr
+++ b/src/csv/lexer/string_based.cr
@@ -2,7 +2,7 @@ require "csv"
 
 # :nodoc:
 class CSV::Lexer::StringBased < CSV::Lexer
-  def initialize(string, separator = DEFAULT_SEPARATOR, quote_char = DEFAULT_QUOTE_CHAR)
+  def initialize(string : String, separator : Char = DEFAULT_SEPARATOR, quote_char : Char = DEFAULT_QUOTE_CHAR)
     super(separator, quote_char)
     @reader = Char::Reader.new(string)
     if @reader.current_char == '\n'
diff --git a/src/deque.cr b/src/deque.cr
index d2154bb462e6..9ab23f605939 100644
--- a/src/deque.cr
+++ b/src/deque.cr
@@ -96,7 +96,7 @@ class Deque(T)
   # ```
   # Deque.new([1, 2, 3]) # => Deque{1, 2, 3}
   # ```
-  def self.new(array : Array(T))
+  def self.new(array : Array(T)) : Deque(Int32) | Deque(String)
     Deque(T).new(array.size) { |i| array[i] }
   end
 
diff --git a/src/digest/adler32.cr b/src/digest/adler32.cr
index 0b31e52a80b1..49d314d617af 100644
--- a/src/digest/adler32.cr
+++ b/src/digest/adler32.cr
@@ -17,16 +17,16 @@ class Digest::Adler32 < ::Digest
     LibZ.adler32(0, nil, 0).to_u32
   end
 
-  def self.checksum(data) : UInt32
+  def self.checksum(data : Slice(UInt8) | String) : UInt32
     update(data, initial)
   end
 
-  def self.update(data, adler32 : UInt32) : UInt32
+  def self.update(data : Slice(UInt8) | String, adler32 : UInt32) : UInt32
     slice = data.to_slice
     LibZ.adler32(adler32, slice, slice.size).to_u32
   end
 
-  def self.combine(adler1 : UInt32, adler2 : UInt32, len) : UInt32
+  def self.combine(adler1 : UInt32, adler2 : UInt32, len : Int32) : UInt32
     LibZ.adler32_combine(adler1, adler2, len).to_u32
   end
 
diff --git a/src/digest/crc32.cr b/src/digest/crc32.cr
index b32ecd68e41f..807ba630e19f 100644
--- a/src/digest/crc32.cr
+++ b/src/digest/crc32.cr
@@ -17,16 +17,16 @@ class Digest::CRC32 < ::Digest
     LibZ.crc32(0, nil, 0).to_u32
   end
 
-  def self.checksum(data) : UInt32
+  def self.checksum(data : String) : UInt32
     update(data, initial)
   end
 
-  def self.update(data, crc32 : UInt32) : UInt32
+  def self.update(data : Slice(UInt8) | String, crc32 : UInt32) : UInt32
     slice = data.to_slice
     LibZ.crc32(crc32, slice, slice.size).to_u32
   end
 
-  def self.combine(crc1 : UInt32, crc2 : UInt32, len) : UInt32
+  def self.combine(crc1 : UInt32, crc2 : UInt32, len : Int32) : UInt32
     LibZ.crc32_combine(crc1, crc2, len).to_u32
   end
 
diff --git a/src/digest/digest.cr b/src/digest/digest.cr
index c55d2a84ead1..6ddfec3af84c 100644
--- a/src/digest/digest.cr
+++ b/src/digest/digest.cr
@@ -21,7 +21,7 @@ abstract class Digest
   # The modules adds convenient class methods as `Digest::MD5.digest`, `Digest::MD5.hexdigest`.
   module ClassMethods
     # Returns the hash of *data*. *data* must respond to `#to_slice`.
-    def digest(data) : Bytes
+    def digest(data : String) : Bytes
       digest do |ctx|
         ctx.update(data.to_slice)
       end
@@ -52,7 +52,7 @@ abstract class Digest
     #
     # Digest::MD5.hexdigest("foo") # => "acbd18db4cc2f85cedef654fccc4a4d8"
     # ```
-    def hexdigest(data) : String
+    def hexdigest(data : String | Slice(UInt8) | StaticArray(UInt8, 1)) : String
       hexdigest &.update(data)
     end
 
@@ -85,7 +85,7 @@ abstract class Digest
     #
     # Digest::SHA1.base64digest("foo") # => "C+7Hteo/D9vJXQ3UfzxbwnXaijM="
     # ```
-    def base64digest(data) : String
+    def base64digest(data : String) : String
       base64digest &.update(data)
     end
 
@@ -113,7 +113,7 @@ abstract class Digest
 
   @finished = false
 
-  def update(data) : self
+  def update(data : StaticArray(UInt8, 16) | String | StaticArray(UInt8, 1)) : self
     update data.to_slice
   end
 
@@ -229,7 +229,7 @@ abstract class Digest
   end
 
   # :ditto:
-  def <<(data) : self
+  def <<(data : File | Slice(UInt8) | String | IO::FileDescriptor) : self
     update(data)
   end
 
diff --git a/src/dir.cr b/src/dir.cr
index 121a77172789..5754704091c2 100644
--- a/src/dir.cr
+++ b/src/dir.cr
@@ -284,14 +284,14 @@ class Dir
   # Dir.mkdir("testdir")
   # Dir.exists?("testdir") # => true
   # ```
-  def self.mkdir(path : Path | String, mode = 0o777) : Nil
+  def self.mkdir(path : Path | String, mode : Int32 = 0o777) : Nil
     Crystal::System::Dir.create(path.to_s, mode)
   end
 
   # Creates a new directory at the given path, including any non-existing
   # intermediate directories. The linux-style permission mode can be specified,
   # with a default of 777 (0o777).
-  def self.mkdir_p(path : Path | String, mode = 0o777) : Nil
+  def self.mkdir_p(path : Path | String, mode : Int32 = 0o777) : Nil
     return if Dir.exists?(path)
 
     path = Path.new path
diff --git a/src/dir/glob.cr b/src/dir/glob.cr
index 8b49bf3cb307..af7f2bb41889 100644
--- a/src/dir/glob.cr
+++ b/src/dir/glob.cr
@@ -64,7 +64,7 @@ class Dir
   # equivalent to
   # `match: File::MatchOptions.glob_default | File::MatchOptions::DotFiles`.
   @[Deprecated("Use the overload with a `match` parameter instead")]
-  def self.glob(*patterns : Path | String, match_hidden, follow_symlinks = false) : Array(String)
+  def self.glob(*patterns : Path | String, match_hidden : Bool, follow_symlinks : Bool = false) : Array(String)
     glob(patterns, match: match_hidden_to_options(match_hidden), follow_symlinks: follow_symlinks)
   end
 
@@ -127,7 +127,7 @@ class Dir
     end
   end
 
-  private def self.match_hidden_to_options(match_hidden)
+  private def self.match_hidden_to_options(match_hidden : Bool) : File::MatchOptions
     options = File::MatchOptions.glob_default
     options |= File::MatchOptions::DotFiles if match_hidden
     options
@@ -138,7 +138,7 @@ class Dir
     record DirectoriesOnly
     record ConstantEntry, path : String, merged : Bool
     record EntryMatch, pattern : String do
-      def matches?(string) : Bool
+      def matches?(string : String) : Bool
         File.match?(pattern, string)
       end
     end
@@ -146,7 +146,7 @@ class Dir
     record ConstantDirectory, path : String
     record RootDirectory
     record DirectoryMatch, pattern : String do
-      def matches?(string) : Bool
+      def matches?(string : String) : Bool
         File.match?(pattern, string)
       end
     end
@@ -173,7 +173,7 @@ class Dir
       end
     end
 
-    private def self.compile(pattern)
+    private def self.compile(pattern : String) : Array(Array(Dir::Globber::ConstantDirectory | Dir::Globber::ConstantEntry | Dir::Globber::DirectoriesOnly | Dir::Globber::DirectoryMatch | Dir::Globber::EntryMatch | Dir::Globber::RecursiveDirectories | Dir::Globber::RootDirectory))
       expanded_patterns = [] of String
       expand_brace_pattern(pattern, expanded_patterns)
 
@@ -182,7 +182,7 @@ class Dir
       end
     end
 
-    private def self.single_compile(glob)
+    private def self.single_compile(glob : String) : Array(Dir::Globber::ConstantDirectory | Dir::Globber::ConstantEntry | Dir::Globber::DirectoriesOnly | Dir::Globber::DirectoryMatch | Dir::Globber::EntryMatch | Dir::Globber::RecursiveDirectories | Dir::Globber::RootDirectory)
       list = [] of PatternType
       return list if glob.empty?
 
@@ -225,7 +225,7 @@ class Dir
       list
     end
 
-    private def self.constant_entry?(file)
+    private def self.constant_entry?(file : String) : Bool
       file.each_char do |char|
         return false if char.in?('*', '?', '[', '\\')
       end
@@ -378,11 +378,11 @@ class Dir
       end
     end
 
-    private def self.root
+    private def self.root : String
       Path[Dir.current].anchor.not_nil!.to_s
     end
 
-    private def self.dir?(path, follow_symlinks)
+    private def self.dir?(path : String, follow_symlinks : Bool) : Bool
       if info = File.info?(path, follow_symlinks: follow_symlinks)
         info.type.directory?
       else
@@ -390,7 +390,7 @@ class Dir
       end
     end
 
-    private def self.join(path, entry)
+    private def self.join(path : String?, entry : String) : String
       return entry unless path
       return "#{root}#{entry}" if path == File::SEPARATOR_STRING
 
@@ -407,7 +407,7 @@ class Dir
     rescue exc : File::NotFoundError
     end
 
-    private def self.read_entry(dir)
+    private def self.read_entry(dir : Dir) : Crystal::System::Dir::Entry?
       return unless dir
 
       # By doing this we get an Entry struct which already tells us
@@ -416,7 +416,7 @@ class Dir
       Crystal::System::Dir.next_entry(dir.@dir, dir.path)
     end
 
-    private def self.matches_file?(entry, match)
+    private def self.matches_file?(entry : Crystal::System::Dir::Entry, match : File::MatchOptions) : Bool
       return false if entry.name.starts_with?('.') && !match.dot_files?
       return false if entry.native_hidden? && !match.native_hidden?
       return false if entry.os_hidden? && !match.os_hidden?
@@ -425,7 +425,7 @@ class Dir
 
     # :nodoc:
     # FIXME: The expansion mechanism does not work for complex brace patterns.
-    private def self.expand_brace_pattern(pattern : String, expanded) : Array(String)?
+    private def self.expand_brace_pattern(pattern : String, expanded : Array(String)) : Array(String)?
       reader = Char::Reader.new(pattern)
 
       lbrace = nil
diff --git a/src/ecr/lexer.cr b/src/ecr/lexer.cr
index 81fedac17087..bb6508785d5e 100644
--- a/src/ecr/lexer.cr
+++ b/src/ecr/lexer.cr
@@ -29,12 +29,12 @@ class ECR::Lexer
     getter line_number : Int32
     getter column_number : Int32
 
-    def initialize(message, @line_number, @column_number)
+    def initialize(message : String, @line_number : Int32, @column_number : Int32)
       super(message)
     end
   end
 
-  def initialize(string)
+  def initialize(string : String)
     @reader = Char::Reader.new(string)
     @token = Token.new
     @line_number = 1
@@ -78,7 +78,7 @@ class ECR::Lexer
     consume_string
   end
 
-  private def consume_string
+  private def consume_string : ECR::Lexer::Token
     start_pos = current_pos
     while true
       case current_char
@@ -102,7 +102,7 @@ class ECR::Lexer
     @token
   end
 
-  private def consume_control(is_output, is_escape, suppress_leading)
+  private def consume_control(is_output : Bool?, is_escape : Bool?, suppress_leading : Bool) : ECR::Lexer::Token
     start_pos = current_pos
     while true
       case current_char
@@ -158,7 +158,7 @@ class ECR::Lexer
     @token
   end
 
-  private def setup_control_token(start_pos, is_escape, suppress_leading, suppress_trailing)
+  private def setup_control_token(start_pos : Int32, is_escape : Bool?, suppress_leading : Bool, suppress_trailing : Bool) : Char
     @token.suppress_leading = !is_escape && suppress_leading
     @token.suppress_trailing = !is_escape && suppress_trailing
     @token.value =
@@ -174,41 +174,41 @@ class ECR::Lexer
     next_char
   end
 
-  private def copy_location_info_to_token
+  private def copy_location_info_to_token : Int32
     @token.line_number = @line_number
     @token.column_number = @column_number
   end
 
-  private def current_char
+  private def current_char : Char
     @reader.current_char
   end
 
-  private def next_char
+  private def next_char : Char
     @column_number += 1
     next_char_no_column_increment
   end
 
-  private def next_char_no_column_increment
+  private def next_char_no_column_increment : Char
     @reader.next_char
   end
 
-  private def peek_next_char
+  private def peek_next_char : Char
     @reader.peek_next_char
   end
 
-  private def current_pos
+  private def current_pos : Int32
     @reader.pos
   end
 
-  private def string_range(start_pos)
+  private def string_range(start_pos : Int32) : String
     string_range(start_pos, current_pos)
   end
 
-  private def string_range(start_pos, end_pos)
+  private def string_range(start_pos : Int32, end_pos : Int32) : String
     @reader.string.byte_slice(start_pos, end_pos - start_pos)
   end
 
-  private def raise(message : String)
+  private def raise(message : String) : Nil
     raise SyntaxException.new(message, @line_number, @column_number)
   end
 end
diff --git a/src/ecr/processor.cr b/src/ecr/processor.cr
index 786879e94273..058801dd56e0 100644
--- a/src/ecr/processor.cr
+++ b/src/ecr/processor.cr
@@ -11,7 +11,7 @@ module ECR
   end
 
   # :nodoc:
-  def process_string(string, filename, buffer_name = DefaultBufferName) : String
+  def process_string(string : String, filename : String, buffer_name : String = DefaultBufferName) : String
     lexer = Lexer.new string
     token = lexer.next_token
 
@@ -65,7 +65,7 @@ module ECR
     end
   end
 
-  private def suppress_leading_indentation(token, string)
+  private def suppress_leading_indentation(token : ECR::Lexer::Token, string : String) : String
     # To suppress leading indentation we find the last index of a newline and
     # then check if all chars after that are whitespace.
     # We use a Char::Reader for this for maximum efficiency.
@@ -85,14 +85,14 @@ module ECR
     string
   end
 
-  private def suppress_trailing_whitespace(token, suppress_trailing)
+  private def suppress_trailing_whitespace(token : ECR::Lexer::Token, suppress_trailing : Bool) : String?
     if suppress_trailing && token.type.string?
       newline_index = token.value.index('\n')
       token.value = token.value[newline_index + 1..-1] if newline_index
     end
   end
 
-  private def append_loc(str, filename, line_number, column_number)
+  private def append_loc(str : String::Builder, filename : String, line_number : Int32, column_number : Int32) : String::Builder
     str << %(#<loc:")
     str << filename
     str << %(",)
diff --git a/src/env.cr b/src/env.cr
index 13779f3051aa..20f6d15107ad 100644
--- a/src/env.cr
+++ b/src/env.cr
@@ -34,7 +34,7 @@ module ENV
   # If *value* is `nil`, the environment variable is deleted.
   #
   # If *key* or *value* contains a null-byte an `ArgumentError` is raised.
-  def self.[]=(key : String, value : String?)
+  def self.[]=(key : String, value : String?) : String?
     Crystal::System::Env.set(key, value)
 
     value
@@ -52,7 +52,7 @@ module ENV
 
   # Retrieves a value corresponding to the given *key*. Raises a `KeyError` exception if the
   # key does not exist.
-  def self.fetch(key) : String
+  def self.fetch(key : String) : String
     fetch(key) do
       raise KeyError.new "Missing ENV key: #{key.inspect}"
     end
@@ -60,7 +60,7 @@ module ENV
 
   # Retrieves a value corresponding to the given *key*. Return the second argument's value
   # if the *key* does not exist.
-  def self.fetch(key, default : T) : String | T forall T
+  def self.fetch(key : String, default : T) : String | T forall T
     fetch(key) { default }
   end
 
diff --git a/src/exception.cr b/src/exception.cr
index d3ab6356d188..e7fd30007835 100644
--- a/src/exception.cr
+++ b/src/exception.cr
@@ -34,7 +34,7 @@ class Exception
   # Returns any backtrace associated with the exception if the call stack exists.
   # The backtrace is an array of strings, each containing
   # “0xAddress: Function at File Line Column”.
-  def backtrace?
+  def backtrace? : Array(String)?
     @callstack.try &.printable_backtrace
   end
 
@@ -76,7 +76,7 @@ end
 # a[2] # raises IndexError
 # ```
 class IndexError < Exception
-  def initialize(message = "Index out of bounds")
+  def initialize(message : String = "Index out of bounds")
     super(message)
   end
 end
@@ -87,7 +87,7 @@ end
 # [1, 2, 3].first(-4) # raises ArgumentError (attempt to take negative size)
 # ```
 class ArgumentError < Exception
-  def initialize(message = "Argument error")
+  def initialize(message : String = "Argument error")
     super(message)
   end
 end
@@ -104,7 +104,7 @@ class TypeCastError < Exception
 end
 
 class InvalidByteSequenceError < Exception
-  def initialize(message = "Invalid byte sequence in UTF-8 string")
+  def initialize(message : String = "Invalid byte sequence in UTF-8 string")
     super(message)
   end
 end
@@ -124,7 +124,7 @@ end
 # 1 // 0 # raises DivisionByZeroError (Division by 0)
 # ```
 class DivisionByZeroError < Exception
-  def initialize(message = "Division by 0")
+  def initialize(message : String = "Division by 0")
     super(message)
   end
 end
@@ -138,7 +138,7 @@ end
 # Float64::MAX.to_f32 # raises OverflowError (Arithmetic overflow)
 # ```
 class OverflowError < Exception
-  def initialize(message = "Arithmetic overflow")
+  def initialize(message : String = "Arithmetic overflow")
     super(message)
   end
 end
@@ -148,7 +148,7 @@ end
 # This can be used either to stub out method bodies, or when the method is not
 # implemented on the current platform.
 class NotImplementedError < Exception
-  def initialize(item)
+  def initialize(item : String)
     super("Not Implemented: #{item}")
   end
 end
@@ -159,7 +159,7 @@ end
 # "hello".index('x').not_nil! # raises NilAssertionError ("hello" does not contain 'x')
 # ```
 class NilAssertionError < Exception
-  def initialize(message = "Nil assertion failed")
+  def initialize(message : String = "Nil assertion failed")
     super(message)
   end
 end
diff --git a/src/exception/call_stack.cr b/src/exception/call_stack.cr
index e49cefe151d6..8b81a03c9bdf 100644
--- a/src/exception/call_stack.cr
+++ b/src/exception/call_stack.cr
@@ -22,7 +22,7 @@ struct Exception::CallStack
 
   @@skip = [] of String
 
-  def self.skip(filename) : Nil
+  def self.skip(filename : String) : Nil
     @@skip << filename
   end
 
@@ -40,7 +40,7 @@ struct Exception::CallStack
     @backtrace ||= decode_backtrace
   end
 
-  private def decode_backtrace
+  private def decode_backtrace : Array(String)
     {% if flag?(:wasm32) %}
       [] of String
     {% else %}
diff --git a/src/exception/call_stack/dwarf.cr b/src/exception/call_stack/dwarf.cr
index 253a72a38ebc..5976c7b5728b 100644
--- a/src/exception/call_stack/dwarf.cr
+++ b/src/exception/call_stack/dwarf.cr
@@ -30,7 +30,7 @@ struct Exception::CallStack
     end
   end
 
-  protected def self.decode_line_number(pc)
+  protected def self.decode_line_number(pc : UInt64) : Tuple(String, Int32, Int32)
     load_debug_info
     if ln = @@dwarf_line_numbers
       if row = ln.find(pc)
@@ -40,7 +40,7 @@ struct Exception::CallStack
     {"??", 0, 0}
   end
 
-  protected def self.decode_function_name(pc)
+  protected def self.decode_function_name(pc : UInt64) : String?
     load_debug_info
     if fn = @@dwarf_function_names
       fn.each do |(low_pc, high_pc, function_name)|
diff --git a/src/exception/call_stack/elf.cr b/src/exception/call_stack/elf.cr
index c4d90a6e879b..e984be7535c3 100644
--- a/src/exception/call_stack/elf.cr
+++ b/src/exception/call_stack/elf.cr
@@ -52,7 +52,7 @@ struct Exception::CallStack
     {% end %}
   end
 
-  protected def self.read_dwarf_sections(program, base_address = 0)
+  protected def self.read_dwarf_sections(program : String, base_address : UInt64 | Int32 = 0) : Array(Tuple(UInt64, UInt64, String))?
     {{ flag?(:win32) ? Crystal::PE : Crystal::ELF }}.open(program) do |image|
       {% if flag?(:win32) %}
         base_address -= image.original_image_base
@@ -101,7 +101,7 @@ struct Exception::CallStack
     end
   end
 
-  protected def self.decode_address(ip)
+  protected def self.decode_address(ip : Pointer(Void)) : UInt64
     ip.address
   end
 end
diff --git a/src/exception/call_stack/libunwind.cr b/src/exception/call_stack/libunwind.cr
index c0f75867aeba..f394c14644ab 100644
--- a/src/exception/call_stack/libunwind.cr
+++ b/src/exception/call_stack/libunwind.cr
@@ -34,7 +34,7 @@ struct Exception::CallStack
     end
   {% end %}
 
-  def self.setup_crash_handler
+  def self.setup_crash_handler : Int32?
     {% if flag?(:win32) %}
       Crystal::System::Signal.setup_seh_handler
     {% else %}
@@ -78,7 +78,7 @@ struct Exception::CallStack
       @count = 0
     end
 
-    def incr
+    def incr : Int32
       @count += 1
     end
   end
@@ -107,14 +107,14 @@ struct Exception::CallStack
     print_frame(rf)
   end
 
-  private def self.print_frame(repeated_frame)
+  private def self.print_frame(repeated_frame : Exception::CallStack::RepeatedFrame) : Nil
     Crystal::System.print_error "[%p] ", repeated_frame.ip
     print_frame_location(repeated_frame)
     Crystal::System.print_error " (%d times)", repeated_frame.count + 1 unless repeated_frame.count == 0
     Crystal::System.print_error "\n"
   end
 
-  private def self.print_frame_location(repeated_frame)
+  private def self.print_frame_location(repeated_frame : Exception::CallStack::RepeatedFrame) : Nil
     {% if flag?(:debug) %}
       if @@dwarf_loaded
         pc = CallStack.decode_address(repeated_frame.ip)
@@ -136,7 +136,7 @@ struct Exception::CallStack
     Crystal::System.print_error "???"
   end
 
-  protected def self.decode_frame(ip)
+  protected def self.decode_frame(ip : Pointer(Void)) : Tuple(Int64, String, String)?
     decode_frame(ip) do |offset, symbol, file|
       symbol = symbol ? String.new(symbol) : "??"
       file = file ? String.new(file) : "??"
diff --git a/src/fiber.cr b/src/fiber.cr
index a9c9c38ca4e5..dd04d09b39c6 100644
--- a/src/fiber.cr
+++ b/src/fiber.cr
@@ -86,7 +86,7 @@ class Fiber
   property list_next : Fiber?
 
   # :nodoc:
-  def self.inactive(fiber : Fiber)
+  def self.inactive(fiber : Fiber) : Nil
     fibers.delete(fiber)
   end
 
@@ -134,7 +134,7 @@ class Fiber
   {% end %}
 
   # :nodoc:
-  def initialize(stack : Void*, thread)
+  def initialize(stack : Void*, thread : Thread)
     @proc = Proc(Void).new { }
 
     # TODO: should creating a new context for the main fiber also be platform specific?
@@ -164,7 +164,7 @@ class Fiber
   end
 
   # :nodoc:
-  def run
+  def run : Nil
     GC.unlock_read
 
     @proc.call
@@ -389,14 +389,14 @@ class Fiber
   # :nodoc:
   #
   # See `Reference#exec_recursive` for details.
-  def exec_recursive_hash
+  def exec_recursive_hash : Hash(Tuple(UInt64, Symbol), Nil)
     @exec_recursive_hash ||= Hash({UInt64, Symbol}, Nil).new
   end
 
   # :nodoc:
   #
   # See `Reference#exec_recursive_clone` for details.
-  def exec_recursive_clone_hash
+  def exec_recursive_clone_hash : Hash(UInt64, UInt64)
     @exec_recursive_clone_hash ||= Hash(UInt64, UInt64).new
   end
 end
diff --git a/src/fiber/context/x86_64-sysv.cr b/src/fiber/context/x86_64-sysv.cr
index f909ce0112b9..562459b58f2a 100644
--- a/src/fiber/context/x86_64-sysv.cr
+++ b/src/fiber/context/x86_64-sysv.cr
@@ -2,7 +2,7 @@
 
 class Fiber
   # :nodoc:
-  def makecontext(stack_ptr, fiber_main) : Nil
+  def makecontext(stack_ptr : Pointer(Pointer(Void)), fiber_main : Proc(Fiber, Nil)) : Nil
     # in x86-64, the context switch push/pop 6 registers + the return address
     # that is left on the stack, we thus reserve space for 7 pointers:
     @context.stack_top = (stack_ptr - 7).as(Void*)
@@ -15,7 +15,7 @@ class Fiber
   # :nodoc:
   @[NoInline]
   @[Naked]
-  def self.swapcontext(current_context, new_context) : Nil
+  def self.swapcontext(current_context : Pointer(Fiber::Context), new_context : Pointer(Fiber::Context)) : Nil
     {% if compare_versions(Crystal::LLVM_VERSION, "9.0.0") >= 0 %}
       #                %rdi           , %rsi
       asm("
diff --git a/src/fiber/stack.cr b/src/fiber/stack.cr
index 9666b506db0c..2fde3633c5a9 100644
--- a/src/fiber/stack.cr
+++ b/src/fiber/stack.cr
@@ -5,7 +5,7 @@ class Fiber
     getter bottom : Void*
     getter? reusable : Bool
 
-    def initialize(@pointer, @bottom, *, @reusable = false)
+    def initialize(@pointer : Pointer(Void), @bottom : Pointer(Void), *, @reusable : Bool = false)
     end
 
     def first_addressable_pointer : Void**
diff --git a/src/fiber/stack_pool.cr b/src/fiber/stack_pool.cr
index 4a36f47f4abd..f1bf302d4803 100644
--- a/src/fiber/stack_pool.cr
+++ b/src/fiber/stack_pool.cr
@@ -24,7 +24,7 @@ class Fiber
       {% end %}
     end
 
-    def finalize
+    def finalize : Nil
       @deque.each do |stack|
         Crystal::System::Fiber.free_stack(stack.pointer, STACK_SIZE)
       end
@@ -32,7 +32,7 @@ class Fiber
 
     # Removes and frees at most *count* stacks from the top of the pool,
     # returning memory to the operating system.
-    def collect(count = lazy_size // 2) : Nil
+    def collect(count : Int32 = lazy_size // 2) : Nil
       count.times do
         if stack = shift?
           Crystal::System::Fiber.free_stack(stack.pointer, STACK_SIZE)
@@ -42,7 +42,7 @@ class Fiber
       end
     end
 
-    def collect_loop(every = 5.seconds) : Nil
+    def collect_loop(every : Time::Span = 5.seconds) : Nil
       loop do
         sleep every
         collect
@@ -77,7 +77,7 @@ class Fiber
       @deque.size
     end
 
-    private def shift?
+    private def shift? : Fiber::Stack?
       {% if flag?(:execution_context) %}
         @lock.sync { @deque.shift? } unless @deque.empty?
       {% else %}
@@ -85,7 +85,7 @@ class Fiber
       {% end %}
     end
 
-    private def pop?
+    private def pop? : Fiber::Stack?
       {% if flag?(:execution_context) %}
         if (stack = Thread.current.dead_fiber_stack?) && stack.reusable?
           stack
diff --git a/src/file/error.cr b/src/file/error.cr
index 355496488bc4..c242b04adaa4 100644
--- a/src/file/error.cr
+++ b/src/file/error.cr
@@ -8,7 +8,7 @@ class File::Error < IO::Error
     target.not_nil!
   end
 
-  private def self.new_from_os_error(message, os_error, **opts)
+  private def self.new_from_os_error(message : String, os_error : Errno, **opts) : File::Error
     case os_error
     when Errno::ENOENT, WinError::ERROR_FILE_NOT_FOUND, WinError::ERROR_PATH_NOT_FOUND
       File::NotFoundError.new(message, **opts)
@@ -23,15 +23,15 @@ class File::Error < IO::Error
     end
   end
 
-  def initialize(message, *, file : String | Path, @other : String? = nil)
+  def initialize(message : String, *, file : String | Path, @other : String? = nil)
     super message, target: file
   end
 
-  protected def self.build_message(message, *, file : String) : String
+  protected def self.build_message(message : String, *, file : String) : String
     "#{message}: '#{file.inspect_unquoted}'"
   end
 
-  protected def self.build_message(message, *, file : String, other : String) : String
+  protected def self.build_message(message : String, *, file : String, other : String) : String
     "#{message}: '#{file.inspect_unquoted}' -> '#{other.inspect_unquoted}'"
   end
 
diff --git a/src/file/info.cr b/src/file/info.cr
index ea5ed2ac82f6..0784d63ccead 100644
--- a/src/file/info.cr
+++ b/src/file/info.cr
@@ -56,7 +56,7 @@ class File
     OwnerRead    = 0o400
     OwnerAll     = 0o700
 
-    def self.new(int : Int)
+    def self.new(int : Int) : File::Permissions
       new(int.to_i16)
     end
 
@@ -127,13 +127,13 @@ class File
 
     # Returns true if this `Info` represents a standard file. Shortcut for
     # `type.file?`.
-    def file?
+    def file? : Bool
       type.file?
     end
 
     # Returns true if this `Info` represents a directory. Shortcut for
     # `type.directory?`.
-    def directory?
+    def directory? : Bool
       type.directory?
     end
 
diff --git a/src/file/tempfile.cr b/src/file/tempfile.cr
index e1b764f35dbf..6e67405d81a2 100644
--- a/src/file/tempfile.cr
+++ b/src/file/tempfile.cr
@@ -13,7 +13,7 @@ class File
   # user-provided values that would result in navigating the directory tree.
   #
   # The path will be placed in *dir* which defaults to the standard temporary directory `Dir.tempdir`.
-  def self.tempname(prefix : String?, suffix : String?, *, dir : String = Dir.tempdir)
+  def self.tempname(prefix : String?, suffix : String?, *, dir : String = Dir.tempdir) : String
     name = String.build do |io|
       if prefix
         io << prefix
@@ -41,7 +41,7 @@ class File
   # File.tempname          # => "/tmp/20171206-1234-449386"
   # File.tempname(".sock") # => "/tmp/20171206-1234-449386.sock"
   # ```
-  def self.tempname(suffix : String? = nil, *, dir : String = Dir.tempdir)
+  def self.tempname(suffix : String? = nil, *, dir : String = Dir.tempdir) : String
     tempname(prefix: nil, suffix: suffix, dir: dir)
   end
 
@@ -63,7 +63,7 @@ class File
   # *encoding* and *invalid* are passed to `IO#set_encoding`.
   #
   # It is the caller's responsibility to remove the file when no longer needed.
-  def self.tempfile(prefix : String?, suffix : String?, *, dir : String = Dir.tempdir, encoding = nil, invalid = nil)
+  def self.tempfile(prefix : String?, suffix : String?, *, dir : String = Dir.tempdir, encoding : Nil = nil, invalid : Nil = nil) : File
     fileno, path = Crystal::System::File.mktemp(prefix, suffix, dir)
     new(path, fileno, blocking: true, encoding: encoding, invalid: invalid)
   end
@@ -86,7 +86,7 @@ class File
   # *encoding* and *invalid* are passed to `IO#set_encoding`.
   #
   # It is the caller's responsibility to remove the file when no longer needed.
-  def self.tempfile(suffix : String? = nil, *, dir : String = Dir.tempdir, encoding = nil, invalid = nil)
+  def self.tempfile(suffix : String? = nil, *, dir : String = Dir.tempdir, encoding : Nil = nil, invalid : Nil = nil) : File
     tempfile(prefix: nil, suffix: suffix, dir: dir, encoding: encoding, invalid: invalid)
   end
 
diff --git a/src/file_utils.cr b/src/file_utils.cr
index 292d911b1630..91774db423c5 100644
--- a/src/file_utils.cr
+++ b/src/file_utils.cr
@@ -124,7 +124,7 @@ module FileUtils
     cp_recursive(src_path, dest_path)
   end
 
-  private def cp_recursive(src_path : Path | String, dest_path : Path | String)
+  private def cp_recursive(src_path : Path | String, dest_path : Path | String) : Nil
     if Dir.exists?(src_path)
       Dir.mkdir(dest_path) unless Dir.exists?(dest_path)
       Dir.each_child(src_path) do |entry|
@@ -257,7 +257,7 @@ module FileUtils
   # ```
   #
   # NOTE: Alias of `Dir.mkdir`
-  def mkdir(path : Path | String, mode = 0o777) : Nil
+  def mkdir(path : Path | String, mode : Int32 = 0o777) : Nil
     Dir.mkdir(path, mode)
   end
 
@@ -269,7 +269,7 @@ module FileUtils
   #
   # FileUtils.mkdir(["foo", "bar"])
   # ```
-  def mkdir(paths : Enumerable(Path | String), mode = 0o777) : Nil
+  def mkdir(paths : Enumerable(Path | String), mode : Int32 = 0o777) : Nil
     paths.each do |path|
       Dir.mkdir(path, mode)
     end
@@ -286,7 +286,7 @@ module FileUtils
   # ```
   #
   # NOTE: Alias of `Dir.mkdir_p`
-  def mkdir_p(path : Path | String, mode = 0o777) : Nil
+  def mkdir_p(path : Path | String, mode : Int32 = 0o777) : Nil
     Dir.mkdir_p(path, mode)
   end
 
@@ -299,7 +299,7 @@ module FileUtils
   #
   # FileUtils.mkdir_p(["foo", "bar", "baz", "dir1", "dir2", "dir3"])
   # ```
-  def mkdir_p(paths : Enumerable(Path | String), mode = 0o777) : Nil
+  def mkdir_p(paths : Enumerable(Path | String), mode : Int32 = 0o777) : Nil
     paths.each do |path|
       Dir.mkdir_p(path, mode)
     end
diff --git a/src/float/fast_float/fast_table.cr b/src/float/fast_float/fast_table.cr
index a2c2b2e9d1c9..c59d8fa30e3a 100644
--- a/src/float/fast_float/fast_table.cr
+++ b/src/float/fast_float/fast_table.cr
@@ -28,7 +28,7 @@ module Float::FastFloat
     # in LLVM, which makes LLVM really slow. The compiler should
     # try to avoid/reuse temporary allocas.
     # Explanation: https://github.com/crystal-lang/crystal/issues/4516#issuecomment-306226171
-    private def self.put(array, value) : Nil
+    private def self.put(array : Array(UInt64), value : UInt64) : Nil
       array << value
     end
 
diff --git a/src/float/printer.cr b/src/float/printer.cr
index 8fa64dd5072c..eb47a2d3a6ce 100644
--- a/src/float/printer.cr
+++ b/src/float/printer.cr
@@ -16,7 +16,7 @@ module Float::Printer
   #
   # *point_range* designates the boundaries of scientific notation which is used
   # for all values whose decimal point position is outside that range.
-  def shortest(v : Float::Primitive, io : IO, *, point_range = -3..15) : Nil
+  def shortest(v : Float::Primitive, io : IO, *, point_range : Range(Nil, Nil) | Range(Int32, Int32) = -3..15) : Nil
     check_finite_float(v, io) do |pos_v|
       if pos_v.zero?
         io << "0.0"
diff --git a/src/float/printer/cached_powers.cr b/src/float/printer/cached_powers.cr
index 71139f5bf5fa..0bc3b5b9f38f 100644
--- a/src/float/printer/cached_powers.cr
+++ b/src/float/printer/cached_powers.cr
@@ -141,7 +141,7 @@ module Float::Printer::CachedPowers
 
   Pow10Cache = {0, 1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000}
 
-  def self.largest_pow10(n, n_bits) : {Int32, Int32}
+  def self.largest_pow10(n : UInt32, n_bits : Int32) : {Int32, Int32}
     # 1233/4096 is approximately 1/lg(10).
     #  We increment to skip over the first entry in the powers cache.
     guess = ((n_bits + 1) * 1233 >> 12) + 1
@@ -154,7 +154,7 @@ module Float::Printer::CachedPowers
 
   # Returns a cached power-of-ten with a binary exponent in the range
   # around *exp* (boundaries included).
-  def self.get_cached_power_for_binary_exponent(exp) : {DiyFP, Int32}
+  def self.get_cached_power_for_binary_exponent(exp : Int32) : {DiyFP, Int32}
     min_exp = MIN_TARGET_EXP - (exp + DiyFP::SIGNIFICAND_SIZE)
     max_exp = MAX_TARGET_EXP - (exp + DiyFP::SIGNIFICAND_SIZE)
     k = ((min_exp + DiyFP::SIGNIFICAND_SIZE - 1) * D_1_LOG2_10).ceil
diff --git a/src/float/printer/dragonbox.cr b/src/float/printer/dragonbox.cr
index 65e090e5d0cc..fc29aaee2842 100644
--- a/src/float/printer/dragonbox.cr
+++ b/src/float/printer/dragonbox.cr
@@ -116,17 +116,17 @@ module Float::Printer::Dragonbox
 
   # Utilities for fast log computation.
   private module Log
-    def self.floor_log10_pow2(e : Int)
+    def self.floor_log10_pow2(e : Int) : Int32
       # Precondition: `-2620 <= e <= 2620`
       (e &* 315653) >> 20
     end
 
-    def self.floor_log2_pow10(e : Int)
+    def self.floor_log2_pow10(e : Int) : Int32
       # Precondition: `-1233 <= e <= 1233`
       (e &* 1741647) >> 19
     end
 
-    def self.floor_log10_pow2_minus_log10_4_over_3(e : Int)
+    def self.floor_log10_pow2_minus_log10_4_over_3(e : Int) : Int32
       # Precondition: `-2985 <= e <= 2936`
       (e &* 631305 &- 261663) >> 21
     end
@@ -199,7 +199,7 @@ module Float::Printer::Dragonbox
     end
 
     # N == 1
-    def self.check_divisibility_and_divide_by_pow10_k1(n : UInt32)
+    def self.check_divisibility_and_divide_by_pow10_k1(n : UInt32) : Tuple(UInt32, Bool)
       n &*= DIVIDE_BY_POW10_INFO_F32::MAGIC_NUMBER
 
       # Mask for the lowest (divisibility_check_bits)-bits.
@@ -212,7 +212,7 @@ module Float::Printer::Dragonbox
     end
 
     # N == 2
-    def self.check_divisibility_and_divide_by_pow10_k2(n : UInt32)
+    def self.check_divisibility_and_divide_by_pow10_k2(n : UInt32) : Tuple(UInt32, Bool)
       n &*= DIVIDE_BY_POW10_INFO_F64::MAGIC_NUMBER
 
       # Mask for the lowest (divisibility_check_bits)-bits.
@@ -226,20 +226,20 @@ module Float::Printer::Dragonbox
   end
 
   private module ImplInfoMethods(D)
-    def extract_exponent_bits(u : D::CarrierUInt)
+    def extract_exponent_bits(u : D::CarrierUInt) : UInt32
       exponent_bits_mask = ~(UInt32::MAX << D::EXPONENT_BITS)
       ((u >> D::SIGNIFICAND_BITS) & exponent_bits_mask).to_u32!
     end
 
-    def remove_exponent_bits(u : D::CarrierUInt, exponent_bits)
+    def remove_exponent_bits(u : D::CarrierUInt, exponent_bits : UInt32) : UInt32 | UInt64
       u ^ (D::CarrierUInt.new!(exponent_bits) << D::SIGNIFICAND_BITS)
     end
 
-    def remove_sign_bit_and_shift(u : D::CarrierUInt)
+    def remove_sign_bit_and_shift(u : D::CarrierUInt) : UInt32 | UInt64
       u << 1
     end
 
-    def check_divisibility_and_divide_by_pow10(n : UInt32)
+    def check_divisibility_and_divide_by_pow10(n : UInt32) : Tuple(UInt32, Bool)
       {% if D::KAPPA == 1 %}
         Div.check_divisibility_and_divide_by_pow10_k1(n)
       {% elsif D::KAPPA == 2 %}
diff --git a/src/float/printer/grisu3.cr b/src/float/printer/grisu3.cr
index d68cf2b7b287..8de06ab1f766 100644
--- a/src/float/printer/grisu3.cr
+++ b/src/float/printer/grisu3.cr
@@ -54,7 +54,7 @@ module Float::Printer::Grisu3
   # representable number to the input.
   #
   # Modifies the generated digits in the buffer to approach (round towards) *w*.
-  def round_weed(buffer_ptr, length, distance_too_high_w, unsafe_interval, rest, ten_kappa, unit) : Bool
+  def round_weed(buffer_ptr : Pointer(UInt8), length : Int32, distance_too_high_w : UInt64, unsafe_interval : UInt64, rest : UInt64, ten_kappa : UInt64, unit : UInt64) : Bool
     buffer = buffer_ptr.to_slice(128)
     small_distance = distance_too_high_w - unit
     big_distance = distance_too_high_w + unit
@@ -209,7 +209,7 @@ module Float::Printer::Grisu3
   # represent 'w' we can stop. Everything inside the interval low - high
   # represents w. However we have to pay attention to low, high and w's
   # imprecision.
-  def digit_gen(low : DiyFP, w : DiyFP, high : DiyFP, buffer_p) : {Bool, Int32, Int32}
+  def digit_gen(low : DiyFP, w : DiyFP, high : DiyFP, buffer_p : Pointer(UInt8)) : {Bool, Int32, Int32}
     buffer = buffer_p.to_slice(128)
     # low, w and high are imprecise, but by less than one ulp (unit in the last
     # place).
@@ -314,7 +314,7 @@ module Float::Printer::Grisu3
   # The last digit will be closest to the actual *v*. That is, even if several
   # digits might correctly yield *v* when read again, the closest will be
   # computed.
-  def grisu3(v : Float64 | Float32, buffer_p) : {Bool, Int32, Int32}
+  def grisu3(v : Float64 | Float32, buffer_p : Pointer(UInt8)) : {Bool, Int32, Int32}
     w = DiyFP.from_f_normalized(v)
 
     # boundary_minus and boundary_plus are the boundaries between v and its
diff --git a/src/float/printer/ieee.cr b/src/float/printer/ieee.cr
index b56b3d5bb454..b36586cb8676 100644
--- a/src/float/printer/ieee.cr
+++ b/src/float/printer/ieee.cr
@@ -82,11 +82,11 @@ module Float::Printer::IEEE
     special?(d32) && (d32 & SIGNIFICAND_MASK_32 == 0)
   end
 
-  def nan?(d64 : UInt64)
+  def nan?(d64 : UInt64) : Bool
     special?(d64) && (d64 & SIGNIFICAND_MASK_64 != 0)
   end
 
-  def nan?(d32 : UInt32)
+  def nan?(d32 : UInt32) : Bool
     special?(d32) && (d32 & SIGNIFICAND_MASK_32 != 0)
   end
 
@@ -176,13 +176,13 @@ module Float::Printer::IEEE
     (d32 & EXPONENT_MASK_32) == 0
   end
 
-  private def exponent(d64 : UInt64)
+  private def exponent(d64 : UInt64) : Int32
     return DENORMAL_EXPONENT_64 if denormal?(d64)
     biased_e = ((d64 & EXPONENT_MASK_64) >> PHYSICAL_SIGNIFICAND_SIZE_64).to_i
     biased_e - EXPONENT_BIAS_64
   end
 
-  private def exponent(d32 : UInt32)
+  private def exponent(d32 : UInt32) : Int32
     return DENORMAL_EXPONENT_32 if denormal?(d32)
     biased_e = ((d32 & EXPONENT_MASK_32) >> PHYSICAL_SIGNIFICAND_SIZE_32).to_i
     biased_e - EXPONENT_BIAS_32
diff --git a/src/float/printer/ryu_printf.cr b/src/float/printer/ryu_printf.cr
index feffb98fdb0e..868b49c8f007 100644
--- a/src/float/printer/ryu_printf.cr
+++ b/src/float/printer/ryu_printf.cr
@@ -63,12 +63,12 @@ module Float::Printer::RyuPrintf
   end
 
   # Returns true if value is divisible by 5^p.
-  private def self.multiple_of_power_of_5?(value : UInt64, p : UInt32)
+  private def self.multiple_of_power_of_5?(value : UInt64, p : UInt32) : Bool
     pow5_factor(value) >= p
   end
 
   # Returns true if value is divisible by 2^p.
-  private def self.multiple_of_power_of_2?(value : UInt64, p : UInt32)
+  private def self.multiple_of_power_of_2?(value : UInt64, p : UInt32) : Bool
     value & ~(UInt64::MAX << p) == 0
   end
 
@@ -103,7 +103,7 @@ module Float::Printer::RyuPrintf
   end
 
   # Returns the low 64 bits of the high 128 bits of the 256-bit product of a and b.
-  private def self.umul256_hi128_lo64(a_hi : UInt64, a_lo : UInt64, b_hi : UInt64, b_lo : UInt64)
+  private def self.umul256_hi128_lo64(a_hi : UInt64, a_lo : UInt64, b_hi : UInt64, b_lo : UInt64) : UInt64
     b00_hi, _ = umul128(a_lo, b_lo)
     b01_hi, b01_lo = umul128(a_lo, b_hi)
     b10_hi, b10_lo = umul128(a_hi, b_lo)
@@ -143,17 +143,17 @@ module Float::Printer::RyuPrintf
     uint128_mod1e9(shiftedhigh, shiftedlow)
   end
 
-  private def self.index_for_exponent(e : UInt32)
+  private def self.index_for_exponent(e : UInt32) : UInt32
     (e &+ 15) // 16
   end
 
   private ADDITIONAL_BITS_2 = 120
 
-  private def self.pow10_bits_for_index(idx : UInt32)
+  private def self.pow10_bits_for_index(idx : UInt32) : UInt32
     idx &* 16 &+ ADDITIONAL_BITS_2
   end
 
-  private def self.length_for_index(idx : UInt32)
+  private def self.length_for_index(idx : UInt32) : UInt32
     # +1 for ceil, +16 for mantissa, +8 to round up when dividing by 9
     (log10_pow2(16 &* idx.to_i32!) &+ 25) // 9
   end
@@ -162,7 +162,7 @@ module Float::Printer::RyuPrintf
   # The caller has to guarantee that:
   #   10^(olength-1) <= digits < 10^olength
   # e.g., by passing `olength` as `decimalLength9(digits)`.
-  private def self.append_n_digits(olength : UInt32, digits : UInt32, result : UInt8*)
+  private def self.append_n_digits(olength : UInt32, digits : UInt32, result : UInt8*) : Pointer(UInt8) | UInt8
     i = 0_u32
     while digits >= 10000
       c = digits &- 10000 &* (digits // 10000)
@@ -191,7 +191,7 @@ module Float::Printer::RyuPrintf
   # dot '.' followed by the remaining digits. The caller has to guarantee that:
   #   10^(olength-1) <= digits < 10^olength
   # e.g., by passing `olength` as `decimalLength9(digits)`.
-  private def self.append_d_digits(olength : UInt32, digits : UInt32, result : UInt8*)
+  private def self.append_d_digits(olength : UInt32, digits : UInt32, result : UInt8*) : UInt8
     i = 0_u32
     while digits >= 10000
       c = digits &- 10000 &* (digits // 10000)
@@ -221,7 +221,7 @@ module Float::Printer::RyuPrintf
 
   # Convert `digits` to decimal and write the last `count` decimal digits to result.
   # If `digits` contains additional digits, then those are silently ignored.
-  private def self.append_c_digits(count : UInt32, digits : UInt32, result : UInt8*)
+  private def self.append_c_digits(count : UInt32, digits : UInt32, result : UInt8*) : UInt8?
     i = 0_u32
 
     # Copy pairs of digits from DIGIT_TABLE.
@@ -241,7 +241,7 @@ module Float::Printer::RyuPrintf
 
   # Convert `digits` to decimal and write the last 9 decimal digits to result.
   # If `digits` contains additional digits, then those are silently ignored.
-  private def self.append_nine_digits(digits : UInt32, result : UInt8*)
+  private def self.append_nine_digits(digits : UInt32, result : UInt8*) : UInt8?
     if digits == 0
       Slice.new(result, 9).fill('0'.ord.to_u8!)
       return
@@ -268,7 +268,7 @@ module Float::Printer::RyuPrintf
   EXPONENT_BITS = 11
 
   # NOTE: in Crystal *d* must be positive and finite
-  private def self.extract_float(d : Float64)
+  private def self.extract_float(d : Float64) : Tuple(Int32, UInt64)
     bits = d.unsafe_as(UInt64)
 
     ieee_mantissa = bits & ~(UInt64::MAX << MANTISSA_BITS)
@@ -285,7 +285,7 @@ module Float::Printer::RyuPrintf
     {e2, m2}
   end
 
-  def self.d2fixed_buffered_n(d : Float64, precision : UInt32, result : UInt8*)
+  def self.d2fixed_buffered_n(d : Float64, precision : UInt32, result : UInt8*) : Int32
     e2, m2 = extract_float(d)
     index = 0
     nonzero = false
@@ -424,7 +424,7 @@ module Float::Printer::RyuPrintf
     index
   end
 
-  def self.d2exp_buffered_n(d : Float64, precision : UInt32, result : UInt8*)
+  def self.d2exp_buffered_n(d : Float64, precision : UInt32, result : UInt8*) : Int32
     if d == 0
       result[0] = '0'.ord.to_u8!
       index = 1
@@ -620,7 +620,7 @@ module Float::Printer::RyuPrintf
   # * https://github.com/ulfjack/ryu/pull/185/files
   # * https://github.com/microsoft/STL/blob/a8888806c6960f1687590ffd4244794c753aa819/stl/inc/charconv#L2324
   # * https://github.com/llvm/llvm-project/blob/701f64790520790f75b1f948a752472d421ddaa3/libcxx/src/include/to_chars_floating_point.h#L836
-  def self.d2gen_buffered_n(d : Float64, precision : UInt32, result : UInt8*, alternative : Bool = false)
+  def self.d2gen_buffered_n(d : Float64, precision : UInt32, result : UInt8*, alternative : Bool = false) : Tuple(Int32, Int32)
     if d == 0
       result[0] = '0'.ord.to_u8!
       return {1, 0}
@@ -682,21 +682,21 @@ module Float::Printer::RyuPrintf
     {(significand_last - result + (exponent_last - exponent_first)).to_i32!, extra_zeros.to_i32!}
   end
 
-  def self.d2fixed(d : Float64, precision : Int)
+  def self.d2fixed(d : Float64, precision : Int) : String
     String.new(2000) do |buffer|
       len = d2fixed_buffered_n(d, precision.to_u32, buffer)
       {len, len}
     end
   end
 
-  def self.d2exp(d : Float64, precision : Int)
+  def self.d2exp(d : Float64, precision : Int) : String
     String.new(2000) do |buffer|
       len = d2exp_buffered_n(d, precision.to_u32, buffer)
       {len, len}
     end
   end
 
-  def self.d2gen(d : Float64, precision : Int)
+  def self.d2gen(d : Float64, precision : Int) : String
     String.new(773) do |buffer|
       len, _ = d2gen_buffered_n(d, precision.to_u32, buffer)
       {len, len}
diff --git a/src/float/printer/ryu_printf_table.cr b/src/float/printer/ryu_printf_table.cr
index aa52d9b6fdd6..42386a95459d 100644
--- a/src/float/printer/ryu_printf_table.cr
+++ b/src/float/printer/ryu_printf_table.cr
@@ -11,7 +11,7 @@ module Float::Printer::RyuPrintf
   # in LLVM, which makes LLVM really slow. The compiler should
   # try to avoid/reuse temporary allocas.
   # Explanation: https://github.com/crystal-lang/crystal/issues/4516#issuecomment-306226171
-  private def self.put(array : Array, values) : Nil
+  private def self.put(array : Array, values : UInt64) : Nil
     array << values
   end
 
diff --git a/src/gc/boehm.cr b/src/gc/boehm.cr
index 3541359b39a9..316b06607944 100644
--- a/src/gc/boehm.cr
+++ b/src/gc/boehm.cr
@@ -303,13 +303,13 @@ module GC
     end
   {% end %}
 
-  def self.collect
+  def self.collect : Nil
     Crystal.trace :gc, "collect" do
       LibGC.collect
     end
   end
 
-  def self.enable
+  def self.enable : Nil
     unless LibGC.is_disabled != 0
       raise "GC is not disabled"
     end
@@ -335,7 +335,7 @@ module GC
     # Nothing
   end
 
-  private def self.add_finalizer_impl(object : T) forall T
+  private def self.add_finalizer_impl(object : T) : Nil forall T
     LibGC.register_finalizer_ignore_self(object.as(Void*),
       ->(obj, data) { obj.as(T).finalize },
       nil, nil, nil)
@@ -347,16 +347,16 @@ module GC
     roots << Pointer(Void).new(object.object_id)
   end
 
-  def self.register_disappearing_link(pointer : Void**)
+  def self.register_disappearing_link(pointer : Void**) : Int32
     base = LibGC.base(pointer.value)
     LibGC.general_register_disappearing_link(pointer, base)
   end
 
-  def self.is_heap_ptr(pointer : Void*)
+  def self.is_heap_ptr(pointer : Void*) : Bool
     LibGC.is_heap_ptr(pointer) != 0
   end
 
-  def self.stats
+  def self.stats : GC::Stats
     LibGC.get_heap_usage_safe(out heap_size, out free_bytes, out unmapped_bytes, out bytes_since_gc, out total_bytes)
     # collections = LibGC.gc_no - 1
     # bytes_found = LibGC.bytes_found
@@ -372,7 +372,7 @@ module GC
     )
   end
 
-  def self.prof_stats
+  def self.prof_stats : GC::ProfStats
     LibGC.get_prof_stats(out stats, sizeof(LibGC::ProfStats))
 
     ProfStats.new(
@@ -415,7 +415,7 @@ module GC
   {% end %}
 
   # :nodoc:
-  def self.current_thread_stack_bottom
+  def self.current_thread_stack_bottom : Tuple(Pointer(Void), Pointer(Void))
     {% if LibGC.has_method?(:get_my_stackbottom) %}
       th = LibGC.get_my_stackbottom(out sb)
       {th, sb.mem_base}
@@ -458,28 +458,28 @@ module GC
   end
 
   # :nodoc:
-  def self.unlock_read
+  def self.unlock_read : Nil
     {% if flag?(:preview_mt) %}
       @@lock.read_unlock
     {% end %}
   end
 
   # :nodoc:
-  def self.lock_write
+  def self.lock_write : Nil
     {% if flag?(:preview_mt) %}
       @@lock.write_lock
     {% end %}
   end
 
   # :nodoc:
-  def self.unlock_write
+  def self.unlock_write : Nil
     {% if flag?(:preview_mt) %}
       @@lock.write_unlock
     {% end %}
   end
 
   # :nodoc:
-  def self.push_stack(stack_top, stack_bottom) : Nil
+  def self.push_stack(stack_top : Pointer(Void), stack_bottom : Pointer(Void)) : Nil
     LibGC.push_all_eager(stack_top, stack_bottom)
   end
 
diff --git a/src/hash.cr b/src/hash.cr
index c145bda36309..fb9c516af26d 100644
--- a/src/hash.cr
+++ b/src/hash.cr
@@ -1797,7 +1797,7 @@ class Hash(K, V)
   # Hash.zip(["key1", "key2", "key3"], ["value1", "value2", "value3"])
   # # => {"key1" => "value1", "key2" => "value2", "key3" => "value3"}
   # ```
-  def self.zip(ary1 : Array(K), ary2 : Array(V))
+  def self.zip(ary1 : Array(K), ary2 : Array(V)) : Hash(Int32, Char)
     hash = {} of K => V
     ary1.each_with_index do |key, i|
       hash[key] = ary2[i]
diff --git a/src/html.cr b/src/html.cr
index 2d1d69926153..cb2c0bb1cebb 100644
--- a/src/html.cr
+++ b/src/html.cr
@@ -121,7 +121,7 @@ module HTML
     end
   end
 
-  private def self.unescape(slice, io)
+  private def self.unescape(slice : Slice(UInt8), io : String::Builder) : Nil
     while bytesize = slice.index('&'.ord)
       io.write(slice[0, bytesize])
       slice += bytesize &+ 1
@@ -133,7 +133,7 @@ module HTML
     io.write slice
   end
 
-  private def self.unescape_entity(ptr, io)
+  private def self.unescape_entity(ptr : Pointer(UInt8), io : String::Builder) : Pointer(UInt8)
     if '#' === ptr.value
       unescape_numbered_entity(ptr, io)
     else
@@ -141,7 +141,7 @@ module HTML
     end
   end
 
-  private def self.unescape_numbered_entity(ptr, io)
+  private def self.unescape_numbered_entity(ptr : Pointer(UInt8), io : String::Builder) : Pointer(UInt8)
     start_ptr = ptr
 
     ptr += 1
@@ -194,7 +194,7 @@ module HTML
   end
 
   # see https://html.spec.whatwg.org/multipage/parsing.html#numeric-character-reference-end-state
-  private def self.decode_codepoint(codepoint)
+  private def self.decode_codepoint(codepoint : UInt32) : Char?
     case codepoint
     when 0x80..0x9F
       # Replace characters from Windows-1252 with UTF-8 equivalents.
@@ -218,7 +218,7 @@ module HTML
     end
   end
 
-  private def self.unescape_named_entity(ptr, io)
+  private def self.unescape_named_entity(ptr : Pointer(UInt8), io : String::Builder) : Pointer(UInt8)
     # Consume the maximum number of characters possible, with the
     # consumed characters matching one of the named references.
     start_ptr = ptr
diff --git a/src/http/content.cr b/src/http/content.cr
index 828015549d3b..4ec83cc2a2a5 100644
--- a/src/http/content.cr
+++ b/src/http/content.cr
@@ -8,12 +8,12 @@ module HTTP
     @continue_sent = false
     setter expects_continue : Bool = false
 
-    def close
+    def close : Nil
       @expects_continue = false
       super
     end
 
-    protected def ensure_send_continue
+    protected def ensure_send_continue : Bool?
       return unless @expects_continue
       return if @continue_sent
       @io << CONTINUE
@@ -73,7 +73,7 @@ module HTTP
       @io.peek
     end
 
-    def skip(bytes_count) : Nil
+    def skip(bytes_count : Int32 | UInt32) : Nil
       ensure_send_continue
       @io.skip(bytes_count)
     end
@@ -164,7 +164,7 @@ module HTTP
       peek
     end
 
-    def skip(bytes_count) : Nil
+    def skip(bytes_count : Int32 | UInt32) : Nil
       ensure_send_continue
       if bytes_count <= @chunk_remaining
         @io.skip(bytes_count)
@@ -176,7 +176,7 @@ module HTTP
 
     # Checks if the last read consumed a chunk and we
     # need to start consuming the next one.
-    private def next_chunk
+    private def next_chunk : Bool?
       return if @chunk_remaining > 0 || @received_final_chunk
 
       # As soon as we finish reading a chunk we return,
@@ -191,7 +191,7 @@ module HTTP
       end
     end
 
-    private def read_crlf
+    private def read_crlf : Nil
       char = @io.read_byte
       if char === '\r'
         char = @io.read_byte
@@ -201,7 +201,7 @@ module HTTP
       end
     end
 
-    private def read_chunk_size
+    private def read_chunk_size : Int32
       line = @io.read_line(@max_headers_size, chomp: true)
 
       if index = line.byte_index(';'.ord)
@@ -213,7 +213,7 @@ module HTTP
       chunk_size.to_i?(16) || raise IO::Error.new("Invalid HTTP chunked content: invalid chunk size")
     end
 
-    private def read_trailer
+    private def read_trailer : Nil
       max_size = @max_headers_size
 
       while true
diff --git a/src/http/formdata.cr b/src/http/formdata.cr
index 7bb32b7519f0..eb01cd764fa9 100644
--- a/src/http/formdata.cr
+++ b/src/http/formdata.cr
@@ -130,7 +130,7 @@ module HTTP::FormData
   # `multipart/form-data` is not compatible with the original definition in
   # [RFC 2183](https://tools.ietf.org/html/rfc2183), but are instead specified
   # in [RFC 2388](https://tools.ietf.org/html/rfc2388).
-  def self.parse_content_disposition(content_disposition) : {String, FileMetadata}
+  def self.parse_content_disposition(content_disposition : String) : {String, FileMetadata}
     filename = nil
     creation_time = nil
     modification_time = nil
diff --git a/src/http/headers.cr b/src/http/headers.cr
index dacae7e9ec88..9a3612954858 100644
--- a/src/http/headers.cr
+++ b/src/http/headers.cr
@@ -39,7 +39,7 @@ struct HTTP::Headers
       true
     end
 
-    private def normalize_byte(byte)
+    private def normalize_byte(byte : UInt8) : Int32 | UInt8
       char = byte.unsafe_chr
 
       return byte if char.ascii_lowercase? || char == '-' # Optimize the common case
@@ -57,24 +57,24 @@ struct HTTP::Headers
     @hash = Hash(Key, String | Array(String)).new
   end
 
-  def []=(key, value : String)
+  def []=(key : String | HTTP::Headers::Key, value : String) : String
     check_invalid_header_content(value)
 
     @hash[wrap(key)] = value
   end
 
-  def []=(key, value : Array(String))
+  def []=(key : String, value : Array(String)) : Array(String)
     value.each { |val| check_invalid_header_content val }
 
     @hash[wrap(key)] = value
   end
 
-  def [](key) : String
+  def [](key : String) : String
     values = @hash[wrap(key)]
     concat values
   end
 
-  def []?(key) : String?
+  def []?(key : String) : String?
     fetch(key, nil)
   end
 
@@ -87,7 +87,7 @@ struct HTTP::Headers
   # headers = HTTP::Headers{"Connection" => "keep-alive, Upgrade"}
   # headers.includes_word?("Connection", "Upgrade") # => true
   # ```
-  def includes_word?(key, word) : Bool
+  def includes_word?(key : String, word : String) : Bool
     return false if word.empty?
 
     values = @hash[wrap(key)]?
@@ -104,7 +104,7 @@ struct HTTP::Headers
     end
   end
 
-  private def includes_word_in_header_value?(word, value)
+  private def includes_word_in_header_value?(word : String, value : String) : Bool
     offset = 0
     while true
       start = value.index(word, offset)
@@ -132,31 +132,31 @@ struct HTTP::Headers
   # headers.add("Connection", "Upgrade")
   # headers["Connection"] # => "keep-alive,Upgrade"
   # ```
-  def add(key, value : String) : self
+  def add(key : String, value : String) : self
     check_invalid_header_content value
     unsafe_add(key, value)
     self
   end
 
-  def add(key, value : Array(String)) : self
+  def add(key : String, value : Array(String)) : self
     value.each { |val| check_invalid_header_content val }
     unsafe_add(key, value)
     self
   end
 
-  def add?(key, value : String) : Bool
+  def add?(key : String, value : String) : Bool
     return false unless valid_value?(value)
     unsafe_add(key, value)
     true
   end
 
-  def add?(key, value : Array(String)) : Bool
+  def add?(key : String, value : Array(String)) : Bool
     value.each { |val| return false unless valid_value?(val) }
     unsafe_add(key, value)
     true
   end
 
-  def fetch(key, default) : String?
+  def fetch(key : String, default : String?) : String?
     fetch(wrap(key)) { default }
   end
 
@@ -165,7 +165,7 @@ struct HTTP::Headers
     values ? concat(values) : yield key
   end
 
-  def has_key?(key) : Bool
+  def has_key?(key : String) : Bool
     @hash.has_key? wrap(key)
   end
 
@@ -173,12 +173,12 @@ struct HTTP::Headers
     @hash.empty?
   end
 
-  def delete(key) : String?
+  def delete(key : String) : String?
     values = @hash.delete wrap(key)
     values ? concat(values) : nil
   end
 
-  def merge!(other) : self
+  def merge!(other : Hash(String, String)) : self
     other.each do |key, value|
       self[wrap(key)] = value
     end
@@ -199,7 +199,7 @@ struct HTTP::Headers
   # HTTP::Headers{"Foo" => "bar"} == HTTP::Headers{"Foo" => ["bar"]} # => true
   # HTTP::Headers{"Foo" => "bar"} == HTTP::Headers{"Foo" => "baz"}   # => false
   # ```
-  def ==(other : self)
+  def ==(other : self) : Bool
     # Adapts `Hash#==` to treat string values equal to a single element array.
 
     return false unless @hash.size == other.@hash.size
@@ -247,11 +247,11 @@ struct HTTP::Headers
     end
   end
 
-  def get(key) : Array(String)
+  def get(key : HTTP::Headers::Key | String) : Array(String)
     cast @hash[wrap(key)]
   end
 
-  def get?(key) : Array(String)?
+  def get?(key : String) : Array(String)?
     @hash[wrap(key)]?.try { |value| cast(value) }
   end
 
@@ -263,7 +263,7 @@ struct HTTP::Headers
     dup
   end
 
-  def clone
+  def clone : HTTP::Headers
     dup
   end
 
@@ -339,13 +339,13 @@ struct HTTP::Headers
     end
   end
 
-  def valid_value?(value) : Bool
+  def valid_value?(value : String) : Bool
     invalid_value_char(value).nil?
   end
 
   forward_missing_to @hash
 
-  private def unsafe_add(key, value : String)
+  private def unsafe_add(key : String, value : String) : Array(String) | String
     key = wrap(key)
     existing = @hash[key]?
     if existing
@@ -359,7 +359,7 @@ struct HTTP::Headers
     end
   end
 
-  private def unsafe_add(key, value : Array(String))
+  private def unsafe_add(key : String, value : Array(String)) : Array(String)
     key = wrap(key)
     existing = @hash[key]?
     if existing
@@ -375,23 +375,23 @@ struct HTTP::Headers
     end
   end
 
-  private def wrap(key)
+  private def wrap(key : String | HTTP::Headers::Key) : HTTP::Headers::Key
     key.is_a?(Key) ? key : Key.new(key)
   end
 
-  private def cast(value : String)
+  private def cast(value : String) : Array(String)
     [value]
   end
 
-  private def cast(value : Array(String))
+  private def cast(value : Array(String)) : Array(String)
     value
   end
 
-  private def concat(values : String)
+  private def concat(values : String) : String
     values
   end
 
-  private def concat(values : Array(String))
+  private def concat(values : Array(String)) : String
     case values.size
     when 0
       ""
@@ -402,13 +402,13 @@ struct HTTP::Headers
     end
   end
 
-  private def check_invalid_header_content(value)
+  private def check_invalid_header_content(value : String) : Nil
     if char = invalid_value_char(value)
       raise ArgumentError.new("Header content contains invalid character #{char.inspect}")
     end
   end
 
-  private def valid_char?(char)
+  private def valid_char?(char : Char) : Bool
     # According to RFC 7230, characters accepted as HTTP header
     # are '\t', ' ', all US-ASCII printable characters and
     # range from '\x80' to '\xff' (but the last is obsoleted.)
@@ -419,7 +419,7 @@ struct HTTP::Headers
     true
   end
 
-  private def invalid_value_char(value)
+  private def invalid_value_char(value : String) : Char?
     value.each_byte do |byte|
       unless valid_char?(char = byte.unsafe_chr)
         return char
diff --git a/src/http/log.cr b/src/http/log.cr
index 7f16c4c63e90..77fd6fe04404 100644
--- a/src/http/log.cr
+++ b/src/http/log.cr
@@ -8,7 +8,7 @@ class HTTP::Client
     yield
   end
 
-  protected def emit_log(request)
+  protected def emit_log(request : HTTP::Request) : Nil
     Log.debug &.emit("Performing request",
       method: request.method,
       host: host,
diff --git a/src/http/server/handler.cr b/src/http/server/handler.cr
index 792879fc949d..069747fc084e 100644
--- a/src/http/server/handler.cr
+++ b/src/http/server/handler.cr
@@ -25,7 +25,7 @@ module HTTP::Handler
 
   abstract def call(context : HTTP::Server::Context)
 
-  def call_next(context : HTTP::Server::Context)
+  def call_next(context : HTTP::Server::Context) : Nil
     if next_handler = @next
       next_handler.call(context)
     else
diff --git a/src/http/server/handlers/compress_handler.cr b/src/http/server/handlers/compress_handler.cr
index e059b697649c..f0969a1fa1b1 100644
--- a/src/http/server/handlers/compress_handler.cr
+++ b/src/http/server/handlers/compress_handler.cr
@@ -10,7 +10,7 @@
 class HTTP::CompressHandler
   include HTTP::Handler
 
-  def call(context)
+  def call(context : HTTP::Server::Context) : Nil
     {% if flag?(:without_zlib) %}
       call_next(context)
     {% else %}
diff --git a/src/http/server/handlers/error_handler.cr b/src/http/server/handlers/error_handler.cr
index f92b1a8d0c7a..916f85a12d15 100644
--- a/src/http/server/handlers/error_handler.cr
+++ b/src/http/server/handlers/error_handler.cr
@@ -14,7 +14,7 @@ class HTTP::ErrorHandler
   def initialize(@verbose : Bool = false, @log = Log.for("http.server"))
   end
 
-  def call(context) : Nil
+  def call(context : HTTP::Server::Context) : Nil
     call_next(context)
   rescue ex : HTTP::Server::ClientError
     @log.debug(exception: ex.cause) { ex.message }
diff --git a/src/http/server/handlers/log_handler.cr b/src/http/server/handlers/log_handler.cr
index 6aa82f6d911a..3bc8428f8fd1 100644
--- a/src/http/server/handlers/log_handler.cr
+++ b/src/http/server/handlers/log_handler.cr
@@ -10,7 +10,7 @@ class HTTP::LogHandler
   def initialize(@log = Log.for("http.server"))
   end
 
-  def call(context) : Nil
+  def call(context : HTTP::Server::Context) : Nil
     start = Time.monotonic
 
     begin
@@ -36,7 +36,7 @@ class HTTP::LogHandler
     end
   end
 
-  private def elapsed_text(elapsed)
+  private def elapsed_text(elapsed : Time::Span) : String
     minutes = elapsed.total_minutes
     return "#{minutes.round(2)}m" if minutes >= 1
 
diff --git a/src/http/server/handlers/static_file_handler.cr b/src/http/server/handlers/static_file_handler.cr
index 665f466c7e46..2f0150d727f5 100644
--- a/src/http/server/handlers/static_file_handler.cr
+++ b/src/http/server/handlers/static_file_handler.cr
@@ -41,7 +41,7 @@ class HTTP::StaticFileHandler
     new(public_dir, fallthrough: !!fallthrough, listing: !!listing)
   end
 
-  def call(context) : Nil
+  def call(context : HTTP::Server::Context) : Nil
     unless context.request.method.in?("GET", "HEAD")
       if @fallthrough
         call_next(context)
@@ -180,7 +180,7 @@ class HTTP::StaticFileHandler
   end
 
   # TODO: Optimize without lots of intermediary strings
-  private def parse_ranges(header, file_size)
+  private def parse_ranges(header : String, file_size : Int64) : Array(Range(Int64, Int64))?
     ranges = [] of Range(Int64, Int64)
     header.split(",") do |range|
       start_string, dash, finish_string = range.lchop(' ').partition("-")
@@ -224,7 +224,7 @@ class HTTP::StaticFileHandler
     path
   end
 
-  private def redirect_to(context, url)
+  private def redirect_to(context : HTTP::Server::Context, url : Path) : Nil
     context.response.redirect url.to_s
   end
 
@@ -251,7 +251,7 @@ class HTTP::StaticFileHandler
     end
   end
 
-  private def etag(modification_time)
+  private def etag(modification_time : Time) : String
     %{W/"#{modification_time.to_unix}"}
   end
 
@@ -265,7 +265,7 @@ class HTTP::StaticFileHandler
     ECR.def_to_s "#{__DIR__}/static_file_handler.html"
   end
 
-  private def directory_listing(io, request_path, path)
+  private def directory_listing(io : HTTP::Server::Response, request_path : Path, path : Path) : Nil
     DirectoryListing.new(request_path.to_s, path.to_s).to_s(io)
   end
 end
diff --git a/src/http/server/handlers/websocket_handler.cr b/src/http/server/handlers/websocket_handler.cr
index 8652a828c1e7..10847056d0c7 100644
--- a/src/http/server/handlers/websocket_handler.cr
+++ b/src/http/server/handlers/websocket_handler.cr
@@ -20,7 +20,7 @@ class HTTP::WebSocketHandler
   def initialize(&@proc : WebSocket, Server::Context ->)
   end
 
-  def call(context) : Nil
+  def call(context : HTTP::Server::Context) : Nil
     unless websocket_upgrade_request? context.request
       return call_next context
     end
@@ -54,7 +54,7 @@ class HTTP::WebSocketHandler
     end
   end
 
-  private def websocket_upgrade_request?(request)
+  private def websocket_upgrade_request?(request : HTTP::Request) : Bool
     return false unless upgrade = request.headers["Upgrade"]?
     return false unless upgrade.compare("websocket", case_insensitive: true) == 0
 
diff --git a/src/http/server/response.cr b/src/http/server/response.cr
index 4dd6968ac560..ccd5199b6569 100644
--- a/src/http/server/response.cr
+++ b/src/http/server/response.cr
@@ -27,7 +27,7 @@ class HTTP::Server
     property output : IO
 
     # :nodoc:
-    def version=(version : String)
+    def version=(version : String) : String
       check_headers
       @version = version
     end
@@ -36,7 +36,7 @@ class HTTP::Server
     # body. If not set, the default value is 200 (OK).
     getter status : HTTP::Status
 
-    def status=(status : HTTP::Status)
+    def status=(status : HTTP::Status) : HTTP::Status
       check_headers
       @status = status
     end
@@ -56,7 +56,7 @@ class HTTP::Server
     end
 
     # :nodoc:
-    def reset
+    def reset : Nil
       # This method is called by RequestProcessor to avoid allocating a new instance for each iteration.
       @headers.clear
       @cookies = nil
@@ -68,13 +68,13 @@ class HTTP::Server
     end
 
     # Convenience method to set the `Content-Type` header.
-    def content_type=(content_type : String)
+    def content_type=(content_type : String) : String
       check_headers
       headers["Content-Type"] = content_type
     end
 
     # Convenience method to set the `Content-Length` header.
-    def content_length=(content_length : Int)
+    def content_length=(content_length : Int) : String
       check_headers
       headers["Content-Length"] = content_length.to_s
     end
@@ -85,7 +85,7 @@ class HTTP::Server
     end
 
     # Convenience method to set the HTTP status code.
-    def status_code=(status_code : Int32)
+    def status_code=(status_code : Int32) : Int32
       self.status = HTTP::Status.new(status_code)
       status_code
     end
@@ -133,7 +133,7 @@ class HTTP::Server
     end
 
     # Sets the status message.
-    def status_message=(status_message : String?)
+    def status_message=(status_message : String?) : String
       check_headers
       @status_message = status_message
     end
@@ -183,7 +183,7 @@ class HTTP::Server
     #
     # Raises `IO::Error` if the response is closed or headers were already
     # sent.
-    def redirect(location : String | URI, status : HTTP::Status = :found)
+    def redirect(location : String | URI, status : HTTP::Status = :found) : Nil
       check_headers
 
       self.status = status
@@ -199,14 +199,14 @@ class HTTP::Server
       close
     end
 
-    private def check_headers
+    private def check_headers : Nil
       raise IO::Error.new "Closed stream" if @original_output.closed?
       if wrote_headers?
         raise IO::Error.new("Headers already sent")
       end
     end
 
-    protected def write_headers
+    protected def write_headers : Bool
       @io << @version << ' ' << @status.code << ' ' << status_message << "\r\n"
       headers.each do |name, values|
         values.each do |value|
@@ -217,11 +217,11 @@ class HTTP::Server
       @wrote_headers = true
     end
 
-    protected def wrote_headers?
+    protected def wrote_headers? : Bool
       @wrote_headers
     end
 
-    protected def has_cookies?
+    protected def has_cookies? : Bool
       !@cookies.nil?
     end
 
@@ -305,7 +305,7 @@ class HTTP::Server
         end
       end
 
-      private def ensure_headers_written
+      private def ensure_headers_written : Bool?
         unless response.wrote_headers?
           if response.has_cookies?
             response.cookies.add_response_headers(response.headers)
diff --git a/src/http/status.cr b/src/http/status.cr
index a02299f9fed7..02b17c2f905e 100644
--- a/src/http/status.cr
+++ b/src/http/status.cr
@@ -81,7 +81,7 @@ enum HTTP::Status
   # HTTP::Status.new(123)  # => 123
   # HTTP::Status.new(1000) # raises ArgumentError
   # ```
-  def self.new(status_code : Int32)
+  def self.new(status_code : Int32) : HTTP::Status
     raise ArgumentError.new("Invalid HTTP status code: #{status_code}") unless 100 <= status_code <= 999
     previous_def(status_code)
   end
diff --git a/src/http/web_socket.cr b/src/http/web_socket.cr
index e2d56b8a7610..1eeff5901771 100644
--- a/src/http/web_socket.cr
+++ b/src/http/web_socket.cr
@@ -6,7 +6,7 @@ class HTTP::WebSocket
   getter? closed = false
 
   # :nodoc:
-  def initialize(io : IO, sync_close = true)
+  def initialize(io : IO, sync_close : Bool = true)
     initialize(Protocol.new(io, sync_close: sync_close))
   end
 
@@ -34,7 +34,7 @@ class HTTP::WebSocket
   # HTTP::WebSocket.new(
   #   URI.parse("ws://user:password@websocket.example.com/chat")) # Creates a new WebSocket to `websocket.example.com` with an HTTP basic auth Authorization header
   # ```
-  def self.new(uri : URI | String, headers = HTTP::Headers.new)
+  def self.new(uri : URI | String, headers : HTTP::Headers = HTTP::Headers.new) : HTTP::WebSocket
     new(Protocol.new(uri, headers: headers))
   end
 
@@ -47,7 +47,7 @@ class HTTP::WebSocket
   # HTTP::WebSocket.new("websocket.example.com", "/chat")            # Creates a new WebSocket to `websocket.example.com`
   # HTTP::WebSocket.new("websocket.example.com", "/chat", tls: true) # Creates a new WebSocket with TLS to `ẁebsocket.example.com`
   # ```
-  def self.new(host : String, path : String, port = nil, tls : HTTP::Client::TLSContext = nil, headers = HTTP::Headers.new)
+  def self.new(host : String, path : String, port : Int32? = nil, tls : HTTP::Client::TLSContext = nil, headers : HTTP::Headers = HTTP::Headers.new) : HTTP::WebSocket
     new(Protocol.new(host, path, port, tls, headers))
   end
 
@@ -62,23 +62,23 @@ class HTTP::WebSocket
   end
 
   # Called when a text message is received.
-  def on_message(&@on_message : String ->)
+  def on_message(&@on_message : String ->) : Proc(String, Nil)
   end
 
   # Called when a binary message is received.
-  def on_binary(&@on_binary : Bytes ->)
+  def on_binary(&@on_binary : Bytes ->) : Proc(Slice(UInt8), Nil)
   end
 
   # Called when the connection is closed by the other party.
-  def on_close(&@on_close : CloseCode, String ->)
+  def on_close(&@on_close : CloseCode, String ->) : Proc(HTTP::WebSocket::CloseCode, String, Nil)
   end
 
-  protected def check_open
+  protected def check_open : Nil
     raise IO::Error.new "Closed socket" if closed?
   end
 
   # Sends a message payload (message).
-  def send(message) : Nil
+  def send(message : String) : Nil
     check_open
     @ws.send(message)
   end
@@ -92,7 +92,7 @@ class HTTP::WebSocket
   end
 
   # Sends a PONG frame, which must be in response to a previously received PING frame from `#on_ping`.
-  def pong(message = nil) : Nil
+  def pong(message : String? = nil) : Nil
     check_open
     @ws.pong(message)
   end
@@ -126,7 +126,7 @@ class HTTP::WebSocket
 
   # Sends a close frame, and closes the connection.
   # The close frame may contain a body (message) that indicates the reason for closing.
-  def close(code : CloseCode | Int? = nil, message = nil) : Nil
+  def close(code : CloseCode | Int? = nil, message : String? = nil) : Nil
     return if closed?
     @closed = true
     @ws.close(code, message)
diff --git a/src/http/web_socket/close_code.cr b/src/http/web_socket/close_code.cr
index 78b469051e4a..e6d2079c91a9 100644
--- a/src/http/web_socket/close_code.cr
+++ b/src/http/web_socket/close_code.cr
@@ -17,7 +17,7 @@ enum HTTP::WebSocket::CloseCode
 
   # Create a new instance with the given close code, or raise an
   # error if the close code given is not inside 0..4999.
-  def self.new(close_code : Int32)
+  def self.new(close_code : Int32) : HTTP::WebSocket::CloseCode
     unless 0 <= close_code <= 4999
       raise ArgumentError.new("Invalid HTTP::WebSocket::CloseCode: #{close_code}")
     end
diff --git a/src/humanize.cr b/src/humanize.cr
index e3e4ed4428c7..c1b7cbc4c26e 100644
--- a/src/humanize.cr
+++ b/src/humanize.cr
@@ -92,7 +92,7 @@ struct Number
   end
 
   # :ditto:
-  def format(separator = '.', delimiter = ',', decimal_places : Int? = nil, *, group : Int = 3, only_significant : Bool = false) : String
+  def format(separator : Char | String = '.', delimiter : Char | String = ',', decimal_places : Int? = nil, *, group : Int = 3, only_significant : Bool = false) : String
     String.build do |io|
       format(io, separator, delimiter, decimal_places, group: group, only_significant: only_significant)
     end
@@ -113,14 +113,14 @@ struct Number
   # ```
   # Number.si_prefix(3) # => 'k'
   # ```
-  def self.si_prefix(magnitude : Int, prefixes = SI_PREFIXES) : Char?
+  def self.si_prefix(magnitude : Int, prefixes : Tuple(Tuple(Char, Char, Char, Char, Char, Char, Char, Char, Char, Char), Tuple(Nil, Char, Char, Char, Char, Char, Char, Char, Char, Char, Char)) | Array(Array(Char)) = SI_PREFIXES) : Char?
     index = (magnitude // 3)
     prefixes = prefixes[magnitude < 0 ? 0 : 1]
     prefixes[index.clamp((-prefixes.size)..(prefixes.size - 1))]
   end
 
   # :nodoc:
-  def self.prefix_index(i : Int32, *, group : Int32 = 3, prefixes = SI_PREFIXES) : Int32
+  def self.prefix_index(i : Int32, *, group : Int32 = 3, prefixes : Tuple(Tuple(Char, Char, Char, Char, Char, Char, Char, Char, Char, Char), Tuple(Nil, Char, Char, Char, Char, Char, Char, Char, Char, Char, Char)) | Array(Array(Char)) = SI_PREFIXES) : Int32
     prefixes = prefixes[i < 0 ? 0 : 1]
     ((i - (i > 0 ? 1 : 0)) // group).clamp((-prefixes.size)..(prefixes.size - 1)) * group
   end
@@ -155,7 +155,7 @@ struct Number
   # Users are encouraged to use a non-breaking space ('\u00A0') to prevent output being split across lines.
   #
   # See `Int#humanize_bytes` to format a file size.
-  def humanize(io : IO, precision = 3, separator = '.', delimiter = ',', *, base = 10 ** 3, significant = true, unit_separator = nil, prefixes : Indexable = SI_PREFIXES) : Nil
+  def humanize(io : IO, precision : Int32 = 3, separator : Char = '.', delimiter : Char = ',', *, base : Int32 = 10 ** 3, significant : Bool = true, unit_separator : Nil | Char | String | Int32 = nil, prefixes : Indexable = SI_PREFIXES) : Nil
     humanize(io, precision, separator, delimiter, base: base, significant: significant, unit_separator: unit_separator) do |magnitude, _|
       magnitude = Number.prefix_index(magnitude, prefixes: prefixes)
       {magnitude, Number.si_prefix(magnitude, prefixes)}
@@ -163,7 +163,7 @@ struct Number
   end
 
   # :ditto:
-  def humanize(precision = 3, separator = '.', delimiter = ',', *, base = 10 ** 3, significant = true, unit_separator = nil, prefixes = SI_PREFIXES) : String
+  def humanize(precision : Int32 = 3, separator : Char = '.', delimiter : Char = ',', *, base : Int32 = 10 ** 3, significant : Bool = true, unit_separator : Char | String | Int32 | Nil = nil, prefixes : Proc(Int32, Float64, Tuple(Int32, String)) | Proc(Int32, Float64, Tuple(Int32, Char | String | Nil)) | Array(Array(Char)) | Tuple(Tuple(Char, Char, Char, Char, Char, Char, Char, Char, Char, Char), Tuple(Nil, Char, Char, Char, Char, Char, Char, Char, Char, Char, Char)) = SI_PREFIXES) : String
     String.build do |io|
       humanize(io, precision, separator, delimiter, base: base, significant: significant, unit_separator: unit_separator, prefixes: prefixes)
     end
@@ -276,7 +276,7 @@ struct Number
   end
 
   # :ditto:
-  def humanize(io : IO, precision = 3, separator = '.', delimiter = ',', *, base = 10 ** 3, significant = true, unit_separator = nil, prefixes : Proc) : Nil
+  def humanize(io : IO, precision : Int32 = 3, separator : Char = '.', delimiter : Char = ',', *, base : Int32 = 10 ** 3, significant : Bool = true, unit_separator : Nil = nil, prefixes : Proc) : Nil
     humanize(io, precision, separator, delimiter, base: base, significant: significant, unit_separator: unit_separator) do |magnitude, number|
       prefixes.call(magnitude, number)
     end
@@ -325,7 +325,7 @@ struct Int
   # ```
   #
   # See `Number#humanize` for more details on the behaviour and arguments.
-  def humanize_bytes(io : IO, precision : Int = 3, separator = '.', *, significant : Bool = true, unit_separator = nil, format : BinaryPrefixFormat = :IEC) : Nil
+  def humanize_bytes(io : IO, precision : Int = 3, separator : Char = '.', *, significant : Bool = true, unit_separator : Char? = nil, format : BinaryPrefixFormat = :IEC) : Nil
     humanize(io, precision, separator, nil, base: 1024, significant: significant) do |magnitude|
       magnitude = Number.prefix_index(magnitude)
 
@@ -344,7 +344,7 @@ struct Int
   end
 
   # :ditto:
-  def humanize_bytes(precision : Int = 3, separator = '.', *, significant : Bool = true, unit_separator = nil, format : BinaryPrefixFormat = :IEC) : String
+  def humanize_bytes(precision : Int = 3, separator : Char = '.', *, significant : Bool = true, unit_separator : Char? = nil, format : BinaryPrefixFormat = :IEC) : String
     String.build do |io|
       humanize_bytes(io, precision, separator, significant: significant, unit_separator: unit_separator, format: format)
     end
diff --git a/src/ini.cr b/src/ini.cr
index d9cea1c82e82..265d205a628e 100644
--- a/src/ini.cr
+++ b/src/ini.cr
@@ -5,7 +5,7 @@ module INI
     getter line_number : Int32
     getter column_number : Int32
 
-    def initialize(message, @line_number, @column_number)
+    def initialize(message : String, @line_number : Int32, @column_number : Int32)
       super "#{message} at line #{@line_number}, column #{@column_number}"
     end
 
@@ -66,12 +66,12 @@ module INI
   #
   # INI.build({"foo" => {"a" => "1"}}, true) # => "[foo]\na = 1\n\n"
   # ```
-  def self.build(ini, space : Bool = false) : String
+  def self.build(ini : Hash(String, Hash(String, Char) | Hash(String, Float64 | Int32) | Hash(String, String)) | NamedTuple(general: NamedTuple(log_level: Char), section1: NamedTuple(foo: Float64, bar: Int32), section2: NamedTuple("x.y.z": String)) | Hash(String, Hash(String, String)), space : Bool = false) : String
     String.build { |str| build str, ini, space }
   end
 
   # Appends INI data to the given IO.
-  def self.build(io : IO, ini, space : Bool = false) : Nil
+  def self.build(io : IO, ini : Hash(String, Hash(String, Char) | Hash(String, Float64 | Int32) | Hash(String, String)) | NamedTuple(general: NamedTuple(log_level: Char), section1: NamedTuple(foo: Float64, bar: Int32), section2: NamedTuple("x.y.z": String)) | Hash(String, Hash(String, String)), space : Bool = false) : Nil
     # An empty section has to be at first, to prevent being included in another one.
     ini[""]?.try &.each do |key, value|
       io << key << (space ? " = " : '=') << value << '\n'
diff --git a/src/intrinsics.cr b/src/intrinsics.cr
index 05c89a47f4c2..b6c834adccab 100644
--- a/src/intrinsics.cr
+++ b/src/intrinsics.cr
@@ -187,7 +187,7 @@ module Intrinsics
     ::LibIntrinsics.debugtrap
   end
 
-  def self.pause
+  def self.pause : Nil
     {% if flag?(:i386) || flag?(:x86_64) %}
       LibIntrinsics.pause
     {% elsif flag?(:aarch64) %}
@@ -211,59 +211,59 @@ module Intrinsics
     LibIntrinsics.read_cycle_counter
   end
 
-  def self.bitreverse8(id) : UInt8
+  def self.bitreverse8(id : UInt8 | Int8) : UInt8
     LibIntrinsics.bitreverse8(id)
   end
 
-  def self.bitreverse16(id) : UInt16
+  def self.bitreverse16(id : UInt16 | Int16) : UInt16
     LibIntrinsics.bitreverse16(id)
   end
 
-  def self.bitreverse32(id) : UInt32
+  def self.bitreverse32(id : UInt32 | Int32) : UInt32
     LibIntrinsics.bitreverse32(id)
   end
 
-  def self.bitreverse64(id) : UInt64
+  def self.bitreverse64(id : UInt64 | Int64) : UInt64
     LibIntrinsics.bitreverse64(id)
   end
 
-  def self.bitreverse128(id) : UInt128
+  def self.bitreverse128(id : UInt128 | Int128) : UInt128
     LibIntrinsics.bitreverse128(id)
   end
 
-  def self.bswap16(id) : UInt16
+  def self.bswap16(id : UInt16 | Int16) : UInt16
     LibIntrinsics.bswap16(id)
   end
 
-  def self.bswap32(id) : UInt32
+  def self.bswap32(id : UInt32 | Int32) : UInt32
     LibIntrinsics.bswap32(id)
   end
 
-  def self.bswap64(id) : UInt64
+  def self.bswap64(id : UInt64 | Int64) : UInt64
     LibIntrinsics.bswap64(id)
   end
 
-  def self.bswap128(id) : UInt128
+  def self.bswap128(id : UInt128 | Int128) : UInt128
     LibIntrinsics.bswap128(id)
   end
 
-  def self.popcount8(src) : Int8
+  def self.popcount8(src : Int8 | UInt8) : Int8
     LibIntrinsics.popcount8(src)
   end
 
-  def self.popcount16(src) : Int16
+  def self.popcount16(src : Int16 | UInt16) : Int16
     LibIntrinsics.popcount16(src)
   end
 
-  def self.popcount32(src) : Int32
+  def self.popcount32(src : UInt32 | Int32) : Int32
     LibIntrinsics.popcount32(src)
   end
 
-  def self.popcount64(src) : Int64
+  def self.popcount64(src : Int64 | UInt64) : Int64
     LibIntrinsics.popcount64(src)
   end
 
-  def self.popcount128(src)
+  def self.popcount128(src : Int128 | UInt128) : Int128
     LibIntrinsics.popcount128(src)
   end
 
@@ -307,43 +307,43 @@ module Intrinsics
     ::LibIntrinsics.counttrailing128({{src}}, {{zero_is_undef}})
   end
 
-  def self.fshl8(a, b, count) : UInt8
+  def self.fshl8(a : Int8 | UInt8, b : Int8 | UInt8, count : Int8 | UInt8) : UInt8
     LibIntrinsics.fshl8(a, b, count)
   end
 
-  def self.fshl16(a, b, count) : UInt16
+  def self.fshl16(a : Int16 | UInt16, b : Int16 | UInt16, count : Int16 | UInt16) : UInt16
     LibIntrinsics.fshl16(a, b, count)
   end
 
-  def self.fshl32(a, b, count) : UInt32
+  def self.fshl32(a : UInt32 | Int32, b : UInt32 | Int32, count : UInt32 | Int32) : UInt32
     LibIntrinsics.fshl32(a, b, count)
   end
 
-  def self.fshl64(a, b, count) : UInt64
+  def self.fshl64(a : UInt64 | Int64, b : UInt64 | Int64, count : UInt64 | Int64) : UInt64
     LibIntrinsics.fshl64(a, b, count)
   end
 
-  def self.fshl128(a, b, count) : UInt128
+  def self.fshl128(a : Int128 | UInt128, b : Int128 | UInt128, count : Int128 | UInt128) : UInt128
     LibIntrinsics.fshl128(a, b, count)
   end
 
-  def self.fshr8(a, b, count) : UInt8
+  def self.fshr8(a : Int8 | UInt8, b : Int8 | UInt8, count : Int8 | UInt8) : UInt8
     LibIntrinsics.fshr8(a, b, count)
   end
 
-  def self.fshr16(a, b, count) : UInt16
+  def self.fshr16(a : Int16 | UInt16, b : Int16 | UInt16, count : Int16 | UInt16) : UInt16
     LibIntrinsics.fshr16(a, b, count)
   end
 
-  def self.fshr32(a, b, count) : UInt32
+  def self.fshr32(a : UInt32 | Int32, b : UInt32 | Int32, count : UInt32 | Int32) : UInt32
     LibIntrinsics.fshr32(a, b, count)
   end
 
-  def self.fshr64(a, b, count) : UInt64
+  def self.fshr64(a : Int64 | UInt64, b : Int64 | UInt64, count : Int64 | UInt64) : UInt64
     LibIntrinsics.fshr64(a, b, count)
   end
 
-  def self.fshr128(a, b, count) : UInt128
+  def self.fshr128(a : Int128 | UInt128, b : Int128 | UInt128, count : Int128 | UInt128) : UInt128
     LibIntrinsics.fshr128(a, b, count)
   end
 
diff --git a/src/io/argf.cr b/src/io/argf.cr
index afa92c2a545b..ba05a5b28d48 100644
--- a/src/io/argf.cr
+++ b/src/io/argf.cr
@@ -44,7 +44,7 @@ class IO::ARGF < IO
     end
   end
 
-  private def peek_next
+  private def peek_next : Slice(UInt8)?
     if !@read_from_stdin && !@argv.empty?
       read_next_argv
       self.peek
@@ -61,7 +61,7 @@ class IO::ARGF < IO
     @path || @argv.first? || "-"
   end
 
-  private def first_initialize
+  private def first_initialize : IO
     # This is the moment where we decide
     # whether we are going to use STDIN or ARGV
     @initialized = true
@@ -73,7 +73,7 @@ class IO::ARGF < IO
     end
   end
 
-  private def read_from_current_io(current_io, slice)
+  private def read_from_current_io(current_io : IO, slice : Slice(UInt8)) : Int32
     read_count = current_io.read slice
     if read_count.zero?
       unless @read_from_stdin
@@ -89,7 +89,7 @@ class IO::ARGF < IO
     read_count
   end
 
-  private def read_next_argv
+  private def read_next_argv : File
     path = @path = @argv.shift
     @current_io = File.open(path)
   end
diff --git a/src/io/delimited.cr b/src/io/delimited.cr
index 4da7074b52bb..4754c3f40527 100644
--- a/src/io/delimited.cr
+++ b/src/io/delimited.cr
@@ -25,7 +25,7 @@ class IO::Delimited < IO
   # byte sequence *read_delimiter* (interpreted as UTF-8) is found. If
   # *sync_close* is set, calling `#close` calls `#close` on the underlying
   # `IO`.
-  def self.new(io : IO, read_delimiter : String, sync_close : Bool = false)
+  def self.new(io : IO, read_delimiter : String, sync_close : Bool = false) : IO::Delimited
     new(io, read_delimiter.to_slice, sync_close)
   end
 
diff --git a/src/io/file_descriptor.cr b/src/io/file_descriptor.cr
index 82c2b8ac232f..c10a09a3dce4 100644
--- a/src/io/file_descriptor.cr
+++ b/src/io/file_descriptor.cr
@@ -39,7 +39,7 @@ class IO::FileDescriptor < IO
     write_timeout
   end
 
-  def initialize(fd : Handle, blocking = nil, *, @close_on_finalize = true)
+  def initialize(fd : Handle, blocking : Bool? = nil, *, @close_on_finalize : Bool = true)
     @volatile_fd = Atomic.new(fd)
     @closed = true # This is necessary so we can reference `self` in `system_closed?` (in case of an exception)
 
@@ -72,13 +72,13 @@ class IO::FileDescriptor < IO
   # This might be different from the internal file descriptor. For example, when
   # `STDIN` is a terminal on Windows, this returns `false` since the underlying
   # blocking reads are done on a completely separate thread.
-  def blocking
+  def blocking : Bool
     emulated = emulated_blocking?
     return emulated unless emulated.nil?
     system_blocking?
   end
 
-  def blocking=(value)
+  def blocking=(value : Bool) : Int32?
     self.system_blocking = value
   end
 
@@ -86,7 +86,7 @@ class IO::FileDescriptor < IO
     system_close_on_exec?
   end
 
-  def close_on_exec=(value : Bool)
+  def close_on_exec=(value : Bool) : Bool
     self.system_close_on_exec = value
   end
 
@@ -94,7 +94,7 @@ class IO::FileDescriptor < IO
     Crystal::System::FileDescriptor.fcntl(fd, cmd, arg)
   end
 
-  def fcntl(cmd, arg = 0)
+  def fcntl(cmd : Int32, arg : Int32 = 0) : Int32
     Crystal::System::FileDescriptor.fcntl(fd, cmd, arg)
   end
 
@@ -199,7 +199,7 @@ class IO::FileDescriptor < IO
   #
   # NOTE: Metadata is flushed even when *flush_metadata* is false on Windows
   # and DragonFly BSD.
-  def fsync(flush_metadata = true) : Nil
+  def fsync(flush_metadata : Bool = true) : Nil
     flush
     system_fsync(flush_metadata)
   end
@@ -217,7 +217,7 @@ class IO::FileDescriptor < IO
 
   # Places a shared advisory lock. More than one process may hold a shared lock for a given file descriptor at a given time.
   # `IO::Error` is raised if *blocking* is set to `false` and an existing exclusive lock is set.
-  def flock_shared(blocking = true) : Nil
+  def flock_shared(blocking : Bool = true) : Nil
     system_flock_shared(blocking)
   end
 
@@ -232,7 +232,7 @@ class IO::FileDescriptor < IO
 
   # Places an exclusive advisory lock. Only one process may hold an exclusive lock for a given file descriptor at a given time.
   # `IO::Error` is raised if *blocking* is set to `false` and any existing lock is set.
-  def flock_exclusive(blocking = true) : Nil
+  def flock_exclusive(blocking : Bool = true) : Nil
     system_flock_exclusive(blocking)
   end
 
@@ -252,7 +252,7 @@ class IO::FileDescriptor < IO
   # Resource release can be disabled with `close_on_finalize = false`.
   #
   # This method is a no-op if the file descriptor has already been closed.
-  def finalize
+  def finalize : Nil
     return if closed? || !close_on_finalize?
 
     Crystal::EventLoop.remove(self)
@@ -267,7 +267,7 @@ class IO::FileDescriptor < IO
     system_tty?
   end
 
-  def reopen(other : IO::FileDescriptor)
+  def reopen(other : IO::FileDescriptor) : File
     return other if self.fd == other.fd
     system_reopen(other)
 
diff --git a/src/io/memory.cr b/src/io/memory.cr
index bd486f0cbdc2..22f0115e8045 100644
--- a/src/io/memory.cr
+++ b/src/io/memory.cr
@@ -45,7 +45,7 @@ class IO::Memory < IO
   # io.read(slice)    # => 6
   # String.new(slice) # => "abcdef"
   # ```
-  def initialize(slice : Bytes, writeable = true)
+  def initialize(slice : Bytes, writeable : Bool = true)
     @buffer = slice.to_unsafe
     @bytesize = @capacity = slice.size.to_i
     @pos = 0
@@ -65,7 +65,7 @@ class IO::Memory < IO
   # io.gets(2)    # => "he"
   # io.print "hi" # raises IO::Error
   # ```
-  def self.new(string : String)
+  def self.new(string : String) : IO::Memory
     new string.to_slice, writeable: false
   end
 
@@ -123,7 +123,7 @@ class IO::Memory < IO
   end
 
   # :nodoc:
-  def gets(delimiter : Char, limit : Int32, chomp = false) : String?
+  def gets(delimiter : Char, limit : Int32, chomp : Bool = false) : String?
     return super if @encoding || delimiter.ord >= 128
 
     check_open
@@ -182,7 +182,7 @@ class IO::Memory < IO
   end
 
   # :nodoc:
-  def skip(bytes_count) : Nil
+  def skip(bytes_count : Int32 | UInt32) : Nil
     check_open
 
     available = @bytesize - @pos
@@ -438,19 +438,19 @@ class IO::Memory < IO
     end
   end
 
-  private def check_writeable
+  private def check_writeable : Nil
     unless @writeable
       raise IO::Error.new "Read-only stream"
     end
   end
 
-  private def check_resizeable
+  private def check_resizeable : Nil
     unless @resizeable
       raise IO::Error.new "Non-resizeable stream"
     end
   end
 
-  private def increase_capacity_by(count)
+  private def increase_capacity_by(count : Int32) : Pointer(UInt8)?
     raise IO::EOFError.new if count >= Int32::MAX - bytesize
 
     new_bytesize = @pos + count
@@ -462,7 +462,7 @@ class IO::Memory < IO
     resize_to_capacity(new_capacity)
   end
 
-  private def calculate_new_capacity(new_bytesize : Int32)
+  private def calculate_new_capacity(new_bytesize : Int32) : Int32
     # If the new bytesize is bigger than 1 << 30, the next power of two would
     # be 1 << 31, which is out of range for Int32.
     # So we limit the capacity to Int32::MAX in order to be able to use the
@@ -472,7 +472,7 @@ class IO::Memory < IO
     Math.pw2ceil(new_bytesize)
   end
 
-  private def resize_to_capacity(capacity)
+  private def resize_to_capacity(capacity : Int32) : Pointer(UInt8)
     @capacity = capacity
     @buffer = GC.realloc(@buffer, @capacity)
   end
diff --git a/src/json.cr b/src/json.cr
index f55aca854c00..2db984dc0a4d 100644
--- a/src/json.cr
+++ b/src/json.cr
@@ -123,7 +123,7 @@ module JSON
     getter line_number : Int32
     getter column_number : Int32
 
-    def initialize(message, @line_number, @column_number, cause = nil)
+    def initialize(message : String?, @line_number : Int32, @column_number : Int32, cause : Nil | Exception | JSON::ParseException = nil)
       super "#{message} at line #{@line_number}, column #{@column_number}", cause
     end
 
diff --git a/src/json/any.cr b/src/json/any.cr
index 61a4118ee932..e4ae1cab35e6 100644
--- a/src/json/any.cr
+++ b/src/json/any.cr
@@ -21,7 +21,7 @@ struct JSON::Any
   alias Type = Nil | Bool | Int64 | Float64 | String | Array(JSON::Any) | Hash(String, JSON::Any)
 
   # Reads a `JSON::Any` value from the given pull parser.
-  def self.new(pull : JSON::PullParser)
+  def self.new(pull : JSON::PullParser) : JSON::Any
     case pull.kind
     when .null?
       new pull.read_null
@@ -58,13 +58,13 @@ struct JSON::Any
   end
 
   # :ditto:
-  def self.new(raw : Int)
+  def self.new(raw : Int) : JSON::Any
     # FIXME: Workaround for https://github.com/crystal-lang/crystal/issues/11645
     new(raw.to_i64)
   end
 
   # :ditto:
-  def self.new(raw : Float)
+  def self.new(raw : Float) : JSON::Any
     # FIXME: Workaround for https://github.com/crystal-lang/crystal/issues/11645
     new(raw.to_f64)
   end
@@ -305,7 +305,7 @@ struct JSON::Any
   def_hash raw
 
   # :nodoc:
-  def to_json(json : JSON::Builder)
+  def to_json(json : JSON::Builder) : Nil
     raw.to_json(json)
   end
 
@@ -319,7 +319,7 @@ struct JSON::Any
   end
 
   # Returns a new JSON::Any instance with the `raw` value `clone`ed.
-  def clone
+  def clone : JSON::Any
     JSON::Any.new(raw.clone)
   end
 end
diff --git a/src/json/lexer.cr b/src/json/lexer.cr
index 3e61179b9844..a7d1f719c30a 100644
--- a/src/json/lexer.cr
+++ b/src/json/lexer.cr
@@ -1,11 +1,11 @@
 require "string_pool"
 
 abstract class JSON::Lexer
-  def self.new(string : String)
+  def self.new(string : String) : JSON::Lexer::StringBased
     StringBased.new(string)
   end
 
-  def self.new(io : IO)
+  def self.new(io : IO) : JSON::Lexer::IOBased
     IOBased.new(io)
   end
 
@@ -76,7 +76,7 @@ abstract class JSON::Lexer
     @token
   end
 
-  private def skip_whitespace
+  private def skip_whitespace : Nil
     while whitespace?(current_char)
       if current_char == '\n'
         @line_number += 1
@@ -86,7 +86,7 @@ abstract class JSON::Lexer
     end
   end
 
-  private def whitespace?(char)
+  private def whitespace?(char : Char) : Bool
     case char
     when ' ', '\t', '\n', '\r'
       true
@@ -95,7 +95,7 @@ abstract class JSON::Lexer
     end
   end
 
-  private def consume_true
+  private def consume_true : JSON::Token::Kind
     if next_char == 'r' && next_char == 'u' && next_char == 'e'
       next_char
       @token.kind = :true
@@ -104,7 +104,7 @@ abstract class JSON::Lexer
     end
   end
 
-  private def consume_false
+  private def consume_false : JSON::Token::Kind
     if next_char == 'a' && next_char == 'l' && next_char == 's' && next_char == 'e'
       next_char
       @token.kind = :false
@@ -113,7 +113,7 @@ abstract class JSON::Lexer
     end
   end
 
-  private def consume_null
+  private def consume_null : JSON::Token::Kind
     if next_char == 'u' && next_char == 'l' && next_char == 'l'
       next_char
       @token.kind = :null
@@ -124,7 +124,7 @@ abstract class JSON::Lexer
 
   # Since we are skipping we don't care about a
   # string's contents, so we just move forward.
-  private def consume_string_skip
+  private def consume_string_skip : Nil
     while true
       case next_char
       when '\0'
@@ -142,7 +142,7 @@ abstract class JSON::Lexer
     end
   end
 
-  private def consume_string_with_buffer
+  private def consume_string_with_buffer : String
     consume_string_with_buffer { }
   end
 
@@ -173,7 +173,7 @@ abstract class JSON::Lexer
     end
   end
 
-  private def consume_string_escape_sequence
+  private def consume_string_escape_sequence : Char
     case char = next_char
     when '\\', '"', '/'
       char
@@ -208,7 +208,7 @@ abstract class JSON::Lexer
     end
   end
 
-  private def read_hex_number
+  private def read_hex_number : Int32
     hexnum = 0
     4.times do
       char = next_char
@@ -217,7 +217,7 @@ abstract class JSON::Lexer
     hexnum
   end
 
-  private def consume_number
+  private def consume_number : String
     number_start
 
     if current_char == '-'
@@ -262,7 +262,7 @@ abstract class JSON::Lexer
     end
   end
 
-  private def consume_float
+  private def consume_float : String
     append_number_char
     char = next_char
 
@@ -283,7 +283,7 @@ abstract class JSON::Lexer
     end
   end
 
-  private def consume_exponent
+  private def consume_exponent : String
     append_number_char
 
     char = next_char
@@ -309,25 +309,25 @@ abstract class JSON::Lexer
     number_end
   end
 
-  private def next_char
+  private def next_char : Char
     @column_number += 1
     next_char_no_column_increment
   end
 
-  private def next_char(kind : Token::Kind)
+  private def next_char(kind : Token::Kind) : Char
     @token.kind = kind
     next_char
   end
 
-  private def number_end
+  private def number_end : String
     @token.raw_value = number_string
   end
 
-  private def unexpected_char(char = current_char)
+  private def unexpected_char(char : Char = current_char) : Nil
     raise "Unexpected char '#{char}'"
   end
 
-  private def raise(msg)
+  private def raise(msg : String) : Nil
     ::raise ParseException.new(msg, @line_number, @column_number)
   end
 end
diff --git a/src/json/lexer/string_based.cr b/src/json/lexer/string_based.cr
index 5696bc6f78b2..89de32d3201c 100644
--- a/src/json/lexer/string_based.cr
+++ b/src/json/lexer/string_based.cr
@@ -1,6 +1,6 @@
 # :nodoc:
 class JSON::Lexer::StringBased < JSON::Lexer
-  def initialize(string)
+  def initialize(string : String)
     super()
     @reader = Char::Reader.new(string)
     @number_start = 0
@@ -39,22 +39,22 @@ class JSON::Lexer::StringBased < JSON::Lexer
     end
   end
 
-  private def consume_string_slow_path(start_pos)
+  private def consume_string_slow_path(start_pos : Int32) : String
     consume_string_with_buffer do
       @buffer.write slice_range(start_pos + 1, current_pos)
       @buffer << consume_string_escape_sequence
     end
   end
 
-  private def current_pos
+  private def current_pos : Int32
     @reader.pos
   end
 
-  def string_range(start_pos, end_pos) : String
+  def string_range(start_pos : Int32, end_pos : Int32) : String
     @reader.string.byte_slice(start_pos, end_pos - start_pos)
   end
 
-  def slice_range(start_pos, end_pos) : Bytes
+  def slice_range(start_pos : Int32, end_pos : Int32) : Bytes
     @reader.string.to_slice[start_pos, end_pos - start_pos]
   end
 
diff --git a/src/json/parser.cr b/src/json/parser.cr
index 865dcd124459..bac4e0f2b79c 100644
--- a/src/json/parser.cr
+++ b/src/json/parser.cr
@@ -15,7 +15,7 @@ class JSON::Parser
     json
   end
 
-  private def parse_value
+  private def parse_value : JSON::Any
     case token.kind
     when .int?
       value_and_next_token token.int_value
@@ -38,7 +38,7 @@ class JSON::Parser
     end
   end
 
-  private def parse_array
+  private def parse_array : JSON::Any
     next_token
 
     ary = [] of Any
@@ -66,7 +66,7 @@ class JSON::Parser
     Any.new(ary)
   end
 
-  private def parse_object
+  private def parse_object : JSON::Any
     next_token_expect_object_key
 
     object = {} of String => Any
@@ -108,20 +108,20 @@ class JSON::Parser
   private delegate next_token, to: @lexer
   private delegate next_token_expect_object_key, to: @lexer
 
-  private def value_and_next_token(value)
+  private def value_and_next_token(value : Int64 | Float64 | String | Nil | Bool) : JSON::Any
     next_token
     Any.new(value)
   end
 
-  private def check(kind : Token::Kind)
+  private def check(kind : Token::Kind) : Nil
     unexpected_token unless token.kind == kind
   end
 
-  private def unexpected_token
+  private def unexpected_token : Nil
     parse_exception "unexpected token '#{token}'"
   end
 
-  private def parse_exception(msg)
+  private def parse_exception(msg : String) : Nil
     raise ParseException.new(msg, token.line_number, token.column_number)
   end
 
diff --git a/src/json/serialization.cr b/src/json/serialization.cr
index 15d948f02f40..6b8675d96b37 100644
--- a/src/json/serialization.cr
+++ b/src/json/serialization.cr
@@ -268,14 +268,14 @@ module JSON
     protected def after_initialize
     end
 
-    protected def on_unknown_json_attribute(pull, key, key_location)
+    protected def on_unknown_json_attribute(pull : JSON::PullParser, key : String, key_location : Tuple(Int32, Int32)) : Nil
       pull.skip
     end
 
-    protected def on_to_json(json : ::JSON::Builder)
+    protected def on_to_json(json : ::JSON::Builder) : Nil
     end
 
-    def to_json(json : ::JSON::Builder)
+    def to_json(json : ::JSON::Builder) : Nil
       {% begin %}
         {% options = @type.annotation(::JSON::Serializable::Options) %}
         {% emit_nulls = options && options[:emit_nulls] %}
@@ -352,7 +352,7 @@ module JSON
     end
 
     module Strict
-      protected def on_unknown_json_attribute(pull, key, key_location)
+      protected def on_unknown_json_attribute(pull : JSON::PullParser, key : String, key_location : Tuple(Int32, Int32)) : Nil
         raise ::JSON::SerializableError.new("Unknown JSON attribute: #{key}", self.class.to_s, nil, *key_location, nil)
       end
     end
@@ -361,7 +361,7 @@ module JSON
       @[JSON::Field(ignore: true)]
       property json_unmapped = Hash(String, JSON::Any).new
 
-      protected def on_unknown_json_attribute(pull, key, key_location)
+      protected def on_unknown_json_attribute(pull : JSON::PullParser, key : String, key_location : Tuple(Int32, Int32)) : JSON::Any
         json_unmapped[key] = begin
           JSON::Any.new(pull)
         rescue exc : ::JSON::ParseException
@@ -369,7 +369,7 @@ module JSON
         end
       end
 
-      protected def on_to_json(json)
+      protected def on_to_json(json : JSON::Builder) : Nil
         json_unmapped.each do |key, value|
           json.field(key) { value.to_json(json) }
         end
@@ -480,7 +480,7 @@ module JSON
     getter klass : String
     getter attribute : String?
 
-    def initialize(message : String?, @klass : String, @attribute : String?, line_number : Int32, column_number : Int32, cause)
+    def initialize(message : String?, @klass : String, @attribute : String?, line_number : Int32, column_number : Int32, cause : JSON::ParseException?)
       message = String.build do |io|
         io << message
         io << "\n  parsing "
diff --git a/src/kernel.cr b/src/kernel.cr
index 054705c55e5b..a29fb3e24760 100644
--- a/src/kernel.cr
+++ b/src/kernel.cr
@@ -168,12 +168,12 @@ end
 # Prints a formatted string to `STDOUT`.
 #
 # For details on the format string, see `sprintf`.
-def printf(format_string, *args) : Nil
+def printf(format_string : String, *args) : Nil
   printf format_string, args
 end
 
 # :ditto:
-def printf(format_string, args : Array | Tuple) : Nil
+def printf(format_string : String, args : Array | Tuple) : Nil
   STDOUT.printf format_string, args
 end
 
@@ -433,12 +433,12 @@ end
 # sprintf "%+g:% g:%-g", 1.23, 1.23, 1.23     # => "+1.23: 1.23:1.23"
 # sprintf "%-3$*1$.*2$s", 10, 5, "abcdefghij" # => "abcde     "
 # ```
-def sprintf(format_string, *args) : String
+def sprintf(format_string : String, *args) : String
   sprintf format_string, args
 end
 
 # :ditto:
-def sprintf(format_string, args : Array | Tuple) : String
+def sprintf(format_string : String, args : Array | Tuple) : String
   String.build(format_string.bytesize) do |str|
     String::Formatter(typeof(args)).new(format_string, args, str).format
   end
@@ -456,7 +456,7 @@ end
 # by a newline. Returns *object*.
 #
 # See also: `Object#inspect(io)`.
-def p(object)
+def p(object : Tuple(Int32, Int32)) : Tuple(Int32, Int32)
   object.inspect(STDOUT)
   puts
   object
@@ -489,7 +489,7 @@ end
 # by a newline. Returns *object*.
 #
 # See also: `Object#pretty_print(pp)`.
-def pp(object)
+def pp(object : Tuple(Int32, Int32)) : Tuple(Int32, Int32)
   PrettyPrint.format(object, STDOUT, 79)
   puts
   object
@@ -563,7 +563,7 @@ end
 # to the invoking environment.
 #
 # Registered `at_exit` procs are executed.
-def exit(status = 0) : NoReturn
+def exit(status : Int32 = 0) : NoReturn
   status = Crystal::AtExitHandlers.run status
   Crystal.ignore_stdio_errors { STDOUT.flush }
   Crystal.ignore_stdio_errors { STDERR.flush }
@@ -572,7 +572,7 @@ end
 
 # Terminates execution immediately, printing *message* to `STDERR` and
 # then calling `exit(status)`.
-def abort(message = nil, status = 1) : NoReturn
+def abort(message = nil, status : Int32 = 1) : NoReturn
   Crystal.ignore_stdio_errors { STDERR.puts message } if message
   exit status
 end
diff --git a/src/levenshtein.cr b/src/levenshtein.cr
index 01ad1bc40784..bcd62e8061ef 100644
--- a/src/levenshtein.cr
+++ b/src/levenshtein.cr
@@ -93,7 +93,7 @@ module Levenshtein
       @tolerance = tolerance || (target.size / 5.0).ceil.to_i
     end
 
-    def test(name : String, value : String = name)
+    def test(name : String, value : String = name) : Levenshtein::Finder::Entry?
       distance = Levenshtein.distance(@target, name)
       if distance <= @tolerance
         if best_entry = @best_entry
diff --git a/src/llvm.cr b/src/llvm.cr
index 170073b9cb56..b3bda7cbde30 100644
--- a/src/llvm.cr
+++ b/src/llvm.cr
@@ -11,7 +11,7 @@ module LLVM
   # information, so the value falls back to `LibLLVM::VERSION` which is
   # determined at compile time and might slightly be out of sync to the
   # dynamic library loaded at runtime.
-  def self.version
+  def self.version : String
     {% if LibLLVM.has_method?(:get_version) %}
       LibLLVM.get_version(out major, out minor, out patch)
       "#{major}.#{minor}.#{patch}"
@@ -169,12 +169,12 @@ module LLVM
     normalized
   end
 
-  def self.to_io(chars, io) : Nil
+  def self.to_io(chars : Pointer(UInt8), io : String::Builder) : Nil
     io.write_string Slice.new(chars, LibC.strlen(chars))
     LibLLVM.dispose_message(chars)
   end
 
-  def self.string_and_dispose(chars) : String
+  def self.string_and_dispose(chars : Pointer(UInt8)) : String
     string = String.new(chars)
     LibLLVM.dispose_message(chars)
     string
diff --git a/src/math/math.cr b/src/math/math.cr
index efeef9ad9e12..6a363884887a 100644
--- a/src/math/math.cr
+++ b/src/math/math.cr
@@ -113,7 +113,7 @@ module Math
   end
 
   # :ditto:
-  def atan2(y, x) : Float64
+  def atan2(y : Int32, x : Int32) : Float64
     atan2(y.to_f, x.to_f)
   end
 
@@ -263,7 +263,7 @@ module Math
   end
 
   # :ditto:
-  def log(value) : Float64
+  def log(value : Int32) : Float64
     log(value.to_f)
   end
 
@@ -293,7 +293,7 @@ module Math
   end
 
   # :ditto:
-  def log2(value) : Float64
+  def log2(value : Int32) : Float64
     log2(value.to_f)
   end
 
@@ -308,12 +308,12 @@ module Math
   end
 
   # :ditto:
-  def log10(value)
+  def log10(value : UInt64 | Int32 | Int64 | BigInt) : Float64
     log10(value.to_f)
   end
 
   # Calculates the logarithm of *value* to the given *base*.
-  def log(value, base)
+  def log(value : Float32 | Float64, base : Int32) : Float64
     log(value) / log(base)
   end
 
@@ -328,12 +328,12 @@ module Math
   end
 
   # :ditto:
-  def sqrt(value) : Float64
+  def sqrt(value : Int32) : Float64
     sqrt(value.to_f)
   end
 
   # Calculates the integer square root of *value*.
-  def isqrt(value : Int::Primitive)
+  def isqrt(value : Int::Primitive) : Int32 | UInt8 | UInt16 | UInt32 | UInt64 | UInt128 | Int8 | Int16 | Int64 | Int128
     raise ArgumentError.new "Input must be non-negative integer" if value < 0
     return value if value < 2
     res = value.class.zero
@@ -410,7 +410,7 @@ module Math
   end
 
   # :ditto:
-  def gamma(value) : Float64
+  def gamma(value : Int32 | Int8) : Float64
     gamma(value.to_f)
   end
 
@@ -423,7 +423,7 @@ module Math
   # ```
   # Math.log(Math.gamma(2.96).abs)
   # ```
-  def lgamma(value : Float32)
+  def lgamma(value : Float32) : Float32
     {% if flag?(:darwin) %}
       LibM.gamma_f64(value).to_f32
     {% else %}
@@ -437,12 +437,12 @@ module Math
   end
 
   # :ditto:
-  def lgamma(value) : Float64
+  def lgamma(value : Int32 | Int8) : Float64
     lgamma(value.to_f)
   end
 
   # Calculates the cylindrical Bessel function of the first kind of *value* for the given *order*.
-  def besselj(order : Int32, value : Float32)
+  def besselj(order : Int32, value : Float32) : Float32
     {% if flag?(:darwin) || flag?(:win32) %}
       LibM.besselj_f64(order, value).to_f32
     {% else %}
@@ -461,7 +461,7 @@ module Math
   end
 
   # Calculates the cylindrical Bessel function of the first kind of *value* for order 0.
-  def besselj0(value : Float32)
+  def besselj0(value : Float32) : Float32
     {% if flag?(:darwin) || flag?(:win32) %}
       LibM.besselj0_f64(value).to_f32
     {% else %}
@@ -480,7 +480,7 @@ module Math
   end
 
   # Calculates the cylindrical Bessel function of the first kind of *value* for order 1.
-  def besselj1(value : Float32)
+  def besselj1(value : Float32) : Float32
     {% if flag?(:darwin) || flag?(:win32) %}
       LibM.besselj1_f64(value).to_f32
     {% else %}
@@ -499,7 +499,7 @@ module Math
   end
 
   # Calculates the cylindrical Bessel function of the second kind of *value* for the given *order*.
-  def bessely(order : Int32, value : Float32)
+  def bessely(order : Int32, value : Float32) : Float32
     {% if flag?(:darwin) || flag?(:win32) %}
       LibM.bessely_f64(order, value).to_f32
     {% else %}
@@ -518,7 +518,7 @@ module Math
   end
 
   # Calculates the cylindrical Bessel function of the second kind of *value* for order 0.
-  def bessely0(value : Float32)
+  def bessely0(value : Float32) : Float32
     {% if flag?(:darwin) || flag?(:win32) %}
       LibM.bessely0_f64(value).to_f32
     {% else %}
@@ -537,7 +537,7 @@ module Math
   end
 
   # Calculates the cylindrical Bessel function of the second kind of *value* for order 1.
-  def bessely1(value : Float32)
+  def bessely1(value : Float32) : Float32
     {% if flag?(:darwin) || flag?(:win32) %}
       LibM.bessely1_f64(value).to_f32
     {% else %}
@@ -676,7 +676,7 @@ module Math
   end
 
   # :ditto:
-  def scalbln(value, exp) : Float64
+  def scalbln(value : Float32 | Float64, exp : Int32) : Float64
     scalbln(value.to_f, exp.to_i64)
   end
 
@@ -714,7 +714,7 @@ module Math
   end
 
   # :ditto:
-  def copysign(value1, value2)
+  def copysign(value1 : Float64 | Int32 | Float32, value2 : Int32 | Float64 | Float32) : Float64
     copysign(value1.to_f, value2.to_f)
   end
 
@@ -729,7 +729,7 @@ module Math
   end
 
   # :ditto:
-  def max(value1, value2)
+  def max(value1 : Int32 | Int64, value2 : UInt32 | UInt64 | Int32) : Int32 | UInt32 | UInt64 | Int64
     value1 >= value2 ? value1 : value2
   end
 
@@ -744,7 +744,7 @@ module Math
   end
 
   # :ditto:
-  def min(value1, value2)
+  def min(value1 : Int32 | UInt32 | UInt64 | Int16 | Int64, value2 : Int32 | Int64 | Int16 | UInt8 | UInt32) : Int32 | Int64 | UInt32 | UInt64 | Int16 | UInt8
     value1 <= value2 ? value1 : value2
   end
 
@@ -759,7 +759,7 @@ module Math
   # Math.pw2ceil(64) # => 64
   # Math.pw2ceil(-5) # => 1
   # ```
-  def pw2ceil(v : Int::Primitive)
+  def pw2ceil(v : Int::Primitive) : Int32 | Int8 | Int16 | Int64 | Int128 | UInt8 | UInt16 | UInt32 | UInt64 | UInt128
     v.next_power_of_two
   end
 end
diff --git a/src/mime.cr b/src/mime.cr
index 7eb65f23818b..b99393b5a35e 100644
--- a/src/mime.cr
+++ b/src/mime.cr
@@ -140,7 +140,7 @@ module MIME
     end
   end
 
-  private def self.initialize_types
+  private def self.initialize_types : Nil
     init unless @@initialized
   end
 
@@ -148,7 +148,7 @@ module MIME
   #
   # A case-sensitive search is tried first, if this yields no result, it is
   # matched case-insensitive. Returns *default* if *extension* is not registered.
-  def self.from_extension(extension : String, default) : String
+  def self.from_extension(extension : String, default : String) : String
     from_extension(extension) { default }
   end
 
@@ -182,7 +182,7 @@ module MIME
   #
   # A case-sensitive search is tried first, if this yields no result, it is
   # matched case-insensitive. Returns *default* if extension is not registered.
-  def self.from_filename(filename : String | Path, default) : String
+  def self.from_filename(filename : String | Path, default : String) : String
     from_extension(File.extname(filename.to_s), default)
   end
 
diff --git a/src/mime/media_type.cr b/src/mime/media_type.cr
index e563f2aa27da..9a50a84c93bd 100644
--- a/src/mime/media_type.cr
+++ b/src/mime/media_type.cr
@@ -64,7 +64,7 @@ module MIME
     # mime_type["foo"] = "bar"
     # mime_type["foo"] # => "bar"
     # ```
-    def []=(key : String, value : String)
+    def []=(key : String, value : String) : String
       raise Error.new("Invalid parameter name") unless MIME::MediaType.token? key
       @params[key] = value
     end
@@ -446,7 +446,7 @@ module MIME
       return reader, value
     end
 
-    private def self.decode_rfc2231(encoded : String)
+    private def self.decode_rfc2231(encoded : String) : String?
       encoding, _, rest = encoded.partition('\'')
       _lang, _, value = rest.partition('\'')
 
@@ -475,7 +475,7 @@ module MIME
       end.rewind.gets_to_end
     end
 
-    private def self.consume_whitespace(reader)
+    private def self.consume_whitespace(reader : Char::Reader) : Char::Reader
       while reader.current_char.ascii_whitespace?
         reader.next_char
       end
@@ -488,12 +488,12 @@ module MIME
     end
 
     # :nodoc:
-    def self.token?(string) : Bool
+    def self.token?(string : String) : Bool
       string.each_char.all? { |char| token? char }
     end
 
     # :nodoc:
-    def self.quote_string(string, io) : Nil
+    def self.quote_string(string : String, io : String::Builder | IO::Memory) : Nil
       string.each_char do |char|
         case char
         when '"', '\\'
diff --git a/src/mime/multipart.cr b/src/mime/multipart.cr
index 943af6a15167..6df5d87c3298 100644
--- a/src/mime/multipart.cr
+++ b/src/mime/multipart.cr
@@ -39,7 +39,7 @@ module MIME::Multipart
   #
   # MIME::Multipart.parse_boundary("multipart/mixed; boundary=\"abcde\"") # => "abcde"
   # ```
-  def self.parse_boundary(content_type) : String?
+  def self.parse_boundary(content_type : String) : String?
     type = MIME::MediaType.parse?(content_type)
 
     if type && type.type == "multipart"
diff --git a/src/mime/multipart/builder.cr b/src/mime/multipart/builder.cr
index 59d38d766da1..18f4608e000a 100644
--- a/src/mime/multipart/builder.cr
+++ b/src/mime/multipart/builder.cr
@@ -33,7 +33,7 @@ module MIME::Multipart
     # builder = MIME::Multipart::Builder.new(io, "a4VF")
     # builder.content_type("mixed") # => "multipart/mixed; boundary=a4VF"
     # ```
-    def content_type(subtype = "mixed") : String
+    def content_type(subtype : String = "mixed") : String
       MIME::MediaType.new("multipart/#{subtype}", {"boundary" => @boundary}).to_s
     end
 
@@ -199,7 +199,7 @@ module MIME::Multipart
       @io.flush
     end
 
-    private def fail(msg)
+    private def fail(msg : String) : Nil
       raise Multipart::Error.new msg
     end
   end
diff --git a/src/mime/multipart/parser.cr b/src/mime/multipart/parser.cr
index 262f500119f0..775f0ff2d1f3 100644
--- a/src/mime/multipart/parser.cr
+++ b/src/mime/multipart/parser.cr
@@ -83,7 +83,7 @@ module MIME::Multipart
       !@state.finished? && !@state.errored?
     end
 
-    private def parse_headers(io)
+    private def parse_headers(io : IO::Delimited) : HTTP::Headers
       headers = HTTP::Headers.new
 
       while line = io.gets(chomp: false)
@@ -104,7 +104,7 @@ module MIME::Multipart
     #
     # If it's not a close delimiter, it eats the transport padding and crlf
     # after a delimiter.
-    private def close_delimiter?
+    private def close_delimiter? : Bool
       transport_padding_crlf = @io.gets("\r\n")
       fail("EOF reading delimiter") unless transport_padding_crlf
 
@@ -122,7 +122,7 @@ module MIME::Multipart
       false
     end
 
-    private def fail(msg)
+    private def fail(msg : String) : Nil
       raise Multipart::Error.new "Failed to parse multipart message: " + msg
     end
   end
diff --git a/src/mutex.cr b/src/mutex.cr
index 14d1aedf7923..fff44ae51e4c 100644
--- a/src/mutex.cr
+++ b/src/mutex.cr
@@ -83,7 +83,7 @@ class Mutex
     @mutex_fiber = Fiber.current unless @protection.unchecked?
   end
 
-  private def try_lock
+  private def try_lock : Bool
     i = 1000
     while @state.swap(LOCKED, :acquire) != UNLOCKED
       while @state.get(:relaxed) != UNLOCKED
diff --git a/src/nil.cr b/src/nil.cr
index 7b97b5f00d3c..d60ddfb109ff 100644
--- a/src/nil.cr
+++ b/src/nil.cr
@@ -55,12 +55,12 @@ struct Nil
   end
 
   # Returns `true`: `Nil` has only one singleton value: `nil`.
-  def ==(other : Nil)
+  def ==(other : Nil) : Bool
     true
   end
 
   # Returns `true`: `Nil` has only one singleton value: `nil`.
-  def same?(other : Nil)
+  def same?(other : Nil) : Bool
     true
   end
 
@@ -127,7 +127,7 @@ struct Nil
     self
   end
 
-  def clone
+  def clone : Nil
     self
   end
 end
diff --git a/src/oauth/authorization_header.cr b/src/oauth/authorization_header.cr
index ad5f2428ef5e..1fe2df4629d5 100644
--- a/src/oauth/authorization_header.cr
+++ b/src/oauth/authorization_header.cr
@@ -6,7 +6,7 @@ struct OAuth::AuthorizationHeader
     @first = true
   end
 
-  def add(key, value)
+  def add(key : String, value : String?) : Bool?
     return unless value
 
     @str << ", " unless @first
diff --git a/src/oauth/consumer.cr b/src/oauth/consumer.cr
index 14ba9407bd41..8d1079792751 100644
--- a/src/oauth/consumer.cr
+++ b/src/oauth/consumer.cr
@@ -72,7 +72,7 @@ class OAuth::Consumer
   # [RFC 5849, Section 2.1](https://tools.ietf.org/html/rfc5849#section-2.1).
   #
   # Raises `OAuth::Error` if there was an error getting the request token.
-  def get_request_token(oauth_callback = "oob") # : RequestToken
+  def get_request_token(oauth_callback : String = "oob") : OAuth::RequestToken
     post(nil, nil, {"oauth_callback" => oauth_callback}, @request_token_uri) do |response|
       RequestToken.from_response(response.body)
     end
@@ -81,7 +81,7 @@ class OAuth::Consumer
   # Returns an authorize URI from a given request token to redirect the user
   # to obtain an access token, as specified by
   # [RFC 5849, Section 2.2](https://tools.ietf.org/html/rfc5849#section-2.2).
-  def get_authorize_uri(request_token, oauth_callback = nil) : String
+  def get_authorize_uri(request_token : OAuth::RequestToken, oauth_callback : String? = nil) : String
     get_authorize_uri(request_token, oauth_callback) { }
   end
 
@@ -116,7 +116,7 @@ class OAuth::Consumer
   # [RFC 5849, Section 2.3](https://tools.ietf.org/html/rfc5849#section-2.3).
   #
   # Raises `OAuth::Error` if there was an error getting the access token.
-  def get_access_token(request_token, oauth_verifier, extra_params = nil) : AccessToken
+  def get_access_token(request_token : OAuth::RequestToken, oauth_verifier : String, extra_params : Hash(String, String)? = nil) : AccessToken
     extra_params ||= {} of String => String
     extra_params["oauth_verifier"] = oauth_verifier
     post(request_token.token, request_token.secret, extra_params, @access_token_uri) do |response|
@@ -151,7 +151,7 @@ class OAuth::Consumer
     end
   end
 
-  private def authenticate(client, token, token_secret, extra_params)
+  private def authenticate(client : HTTP::Client, token : String?, token_secret : String?, extra_params : Hash(String, String)?) : Nil
     OAuth.authenticate(client, token, token_secret, @consumer_key, @consumer_secret, extra_params)
   end
 
diff --git a/src/oauth/oauth.cr b/src/oauth/oauth.cr
index 292055cfc777..c5a0e40a6b10 100644
--- a/src/oauth/oauth.cr
+++ b/src/oauth/oauth.cr
@@ -40,17 +40,17 @@
 module OAuth
   # Sets up an `HTTP::Client` to add an OAuth authorization header to every request performed.
   # Check this module's docs for an example usage.
-  def self.authenticate(client : HTTP::Client, token, token_secret, consumer_key, consumer_secret, extra_params = nil) : Nil
+  def self.authenticate(client : HTTP::Client, token : String?, token_secret : String?, consumer_key : String, consumer_secret : String, extra_params : Hash(String, String)? = nil) : Nil
     client.before_request do |request|
       authenticate client, request, token, token_secret, consumer_key, consumer_secret, extra_params
     end
   end
 
-  private def self.authenticate(client, request, token, token_secret, consumer_key, consumer_secret, extra_params)
+  private def self.authenticate(client : HTTP::Client, request : HTTP::Request, token : String?, token_secret : String?, consumer_key : String, consumer_secret : String, extra_params : Hash(String, String)?) : String
     request.headers["Authorization"] = oauth_header(client, request, token, token_secret, consumer_key, consumer_secret, extra_params)
   end
 
-  private def self.oauth_header(client, request, token, token_secret, consumer_key, consumer_secret, extra_params)
+  private def self.oauth_header(client : HTTP::Client, request : HTTP::Request, token : String?, token_secret : String?, consumer_key : String, consumer_secret : String, extra_params : Hash(String, String)?) : String
     ts = Time.utc.to_unix.to_s
     nonce = Random::Secure.hex
 
diff --git a/src/oauth/params.cr b/src/oauth/params.cr
index 9689d4ac479c..637c39905bda 100644
--- a/src/oauth/params.cr
+++ b/src/oauth/params.cr
@@ -4,13 +4,13 @@ struct OAuth::Params
     @params = [] of {String, String}
   end
 
-  def add(key, value) : Nil
+  def add(key : String, value : String?) : Nil
     if value
       @params << {URI.encode_www_form(key, space_to_plus: false), URI.encode_www_form(value, space_to_plus: false)}
     end
   end
 
-  def add_query(query) : Nil
+  def add_query(query : String) : Nil
     URI::Params.parse(query) do |key, value|
       add key, value
     end
diff --git a/src/oauth/request_token.cr b/src/oauth/request_token.cr
index 0fa79c36fc9c..bbcc6b9b171f 100644
--- a/src/oauth/request_token.cr
+++ b/src/oauth/request_token.cr
@@ -5,7 +5,7 @@ class OAuth::RequestToken
   def initialize(@token : String, @secret : String)
   end
 
-  def self.from_response(response) : self
+  def self.from_response(response : String) : self
     token = nil
     secret = nil
 
diff --git a/src/oauth/signature.cr b/src/oauth/signature.cr
index 2038243f25fa..77c57ed4e1fe 100644
--- a/src/oauth/signature.cr
+++ b/src/oauth/signature.cr
@@ -3,7 +3,7 @@ struct OAuth::Signature
   def initialize(@consumer_key : String, @client_shared_secret : String, @oauth_token : String? = nil, @token_shared_secret : String? = nil, @extra_params : Hash(String, String)? = nil)
   end
 
-  def base_string(request, tls, ts, nonce) : String
+  def base_string(request : HTTP::Request, tls : Nil | OpenSSL::SSL::Context::Client | Bool, ts : String, nonce : String) : String
     base_string request, tls, gather_params(request, ts, nonce)
   end
 
@@ -17,12 +17,12 @@ struct OAuth::Signature
     end
   end
 
-  def compute(request, tls, ts, nonce) : String
+  def compute(request : HTTP::Request, tls : Nil | OpenSSL::SSL::Context::Client | Bool, ts : String, nonce : String) : String
     base_string = base_string(request, tls, ts, nonce)
     Base64.strict_encode(OpenSSL::HMAC.digest :sha1, key, base_string)
   end
 
-  def authorization_header(request, tls, ts, nonce) : String
+  def authorization_header(request : HTTP::Request, tls : Nil | OpenSSL::SSL::Context::Client | Bool, ts : String, nonce : String) : String
     oauth_signature = compute request, tls, ts, nonce
 
     auth_header = AuthorizationHeader.new
@@ -39,7 +39,7 @@ struct OAuth::Signature
     auth_header.to_s
   end
 
-  private def base_string(request, tls, params)
+  private def base_string(request : HTTP::Request, tls : Nil | OpenSSL::SSL::Context::Client | Bool, params : OAuth::Params) : String
     host, port = host_and_port(request, tls)
 
     String.build do |str|
@@ -59,7 +59,7 @@ struct OAuth::Signature
     end
   end
 
-  private def gather_params(request, ts, nonce)
+  private def gather_params(request : HTTP::Request, ts : String, nonce : String) : OAuth::Params
     params = Params.new
     params.add "oauth_consumer_key", @consumer_key
     params.add "oauth_nonce", nonce
@@ -87,7 +87,7 @@ struct OAuth::Signature
     params
   end
 
-  private def host_and_port(request, tls)
+  private def host_and_port(request : HTTP::Request, tls : Nil | OpenSSL::SSL::Context::Client | Bool) : Tuple(String, Int32 | Nil)
     host_header = request.headers["Host"]
     if colon_index = host_header.index ':'
       host = host_header[0...colon_index]
diff --git a/src/oauth2/access_token/access_token.cr b/src/oauth2/access_token/access_token.cr
index 2fe2566aea80..229f8a900787 100644
--- a/src/oauth2/access_token/access_token.cr
+++ b/src/oauth2/access_token/access_token.cr
@@ -2,7 +2,7 @@
 #
 # Use `#authenticate` to authenticate an `HTTP::Client`.
 abstract class OAuth2::AccessToken
-  def self.new(pull : JSON::PullParser)
+  def self.new(pull : JSON::PullParser) : OAuth2::AccessToken
     token_type = nil
     access_token = nil
     expires_in = nil
diff --git a/src/oauth2/access_token/bearer.cr b/src/oauth2/access_token/bearer.cr
index c586dbb2cd47..025586dab9a4 100644
--- a/src/oauth2/access_token/bearer.cr
+++ b/src/oauth2/access_token/bearer.cr
@@ -1,7 +1,7 @@
 require "./access_token"
 
 class OAuth2::AccessToken::Bearer < OAuth2::AccessToken
-  def self.new(pull : JSON::PullParser)
+  def self.new(pull : JSON::PullParser) : OAuth2::AccessToken::Bearer
     OAuth2::AccessToken.new(pull).as(self)
   end
 
diff --git a/src/oauth2/access_token/mac.cr b/src/oauth2/access_token/mac.cr
index 6f7a8f44cbf4..e5d1cb45cda3 100644
--- a/src/oauth2/access_token/mac.cr
+++ b/src/oauth2/access_token/mac.cr
@@ -4,7 +4,7 @@ require "base64"
 require "./access_token"
 
 class OAuth2::AccessToken::Mac < OAuth2::AccessToken
-  def self.new(pull : JSON::PullParser)
+  def self.new(pull : JSON::PullParser) : OAuth2::AccessToken::Mac
     OAuth2::AccessToken.new(pull).as(self)
   end
 
@@ -12,7 +12,7 @@ class OAuth2::AccessToken::Mac < OAuth2::AccessToken
   property mac_key : String
   property issued_at : Int64
 
-  def initialize(access_token, expires_in, @mac_algorithm, @mac_key, refresh_token = nil, scope = nil, @issued_at = Time.utc.to_unix, extra = nil)
+  def initialize(access_token : String, expires_in : Int64 | Nil | Int32, @mac_algorithm : String, @mac_key : String, refresh_token : String? = nil, scope : String? = nil, @issued_at : Int64 = Time.utc.to_unix, extra : Hash(String, String)? = nil)
     super(access_token, expires_in, refresh_token, scope, extra)
   end
 
@@ -34,7 +34,7 @@ class OAuth2::AccessToken::Mac < OAuth2::AccessToken
     request.headers["Authorization"] = header
   end
 
-  def self.signature(ts, nonce, method, uri, host, port, ext, mac_algorithm, mac_key) : String
+  def self.signature(ts : Int64 | Int32, nonce : String, method : String, uri : String, host : String, port : Int32 | String, ext : String, mac_algorithm : String, mac_key : String) : String
     normalized_request_string = "#{ts}\n#{nonce}\n#{method}\n#{uri}\n#{host}\n#{port}\n#{ext}\n"
 
     digest = case mac_algorithm
@@ -59,7 +59,7 @@ class OAuth2::AccessToken::Mac < OAuth2::AccessToken
 
   def_equals_and_hash access_token, expires_in, mac_algorithm, mac_key, refresh_token, scope
 
-  private def host_and_port(request, tls)
+  private def host_and_port(request : HTTP::Request, tls : Bool | Nil | OpenSSL::SSL::Context::Client) : Tuple(String, Int32)
     host_header = request.headers["Host"]
     if colon_index = host_header.index ':'
       host = host_header[0...colon_index]
diff --git a/src/oauth2/client.cr b/src/oauth2/client.cr
index df30eafd8a3b..7cfcc0c622da 100644
--- a/src/oauth2/client.cr
+++ b/src/oauth2/client.cr
@@ -95,7 +95,7 @@ class OAuth2::Client
 
   # Builds an authorize URI, as specified by
   # [RFC 6749, Section 4.1.1](https://tools.ietf.org/html/rfc6749#section-4.1.1).
-  def get_authorize_uri(scope = nil, state = nil) : String
+  def get_authorize_uri(scope : String? = nil, state : String? = nil) : String
     get_authorize_uri(scope, state) { }
   end
 
@@ -139,7 +139,7 @@ class OAuth2::Client
 
   # Gets an access token using the resource owner credentials, as specified by
   # [RFC 6749, Section 4.3.2](https://tools.ietf.org/html/rfc6749#section-4.3.2).
-  def get_access_token_using_resource_owner_credentials(username : String, password : String, scope = nil) : AccessToken
+  def get_access_token_using_resource_owner_credentials(username : String, password : String, scope : String? = nil) : AccessToken
     get_access_token do |form|
       form.add("grant_type", "password")
       form.add("username", username)
@@ -150,7 +150,7 @@ class OAuth2::Client
 
   # Gets an access token using client credentials, as specified by
   # [RFC 6749, Section 4.4.2](https://tools.ietf.org/html/rfc6749#section-4.4.2).
-  def get_access_token_using_client_credentials(scope = nil) : AccessToken
+  def get_access_token_using_client_credentials(scope : String? = nil) : AccessToken
     get_access_token do |form|
       form.add("grant_type", "client_credentials")
       form.add("scope", scope) unless scope.nil?
@@ -159,7 +159,7 @@ class OAuth2::Client
 
   # Gets an access token using a refresh token, as specified by
   # [RFC 6749, Section 6](https://tools.ietf.org/html/rfc6749#section-6).
-  def get_access_token_using_refresh_token(refresh_token, scope = nil) : AccessToken
+  def get_access_token_using_refresh_token(refresh_token : String?, scope : String? = nil) : AccessToken
     get_access_token do |form|
       form.add("grant_type", "refresh_token")
       form.add("refresh_token", refresh_token)
diff --git a/src/oauth2/session.cr b/src/oauth2/session.cr
index 1ba9ec088cee..8334dfee5adf 100644
--- a/src/oauth2/session.cr
+++ b/src/oauth2/session.cr
@@ -18,12 +18,12 @@ class OAuth2::Session
   # Authenticates an `HTTP::Client`, refreshing the access token if it is expired.
   #
   # Invoke this method on an `HTTP::Client` before executing an HTTP request.
-  def authenticate(http_client) : Nil
+  def authenticate(http_client : HTTP::Client) : Nil
     check_refresh_token
     @access_token.authenticate http_client
   end
 
-  private def check_refresh_token
+  private def check_refresh_token : Nil
     if access_token_expired?
       refresh_access_token
 
@@ -31,7 +31,7 @@ class OAuth2::Session
     end
   end
 
-  private def access_token_expired?
+  private def access_token_expired? : Bool
     if expires_at = @expires_at
       Time.utc >= expires_at
     else
@@ -39,7 +39,7 @@ class OAuth2::Session
     end
   end
 
-  private def refresh_access_token
+  private def refresh_access_token : String?
     old_access_token = @access_token
     @access_token = @oauth2_client.get_access_token_using_refresh_token(@access_token.refresh_token)
 
diff --git a/src/openssl.cr b/src/openssl.cr
index 986cd5d29fc9..6f72c13e4d20 100644
--- a/src/openssl.cr
+++ b/src/openssl.cr
@@ -110,7 +110,7 @@ module OpenSSL
       getter error : ErrorType
       getter? underlying_eof : Bool = false
 
-      def initialize(ssl : LibSSL::SSL, return_code : LibSSL::Int, func = nil)
+      def initialize(ssl : LibSSL::SSL, return_code : LibSSL::Int, func : String? = nil)
         @error = LibSSL.ssl_get_error(ssl, return_code)
 
         case @error
diff --git a/src/openssl/algorithm.cr b/src/openssl/algorithm.cr
index 8ad843eb8e40..db2ea4915063 100644
--- a/src/openssl/algorithm.cr
+++ b/src/openssl/algorithm.cr
@@ -17,7 +17,7 @@ module OpenSSL
     #
     # The internal bindings to the `LibCrypto` digest operations sometimes require a hash algorithm
     # implementation to be passed as one of the arguments.
-    def to_evp
+    def to_evp : LibCrypto::EVP_MD
       case self
       when MD4       then LibCrypto.evp_md4
       when MD5       then LibCrypto.evp_md5
diff --git a/src/openssl/bio.cr b/src/openssl/bio.cr
index 705bbab0a05c..2e0c27d3c468 100644
--- a/src/openssl/bio.cr
+++ b/src/openssl/bio.cr
@@ -2,7 +2,7 @@ require "./lib_crypto"
 
 # :nodoc:
 struct OpenSSL::BIO
-  def self.get_data(bio) : Void*
+  def self.get_data(bio : Pointer(LibCrypto::Bio)) : Void*
     {% if LibCrypto.has_method?(:BIO_get_data) %}
       LibCrypto.BIO_get_data(bio)
     {% else %}
@@ -10,7 +10,7 @@ struct OpenSSL::BIO
     {% end %}
   end
 
-  def self.set_data(bio, data : Void*)
+  def self.set_data(bio : Pointer(LibCrypto::Bio), data : Void*) : Nil
     {% if LibCrypto.has_method?(:BIO_set_data) %}
       LibCrypto.BIO_set_data(bio, data)
     {% else %}
@@ -129,7 +129,7 @@ struct OpenSSL::BIO
 
   getter io
 
-  def to_unsafe
+  def to_unsafe : Pointer(LibCrypto::Bio)
     @bio
   end
 end
diff --git a/src/openssl/cipher.cr b/src/openssl/cipher.cr
index c24c821d54dc..1455b57ab257 100644
--- a/src/openssl/cipher.cr
+++ b/src/openssl/cipher.cr
@@ -45,7 +45,7 @@ class OpenSSL::Cipher
   class Error < OpenSSL::Error
   end
 
-  def initialize(name)
+  def initialize(name : String)
     cipher = LibCrypto.evp_get_cipherbyname name
     raise ArgumentError.new "Unsupported cipher algorithm #{name.inspect}" unless cipher
 
@@ -67,13 +67,13 @@ class OpenSSL::Cipher
     cipherinit enc: 0
   end
 
-  def key=(key)
+  def key=(key : String) : String
     raise ArgumentError.new "Key length too short: wanted #{key_len}, got #{key.bytesize}" if key.bytesize < key_len
     cipherinit key: key
     key
   end
 
-  def iv=(iv)
+  def iv=(iv : String) : String
     raise ArgumentError.new "IV length too short: wanted #{iv_len}, got #{iv.bytesize}" if iv.bytesize < iv_len
     cipherinit iv: iv
     iv
@@ -97,7 +97,7 @@ class OpenSSL::Cipher
   end
 
   # Add the data to be encrypted or decrypted to this cipher's buffer.
-  def update(data)
+  def update(data : String | Slice(UInt8)) : Slice(UInt8)
     slice = data.to_slice
     buffer_length = slice.size + block_size
     buffer = Bytes.new(buffer_length)
@@ -148,7 +148,7 @@ class OpenSSL::Cipher
     LibCrypto.evp_cipher_iv_length cipher
   end
 
-  def finalize
+  def finalize : Pointer(Void)
     LibCrypto.evp_cipher_ctx_free(@ctx) if @ctx
     @ctx = typeof(@ctx).null
   end
@@ -157,7 +157,7 @@ class OpenSSL::Cipher
     LibCrypto.evp_cipher_flags(cipher).includes?(LibCrypto::CipherFlags::EVP_CIPH_FLAG_AEAD_CIPHER)
   end
 
-  private def cipherinit(cipher = nil, engine = nil, key = nil, iv = nil, enc = -1)
+  private def cipherinit(cipher : Pointer(Void)? = nil, engine : Nil = nil, key : String? = nil, iv : String? = nil, enc : Int32 = -1) : Nil
     if LibCrypto.evp_cipherinit_ex(@ctx, cipher, engine, key, iv, enc) != 1
       raise Error.new "EVP_CipherInit_ex"
     end
@@ -165,7 +165,7 @@ class OpenSSL::Cipher
     nil
   end
 
-  private def cipher
+  private def cipher : Pointer(Void)
     LibCrypto.evp_cipher_ctx_cipher @ctx
   end
 end
diff --git a/src/openssl/digest.cr b/src/openssl/digest.cr
index 9d0e2523c178..2c7b0fb81c1f 100644
--- a/src/openssl/digest.cr
+++ b/src/openssl/digest.cr
@@ -18,7 +18,7 @@ module OpenSSL
     protected def initialize(@name : String, @ctx : LibCrypto::EVP_MD_CTX)
     end
 
-    private def new_evp_mt_ctx(name)
+    private def new_evp_mt_ctx(name : String) : Pointer(LibCrypto::EVP_MD_CTX_Struct)
       md = LibCrypto.evp_get_digestbyname(name)
       unless md
         raise UnsupportedError.new("Unsupported digest algorithm: #{name}")
@@ -34,7 +34,7 @@ module OpenSSL
       ctx
     end
 
-    def finalize
+    def finalize : Nil
       LibCrypto.evp_md_ctx_free(self)
     end
 
@@ -42,7 +42,7 @@ module OpenSSL
       Digest.new(@name, dup_ctx)
     end
 
-    protected def dup_ctx
+    protected def dup_ctx : Pointer(LibCrypto::EVP_MD_CTX_Struct)
       ctx = LibCrypto.evp_md_ctx_new
       if LibCrypto.evp_md_ctx_copy(ctx, @ctx) == 0
         LibCrypto.evp_md_ctx_free(ctx)
@@ -81,11 +81,11 @@ module OpenSSL
       LibCrypto.evp_md_block_size(to_unsafe_md).to_i
     end
 
-    def to_unsafe_md
+    def to_unsafe_md : LibCrypto::EVP_MD
       LibCrypto.evp_md_ctx_md(self)
     end
 
-    def to_unsafe
+    def to_unsafe : Pointer(LibCrypto::EVP_MD_CTX_Struct)
       @ctx
     end
   end
diff --git a/src/openssl/error.cr b/src/openssl/error.cr
index ff12c33812ab..fd5a9cf2db2f 100644
--- a/src/openssl/error.cr
+++ b/src/openssl/error.cr
@@ -5,7 +5,7 @@ module OpenSSL
   class Error < Exception
     getter! code : LibCrypto::ULong
 
-    def initialize(message = nil, fetched = false, cause : Exception? = nil)
+    def initialize(message : String? = nil, fetched : Bool = false, cause : Exception? = nil)
       @code ||= LibCrypto::ULong.new(0)
 
       if fetched
@@ -16,13 +16,13 @@ module OpenSSL
       end
     end
 
-    protected def fetch_error_details
+    protected def fetch_error_details : Tuple(UInt64, String)
       code = LibCrypto.err_get_error
       message = String.new(LibCrypto.err_error_string(code, nil)) unless code == 0
       {code, message || "Unknown or no error"}
     end
 
-    protected def get_reason(code)
+    protected def get_reason(code : UInt64) : Int32
       {% if LibCrypto.has_constant?(:ERR_REASON_MASK) %}
         if (code & LibCrypto::ERR_SYSTEM_FLAG) != 0
           (code & LibCrypto::ERR_SYSTEM_MASK).to_i
diff --git a/src/openssl/hmac.cr b/src/openssl/hmac.cr
index 187cbe40bd6e..eb51e46a0841 100644
--- a/src/openssl/hmac.cr
+++ b/src/openssl/hmac.cr
@@ -15,7 +15,7 @@ class OpenSSL::HMAC
   # It may contain non-ASCII bytes, including NUL bytes.
   #
   # *algorithm* specifies which `OpenSSL::Algorithm` is to be used.
-  def self.digest(algorithm : OpenSSL::Algorithm, key, data) : Bytes
+  def self.digest(algorithm : OpenSSL::Algorithm, key : String, data : String) : Bytes
     evp = algorithm.to_evp
     key_slice = key.to_slice
     data_slice = data.to_slice
@@ -29,7 +29,7 @@ class OpenSSL::HMAC
   # the digest where binary messages are not allowed.
   #
   # See also `#digest`.
-  def self.hexdigest(algorithm : OpenSSL::Algorithm, key, data) : String
+  def self.hexdigest(algorithm : OpenSSL::Algorithm, key : String, data : String) : String
     digest(algorithm, key, data).hexstring
   end
 end
diff --git a/src/openssl/pkcs5.cr b/src/openssl/pkcs5.cr
index d1bca87d2095..a432b1753f0d 100644
--- a/src/openssl/pkcs5.cr
+++ b/src/openssl/pkcs5.cr
@@ -2,7 +2,7 @@ require "openssl"
 require "openssl/algorithm"
 
 module OpenSSL::PKCS5
-  def self.pbkdf2_hmac_sha1(secret, salt, iterations = 2**16, key_size = 64) : Bytes
+  def self.pbkdf2_hmac_sha1(secret : String, salt : String, iterations : Int32 = 2**16, key_size : Int32 = 64) : Bytes
     buffer = Bytes.new(key_size)
     if LibCrypto.pkcs5_pbkdf2_hmac_sha1(secret, secret.bytesize, salt, salt.bytesize, iterations, key_size, buffer) != 1
       raise OpenSSL::Error.new "pkcs5_pbkdf2_hmac"
@@ -10,7 +10,7 @@ module OpenSSL::PKCS5
     buffer
   end
 
-  def self.pbkdf2_hmac(secret, salt, iterations = 2**16, algorithm : OpenSSL::Algorithm = OpenSSL::Algorithm::SHA1, key_size = 64) : Bytes
+  def self.pbkdf2_hmac(secret : String, salt : String, iterations : Int32 = 2**16, algorithm : OpenSSL::Algorithm = OpenSSL::Algorithm::SHA1, key_size : Int32 = 64) : Bytes
     {% if LibCrypto.has_method?(:pkcs5_pbkdf2_hmac) %}
       evp = algorithm.to_evp
       buffer = Bytes.new(key_size)
diff --git a/src/openssl/sha1.cr b/src/openssl/sha1.cr
index 54c45817739e..7e5e186ef231 100644
--- a/src/openssl/sha1.cr
+++ b/src/openssl/sha1.cr
@@ -7,11 +7,11 @@ require "./lib_crypto"
 # `Crypto::Bcrypt::Password`. For a generic cryptographic hash, use SHA-256 via
 # `OpenSSL::Digest.new("SHA256")`.
 class OpenSSL::SHA1
-  def self.hash(data : String)
+  def self.hash(data : String) : StaticArray(UInt8, 20)
     hash(data.to_unsafe, LibC::SizeT.new(data.bytesize))
   end
 
-  def self.hash(data : UInt8*, bytesize : LibC::SizeT)
+  def self.hash(data : UInt8*, bytesize : LibC::SizeT) : StaticArray(UInt8, 20)
     buffer = uninitialized UInt8[20]
     LibCrypto.sha1(data, bytesize, buffer)
     buffer
diff --git a/src/openssl/ssl/context.cr b/src/openssl/ssl/context.cr
index 38e0054cba17..ad4c61c8f53d 100644
--- a/src/openssl/ssl/context.cr
+++ b/src/openssl/ssl/context.cr
@@ -11,7 +11,7 @@ require "log"
 # appropriately.
 abstract class OpenSSL::SSL::Context
   # :nodoc:
-  def self.default_method
+  def self.default_method : LibSSL::SSLMethod
     {% if LibSSL.has_method?(:tls_method) %}
       LibSSL.tls_method
     {% else %}
@@ -73,7 +73,7 @@ abstract class OpenSSL::SSL::Context
     # * `ca`: Path to a file containing the CA certificate chain or a directory containing all CA certificates.
     #    See `#ca_certificates=` and `#ca_certificates_path=`, respectively.
     #    Required if `verify_mode` is `peer`, `force-peer` or empty.
-    def self.from_hash(params) : self
+    def self.from_hash(params : Hash(String, String)) : self
       super(params)
     end
 
@@ -98,7 +98,7 @@ abstract class OpenSSL::SSL::Context
       }, hostname.as(Void*))
     end
 
-    private def alpn_protocol=(protocol : Bytes)
+    private def alpn_protocol=(protocol : Bytes) : Int32
       {% if LibSSL.has_method?(:ssl_ctx_set_alpn_protos) %}
         LibSSL.ssl_ctx_set_alpn_protos(@handle, protocol, protocol.size)
       {% else %}
@@ -156,7 +156,7 @@ abstract class OpenSSL::SSL::Context
     # * `ca`: Path to a file containing the CA certificate chain or a directory containing all CA certificates.
     #    See `#ca_certificates=` and `#ca_certificates_path=`, respectively.
     #    Required if `verify_mode` is `peer` or `force-peer`.
-    def self.from_hash(params) : self
+    def self.from_hash(params : URI::Params | Hash(String, String)) : self
       super(params)
     end
 
@@ -174,7 +174,7 @@ abstract class OpenSSL::SSL::Context
       {% end %}
     end
 
-    private def alpn_protocol=(protocol : Bytes)
+    private def alpn_protocol=(protocol : Bytes) : Nil
       {% if LibSSL.has_method?(:ssl_ctx_set_alpn_select_cb) %}
         alpn_cb = ->(ssl : LibSSL::SSL, o : LibC::Char**, olen : LibC::Char*, i : LibC::Char*, ilen : LibC::Int, data : Void*) {
           proto = Box(Bytes).unbox(data)
@@ -228,7 +228,7 @@ abstract class OpenSSL::SSL::Context
   # versions public too. So to provide insecure in the child classes, we need
   # a second constructor that we call from there without getting the
   # overridden ones of the childs.
-  protected def _initialize_insecure(method : LibSSL::SSLMethod)
+  protected def _initialize_insecure(method : LibSSL::SSLMethod) : Nil
     @handle = LibSSL.ssl_ctx_new(method)
     raise OpenSSL::Error.new("SSL_CTX_new") if @handle.null?
 
@@ -239,25 +239,25 @@ abstract class OpenSSL::SSL::Context
     {% end %}
   end
 
-  protected def self.insecure(method : LibSSL::SSLMethod)
+  protected def self.insecure(method : LibSSL::SSLMethod) : OpenSSL::SSL::Context::Client | OpenSSL::SSL::Context::Server
     obj = allocate
     obj._initialize_insecure(method)
     GC.add_finalizer(obj)
     obj
   end
 
-  def finalize
+  def finalize : Nil
     LibSSL.ssl_ctx_free(@handle)
   end
 
   # Sets the default paths for `ca_certificates=` and `ca_certificates_path=`.
-  def set_default_verify_paths
+  def set_default_verify_paths : Int32
     LibSSL.ssl_ctx_set_default_verify_paths(@handle)
   end
 
   # Sets the path to a file containing all CA certificates, in PEM format, used to
   # validate the peers certificate.
-  def ca_certificates=(file_path : String)
+  def ca_certificates=(file_path : String) : Nil
     ret = LibSSL.ssl_ctx_load_verify_locations(@handle, file_path, nil)
     raise OpenSSL::Error.new("SSL_CTX_load_verify_locations") unless ret == 1
   end
@@ -265,21 +265,21 @@ abstract class OpenSSL::SSL::Context
   # Sets the path to a directory containing all CA certificates used to
   # validate the peers certificate. The certificates should be in PEM format
   # and the `c_rehash(1)` utility must have been run in the directory.
-  def ca_certificates_path=(dir_path : String)
+  def ca_certificates_path=(dir_path : String) : Nil
     ret = LibSSL.ssl_ctx_load_verify_locations(@handle, nil, dir_path)
     raise OpenSSL::Error.new("SSL_CTX_load_verify_locations") unless ret == 1
   end
 
   # Specify the path to the certificate chain file to use. In server mode this
   # is presented to the client, in client mode this used as client certificate.
-  def certificate_chain=(file_path : String)
+  def certificate_chain=(file_path : String) : Nil
     ret = LibSSL.ssl_ctx_use_certificate_chain_file(@handle, file_path)
     raise OpenSSL::Error.new("SSL_CTX_use_certificate_chain_file") unless ret == 1
   end
 
   # Specify the path to the private key to use. The key must in PEM format.
   # The key must correspond to the entity certificate set by `certificate_chain=`.
-  def private_key=(file_path : String)
+  def private_key=(file_path : String) : Nil
     ret = LibSSL.ssl_ctx_use_privatekey_file(@handle, file_path, LibSSL::SSLFileType::PEM)
     raise OpenSSL::Error.new("SSL_CTX_use_PrivateKey_file") unless ret == 1
   end
@@ -295,7 +295,7 @@ abstract class OpenSSL::SSL::Context
   #       linked version of the system SSL library. A comprehensive list
   #       of ciphers can be found in the
   #       [OpenSSL Cipher documentation](https://www.openssl.org/docs/man3.0/man1/openssl-ciphers.html#CIPHER-STRINGS).
-  def ciphers=(ciphers : String)
+  def ciphers=(ciphers : String) : String
     ret = LibSSL.ssl_ctx_set_cipher_list(@handle, ciphers)
     raise OpenSSL::Error.new("SSL_CTX_set_cipher_list") if ret == 0
     ciphers
@@ -309,7 +309,7 @@ abstract class OpenSSL::SSL::Context
   #       linked version of the system SSL library. A comprehensive list
   #       of ciphersuites can be found in the
   #       [OpenSSL Cipher documentation](https://www.openssl.org/docs/man3.0/man1/openssl-ciphers.html#TLS-v1.3-cipher-suites).
-  def cipher_suites=(cipher_suites : String)
+  def cipher_suites=(cipher_suites : String) : String
     {% if LibSSL.has_method?(:ssl_ctx_set_ciphersuites) %}
       ret = LibSSL.ssl_ctx_set_ciphersuites(@handle, cipher_suites)
       raise OpenSSL::Error.new("SSL_CTX_set_ciphersuites") if ret == 0
@@ -325,7 +325,7 @@ abstract class OpenSSL::SSL::Context
   # WARNING: Does nothing as of Crystal 1.13.
   # WARNING: Didn't work as expected as of OpenSSL 1.1 (didn't configure TLSv1.2 and below).
   @[Deprecated("Deprecated with no replacement. Prefer #security_level, global system configuration or build your own from https://wiki.mozilla.org/Security/Server_Side_TLS")]
-  def set_modern_ciphers
+  def set_modern_ciphers : Nil
   end
 
   # Sets the current ciphers and ciphers suites to **intermediate** compatibility level as per Mozilla
@@ -334,7 +334,7 @@ abstract class OpenSSL::SSL::Context
   # WARNING: Does nothing as of Crystal 1.13.
   # WARNING: Didn't work as expected as of OpenSSL 1.1 (didn't configure TLSv1.2 and below).
   @[Deprecated("Deprecated with no replacement. Prefer #security_level, global system configuration or build your own from https://wiki.mozilla.org/Security/Server_Side_TLS")]
-  def set_intermediate_ciphers
+  def set_intermediate_ciphers : Nil
   end
 
   # Sets the current ciphers and ciphers suites to **old** compatibility level as per Mozilla
@@ -343,7 +343,7 @@ abstract class OpenSSL::SSL::Context
   # WARNING: Does nothing as of Crystal 1.13.
   # WARNING: Didn't work as expected as of OpenSSL 1.1 (didn't configure TLSv1.2 and below).
   @[Deprecated("Deprecated with no replacement. Prefer #security_level, global system configuration or build your own from https://wiki.mozilla.org/Security/Server_Side_TLS")]
-  def set_old_ciphers
+  def set_old_ciphers : Nil
   end
 
   # Returns the security level used by this TLS context.
@@ -361,7 +361,7 @@ abstract class OpenSSL::SSL::Context
   #
   # * https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set_security_level.html
   # * https://wiki.debian.org/ContinuousIntegration/TriagingTips/openssl-1.1.1
-  def security_level=(value : Int32)
+  def security_level=(value : Int32) : Int32
     {% if LibSSL.has_method?(:ssl_ctx_set_security_level) %}
       LibSSL.ssl_ctx_set_security_level(@handle, value)
     {% else %}
@@ -372,7 +372,7 @@ abstract class OpenSSL::SSL::Context
 
   # Adds a temporary ECDH key curve to the TLS context. This is required to
   # enable the EECDH cipher suites. By default the prime256 curve will be used.
-  def set_tmp_ecdh_key(curve = LibCrypto::NID_X9_62_prime256v1) : Nil
+  def set_tmp_ecdh_key(curve : Int32 = LibCrypto::NID_X9_62_prime256v1) : Nil
     key = LibCrypto.ec_key_new_by_curve_name(curve)
     raise OpenSSL::Error.new("ec_key_new_by_curve_name") if key.null?
     LibSSL.ssl_ctx_ctrl(@handle, LibSSL::SSL_CTRL_SET_TMP_ECDH, 0, key)
@@ -385,12 +385,12 @@ abstract class OpenSSL::SSL::Context
   end
 
   # Adds modes to the TLS context.
-  def add_modes(mode : OpenSSL::SSL::Modes)
+  def add_modes(mode : OpenSSL::SSL::Modes) : LibSSL::Modes
     OpenSSL::SSL::Modes.new LibSSL.ssl_ctx_ctrl(@handle, LibSSL::SSL_CTRL_MODE, mode, nil)
   end
 
   # Removes modes from the TLS context.
-  def remove_modes(mode : OpenSSL::SSL::Modes)
+  def remove_modes(mode : OpenSSL::SSL::Modes) : LibSSL::Modes
     OpenSSL::SSL::Modes.new LibSSL.ssl_ctx_ctrl(@handle, LibSSL::SSL_CTRL_CLEAR_MODE, mode, nil)
   end
 
@@ -414,7 +414,7 @@ abstract class OpenSSL::SSL::Context
   #   OpenSSL::SSL::Options::NO_SSL_V3   # disable deprecated SSLv3
   # )
   # ```
-  def add_options(options : OpenSSL::SSL::Options)
+  def add_options(options : OpenSSL::SSL::Options) : LibSSL::Options
     opts = {% if LibSSL.has_method?(:ssl_ctx_set_options) %}
              LibSSL.ssl_ctx_set_options(@handle, options)
            {% else %}
@@ -429,7 +429,7 @@ abstract class OpenSSL::SSL::Context
   # ```
   # context.remove_options(OpenSSL::SSL::Options::NO_SSL_V3)
   # ```
-  def remove_options(options : OpenSSL::SSL::Options)
+  def remove_options(options : OpenSSL::SSL::Options) : LibSSL::Options
     opts = {% if LibSSL.has_method?(:ssl_ctx_clear_options) %}
              LibSSL.ssl_ctx_clear_options(@handle, options)
            {% else %}
@@ -444,7 +444,7 @@ abstract class OpenSSL::SSL::Context
   end
 
   # Sets the verify mode. See the `SSL_CTX_set_verify(3)` manpage for more details.
-  def verify_mode=(mode : OpenSSL::SSL::VerifyMode)
+  def verify_mode=(mode : OpenSSL::SSL::VerifyMode) : Nil
     LibSSL.ssl_ctx_set_verify(@handle, mode, nil)
   end
 
@@ -458,7 +458,7 @@ abstract class OpenSSL::SSL::Context
   # ```
   # context.alpn_protocol = "h2"
   # ```
-  def alpn_protocol=(protocol : String)
+  def alpn_protocol=(protocol : String) : Int32?
     proto = Bytes.new(protocol.bytesize + 1)
     proto[0] = protocol.bytesize.to_u8
     protocol.to_slice.copy_to(proto.to_unsafe + 1, protocol.bytesize)
@@ -492,11 +492,11 @@ abstract class OpenSSL::SSL::Context
     {% end %}
   end
 
-  def to_unsafe
+  def to_unsafe : LibSSL::SSLContext
     @handle
   end
 
-  private def self.from_hash(params)
+  private def self.from_hash(params : Hash(String, String) | URI::Params) : OpenSSL::SSL::Context::Client | OpenSSL::SSL::Context::Server
     context = new
     if key = params["key"]?
       context.private_key = key
diff --git a/src/openssl/ssl/hostname_validation.cr b/src/openssl/ssl/hostname_validation.cr
index 8c0a787e6541..190f130328ba 100644
--- a/src/openssl/ssl/hostname_validation.cr
+++ b/src/openssl/ssl/hostname_validation.cr
@@ -27,7 +27,7 @@ module OpenSSL::SSL::HostnameValidation
   # Matches hostname against Subject Alternate Name (SAN) entries of certificate.
   #
   # Adapted from https://wiki.openssl.org/index.php/Hostname_validation
-  def self.matches_subject_alternative_name(hostname, server_cert : LibCrypto::X509) : Result
+  def self.matches_subject_alternative_name(hostname : String, server_cert : LibCrypto::X509) : Result
     san_names = LibCrypto.x509_get_ext_d2i(server_cert, LibCrypto::NID_subject_alt_name, nil, nil)
     return Result::NoSANPresent if san_names.null?
 
@@ -78,7 +78,7 @@ module OpenSSL::SSL::HostnameValidation
   # called if no SAN entries could be found in certificate.
   #
   # Adapted from https://wiki.openssl.org/index.php/Hostname_validation
-  def self.matches_common_name(hostname, server_cert : LibCrypto::X509) : Result
+  def self.matches_common_name(hostname : String, server_cert : LibCrypto::X509) : Result
     subject = LibCrypto.x509_get_subject_name(server_cert)
 
     index = LibCrypto.x509_name_get_index_by_nid(subject, LibCrypto::NID_commonName, -1)
@@ -121,7 +121,7 @@ module OpenSSL::SSL::HostnameValidation
   # Adapted from cURL:
   # Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.
   # https://github.com/curl/curl/blob/curl-7_41_0/lib/hostcheck.c
-  def self.matches_hostname?(pattern, hostname) : Bool
+  def self.matches_hostname?(pattern : String, hostname : String) : Bool
     pattern = pattern.chomp('.').downcase
     hostname = hostname.chomp('.').downcase
 
diff --git a/src/openssl/ssl/server.cr b/src/openssl/ssl/server.cr
index 97132f8c4810..1784ac823f8b 100644
--- a/src/openssl/ssl/server.cr
+++ b/src/openssl/ssl/server.cr
@@ -75,7 +75,7 @@ class OpenSSL::SSL::Server
     end
   end
 
-  private def new_ssl_socket(io)
+  private def new_ssl_socket(io : IO) : OpenSSL::SSL::Socket::Server
     OpenSSL::SSL::Socket::Server.new(io, @context, sync_close: @sync_close, accept: @start_immediately)
   end
 
diff --git a/src/openssl/ssl/socket.cr b/src/openssl/ssl/socket.cr
index 8bff5a131410..42b2d5c4500d 100644
--- a/src/openssl/ssl/socket.cr
+++ b/src/openssl/ssl/socket.cr
@@ -1,6 +1,6 @@
 abstract class OpenSSL::SSL::Socket < IO
   class Client < Socket
-    def initialize(io, context : Context::Client = Context::Client.new, sync_close : Bool = false, hostname : String? = nil)
+    def initialize(io : TCPSocket, context : Context::Client = Context::Client.new, sync_close : Bool = false, hostname : String? = nil)
       super(io, context, sync_close)
       begin
         if hostname
@@ -56,7 +56,7 @@ abstract class OpenSSL::SSL::Socket < IO
   end
 
   class Server < Socket
-    def initialize(io, context : Context::Server = Context::Server.new,
+    def initialize(io : IO | TCPSocket, context : Context::Server = Context::Server.new,
                    sync_close : Bool = false, accept : Bool = true)
       super(io, context, sync_close)
 
@@ -97,7 +97,7 @@ abstract class OpenSSL::SSL::Socket < IO
 
   getter? closed : Bool
 
-  protected def initialize(io, context : Context, @sync_close : Bool = false)
+  protected def initialize(io : TCPSocket | IO, context : Context, @sync_close : Bool = false)
     @closed = false
 
     @ssl = LibSSL.ssl_new(context)
@@ -116,7 +116,7 @@ abstract class OpenSSL::SSL::Socket < IO
     LibSSL.ssl_set_bio(@ssl, @bio, @bio)
   end
 
-  def finalize
+  def finalize : Nil
     LibSSL.ssl_free(@ssl)
   end
 
@@ -157,7 +157,7 @@ abstract class OpenSSL::SSL::Socket < IO
 
   # Returns the negotiated ALPN protocol (eg: `"h2"`) of `nil` if no protocol was
   # negotiated.
-  def alpn_protocol
+  def alpn_protocol : String?
     {% if LibSSL.has_method?(:ssl_get0_alpn_selected) %}
       LibSSL.ssl_get0_alpn_selected(@ssl, out protocol, out len)
       String.new(protocol, len) unless protocol.null?
diff --git a/src/openssl/x509/certificate.cr b/src/openssl/x509/certificate.cr
index 593f3fb4dc4b..1d6fe9344bed 100644
--- a/src/openssl/x509/certificate.cr
+++ b/src/openssl/x509/certificate.cr
@@ -16,7 +16,7 @@ module OpenSSL::X509
       raise Error.new("X509_dup") if @cert.null?
     end
 
-    def finalize
+    def finalize : Nil
       LibCrypto.x509_free(@cert)
     end
 
@@ -24,7 +24,7 @@ module OpenSSL::X509
       self.class.new(@cert)
     end
 
-    def to_unsafe
+    def to_unsafe : LibCrypto::X509
       @cert
     end
 
@@ -50,11 +50,11 @@ module OpenSSL::X509
     # Sets the subject.
     #
     # Refer to `Name.parse` for the format.
-    def subject=(subject : String)
+    def subject=(subject : String) : OpenSSL::X509::Name
       self.subject = Name.parse(subject)
     end
 
-    def subject=(subject : Name)
+    def subject=(subject : Name) : OpenSSL::X509::Name
       ret = LibCrypto.x509_set_subject_name(@cert, subject)
       raise Error.new("X509_set_subject_name") if ret == 0
       subject
@@ -67,7 +67,7 @@ module OpenSSL::X509
       end
     end
 
-    def add_extension(extension : Extension)
+    def add_extension(extension : Extension) : OpenSSL::X509::Extension
       ret = LibCrypto.x509_add_ext(@cert, extension, -1)
       raise Error.new("X509_add_ext") if ret.null?
       extension
diff --git a/src/openssl/x509/extension.cr b/src/openssl/x509/extension.cr
index b326a35f8309..ddf841813444 100644
--- a/src/openssl/x509/extension.cr
+++ b/src/openssl/x509/extension.cr
@@ -4,14 +4,14 @@ require "openssl/lib_crypto"
 module OpenSSL::X509
   # :nodoc:
   class Extension
-    def self.new(oid : String, value : String, critical = false)
+    def self.new(oid : String, value : String, critical : Bool = false) : OpenSSL::X509::Extension
       nid = LibCrypto.obj_ln2nid(oid)
       nid = LibCrypto.obj_sn2nid(oid) if nid == LibCrypto::NID_undef
       raise Error.new("OBJ_sn2nid") if nid == LibCrypto::NID_undef
       new(nid, value, critical)
     end
 
-    def initialize(nid : Int32, value : String, critical = false)
+    def initialize(nid : Int32, value : String, critical : Bool = false)
       valstr = String.build do |str|
         str << "critical," if critical
         str << value
@@ -25,7 +25,7 @@ module OpenSSL::X509
       raise Error.new("X509_EXTENSION_dup") if @ext.null?
     end
 
-    def finalize
+    def finalize : Nil
       LibCrypto.x509_extension_free(@ext)
     end
 
@@ -33,7 +33,7 @@ module OpenSSL::X509
       self.class.new(@ext)
     end
 
-    def to_unsafe
+    def to_unsafe : LibCrypto::X509_EXTENSION
       @ext
     end
 
diff --git a/src/openssl/x509/name.cr b/src/openssl/x509/name.cr
index dacb04d67c2c..1582da47eb41 100644
--- a/src/openssl/x509/name.cr
+++ b/src/openssl/x509/name.cr
@@ -31,7 +31,7 @@ module OpenSSL::X509
       raise Error.new("X509_NAME_dup") if @name.null?
     end
 
-    def finalize
+    def finalize : Nil
       LibCrypto.x509_name_free(@name)
     end
 
@@ -39,7 +39,7 @@ module OpenSSL::X509
       self.class.new(@name)
     end
 
-    def to_unsafe
+    def to_unsafe : LibCrypto::X509_NAME
       @name
     end
 
diff --git a/src/option_parser.cr b/src/option_parser.cr
index 5f61e73cd58d..031c23d9ff94 100644
--- a/src/option_parser.cr
+++ b/src/option_parser.cr
@@ -85,13 +85,13 @@ class OptionParser
   end
 
   class InvalidOption < Exception
-    def initialize(option)
+    def initialize(option : String)
       super("Invalid option: #{option}")
     end
   end
 
   class MissingOption < Exception
-    def initialize(option)
+    def initialize(option : String)
       super("Missing option: #{option}")
     end
   end
@@ -207,7 +207,7 @@ class OptionParser
   # Examples of valid subcommands:
   #
   # * `foo`, `run`
-  def on(flag : String, description : String, &block : String ->)
+  def on(flag : String, description : String, &block : String ->) : OptionParser::Handler
     append_flag flag, description
 
     flag, value_type = parse_flag_definition(flag)
@@ -218,7 +218,7 @@ class OptionParser
   #
   # See the other definition of `on` for examples. This method does not support
   # subcommands.
-  def on(short_flag : String, long_flag : String, description : String, &block : String ->)
+  def on(short_flag : String, long_flag : String, description : String, &block : String ->) : OptionParser::Handler
     check_starts_with_dash short_flag, "short_flag", allow_empty: true
     check_starts_with_dash long_flag, "long_flag"
 
@@ -240,7 +240,7 @@ class OptionParser
     @handlers[short_flag] = @handlers[long_flag] = handler
   end
 
-  private def parse_flag_definition(flag : String)
+  private def parse_flag_definition(flag : String) : Tuple(String, OptionParser::FlagValue)
     case flag
     when /\A--(\S+)\s+\[\S+\]\z/
       {"--#{$1}", FlagValue::Optional}
@@ -264,7 +264,7 @@ class OptionParser
   # before, and the flags registered after the call.
   #
   # This way, you can group the different options in an easier to read way.
-  def separator(message = "") : Nil
+  def separator(message : String = "") : Nil
     @flags << message.to_s
   end
 
@@ -274,21 +274,21 @@ class OptionParser
   # that your program expects (for example, filenames). The default behaviour
   # is to do nothing. The arguments can also be extracted from the *args* array
   # passed to `#parse` after parsing.
-  def unknown_args(&@unknown_args : Array(String), Array(String) ->)
+  def unknown_args(&@unknown_args : Array(String), Array(String) ->) : Proc(Array(String), Array(String), Nil)
   end
 
   # Sets a handler for when a option that expects an argument wasn't given any.
   #
   # You typically use this to display a help message.
   # The default behaviour is to raise `MissingOption`.
-  def missing_option(&@missing_option : String ->)
+  def missing_option(&@missing_option : String ->) : Proc(String, Nil)
   end
 
   # Sets a handler for option arguments that didn't match any of the setup options.
   #
   # You typically use this to display a help message.
   # The default behaviour is to raise `InvalidOption`.
-  def invalid_option(&@invalid_option : String ->)
+  def invalid_option(&@invalid_option : String ->) : Proc(String, Nil)
   end
 
   # Sets a handler which runs before each argument is parsed. This callback is
@@ -297,7 +297,7 @@ class OptionParser
   #
   # You typically use this to implement advanced option parsing behaviour such
   # as treating all options after a filename differently (along with `#stop`).
-  def before_each(&@before_each : String ->)
+  def before_each(&@before_each : String ->) : Proc(String, Nil)
   end
 
   # Stops the current parse and returns immediately, leaving the remaining flags
@@ -316,7 +316,7 @@ class OptionParser
     @flags.join io, '\n'
   end
 
-  private def append_flag(flag, description)
+  private def append_flag(flag : String, description : String) : Array(String)
     indent = " " * 37
     description = description.gsub("\n", "\n#{indent}")
     if flag.size >= 33
@@ -326,7 +326,7 @@ class OptionParser
     end
   end
 
-  private def check_starts_with_dash(arg, name, allow_empty = false)
+  private def check_starts_with_dash(arg : String, name : String, allow_empty : Bool = false) : Nil
     return if allow_empty && arg.empty?
 
     unless arg.starts_with?('-')
@@ -358,7 +358,7 @@ class OptionParser
   end
 
   # Parses the passed *args* (defaults to `ARGV`), running the handlers associated to each option.
-  def parse(args = ARGV) : Nil
+  def parse(args : Array(String) = ARGV) : Nil
     with_preserved_state do
       # List of indexes in `args` which have been handled and must be deleted
       handled_args = [] of Int32
diff --git a/src/path.cr b/src/path.cr
index f9cd35efaeaf..f4e1e965766c 100644
--- a/src/path.cr
+++ b/src/path.cr
@@ -83,7 +83,7 @@ struct Path
   SEPARATORS = separators(Kind.native)
 
   # :nodoc:
-  def self.separators(kind)
+  def self.separators(kind : Path::Kind) : Tuple(Char) | Tuple(Char, Char)
     if kind.windows?
       {'\\', '/'}
     else
@@ -169,7 +169,7 @@ struct Path
   end
 
   # Internal helper method to create a new `Path` of the same kind as `self`.
-  private def new_instance(string : String, kind = @kind) : Path
+  private def new_instance(string : String, kind : Path::Kind = @kind) : Path
     Path.new(string, kind)
   end
 
@@ -556,7 +556,7 @@ struct Path
   end
 
   # :nodoc:
-  def self.next_part_separator_index(reader : Char::Reader, last_was_separator, separators)
+  def self.next_part_separator_index(reader : Char::Reader, last_was_separator : Bool, separators : Tuple(Char) | Tuple(Char, Char)) : Tuple(Char::Reader, Bool, Int32)?
     start_pos = reader.pos
 
     reader.each do |char|
@@ -596,7 +596,7 @@ struct Path
       @path.@name.byte_slice(start_pos, @reader.pos - start_pos)
     end
 
-    private def next_pos
+    private def next_pos : Int32?
       unless @anchor_processed
         @anchor_processed = true
         if anchor_pos = process_anchor
@@ -612,7 +612,7 @@ struct Path
       start_pos
     end
 
-    private def process_anchor
+    private def process_anchor : Int32?
       anchor = @path.anchor
       return unless anchor
 
@@ -629,7 +629,7 @@ struct Path
     end
   end
 
-  private def windows_drive?
+  private def windows_drive? : Bool
     @name.byte_at?(1) === ':' && @name.char_at(0).ascii_letter?
   end
 
@@ -714,7 +714,7 @@ struct Path
   # See `#to_windows` and `#to_posix` for details.
   #
   # * `#to_native` converts to the native path semantics.
-  def to_kind(kind, *, mappings : Bool = true) : Path
+  def to_kind(kind : Path::Kind, *, mappings : Bool = true) : Path
     if kind.posix?
       to_posix(mappings: mappings)
     else
@@ -740,7 +740,7 @@ struct Path
   # If *expand_base* is `true`, *base* itself will be expanded in `Dir.current`
   # if it is not an absolute path. This guarantees the method returns an absolute
   # path (assuming that `Dir.current` is absolute).
-  def expand(base : Path | String = Dir.current, *, home : Path | String | Bool = false, expand_base = true) : Path
+  def expand(base : Path | String = Dir.current, *, home : Path | String | Bool = false, expand_base : Bool = true) : Path
     base = Path.new(base) unless base.is_a?(Path)
     base = base.to_kind(@kind)
     if base == self
@@ -809,7 +809,7 @@ struct Path
     expanded.normalize(remove_final_separator: false)
   end
 
-  private def resolve_home(home)
+  private def resolve_home(home : Bool | String | Path) : Path
     case home
     when String then home = Path[home]
     when Bool   then home = Path.home
@@ -836,7 +836,7 @@ struct Path
   # Path["a/b/"].join("")  # => Path["a/b/"]
   # Path["a/b/"].join("c") # => Path["a/b/c"]
   # ```
-  def join(part) : Path
+  def join(part : String | Path) : Path
     # If we are joining a single part we can use `String.new` instead of
     # `String.build` which avoids an extra allocation.
     # Given that `File.join(arg1, arg2)` is the most common usage
@@ -954,7 +954,7 @@ struct Path
     end
   end
 
-  private def empty?
+  private def empty? : Bool
     @name.empty? || @name == "."
   end
 
@@ -1096,7 +1096,7 @@ struct Path
   # Path.posix("foo") == Path.posix("FOO")     # => false
   # Path.windows("foo") == Path.windows("FOO") # => true
   # ```
-  def ==(other : self)
+  def ==(other : self) : Bool
     return false if @kind != other.@kind
 
     @name.compare(other.@name, case_insensitive: windows? || other.windows?) == 0
@@ -1205,7 +1205,7 @@ struct Path
     end
   end
 
-  private def unc_share?
+  private def unc_share? : Tuple(Int32, Int32 | Nil)?
     # Test for UNC share
     # path: //share/share
     # part: 1122222 33333
@@ -1322,7 +1322,7 @@ struct Path
   end
 
   # :nodoc:
-  def separators
+  def separators : Tuple(Char) | Tuple(Char, Char)
     Path.separators(@kind)
   end
 
@@ -1330,11 +1330,11 @@ struct Path
     ends_with_separator?(@name)
   end
 
-  private def ends_with_separator?(name)
+  private def ends_with_separator?(name : String) : Bool
     separators.any? { |separator| name.ends_with?(separator) }
   end
 
-  private def starts_with_separator?(name = @name)
+  private def starts_with_separator?(name : String = @name) : Bool
     separators.any? { |separator| name.starts_with?(separator) }
   end
 
diff --git a/src/process/executable_path.cr b/src/process/executable_path.cr
index 1a7b0c44d55d..dbb15f5fc831 100644
--- a/src/process/executable_path.cr
+++ b/src/process/executable_path.cr
@@ -41,7 +41,7 @@ class Process
     end
   end
 
-  private def self.file_executable?(path)
+  private def self.file_executable?(path : Path) : Bool
     unless File.info?(path, follow_symlinks: true).try &.file?
       return false
     end
diff --git a/src/process/shell.cr b/src/process/shell.cr
index e206c6c19a5a..80a3c844cc0e 100644
--- a/src/process/shell.cr
+++ b/src/process/shell.cr
@@ -64,7 +64,7 @@ class Process
     String.build { |io| quote_windows(io, args) }
   end
 
-  private def self.quote_windows(io : IO, args)
+  private def self.quote_windows(io : IO, args : Tuple(String) | Array(String)) : Nil
     args.join(io, ' ') do |arg|
       need_quotes = arg.empty? || arg.includes?(' ') || arg.includes?('\t')
 
diff --git a/src/process/status.cr b/src/process/status.cr
index 32b26c735146..979106967651 100644
--- a/src/process/status.cr
+++ b/src/process/status.cr
@@ -95,7 +95,7 @@ enum Process::ExitReason
   # Returns `true` if the process exited abnormally.
   #
   # This includes all values except `Normal`.
-  def abnormal?
+  def abnormal? : Bool
     !normal?
   end
 
@@ -107,7 +107,7 @@ enum Process::ExitReason
   # ```
   #
   # `Status#description` provides more detail for a specific process status.
-  def description
+  def description : String
     case self
     in .normal?
       "Process exited normally"
@@ -309,7 +309,7 @@ class Process::Status
     exit_code? == 0
   end
 
-  private def signal_code
+  private def signal_code : Int32
     # define __WTERMSIG(status) ((status) & 0x7f)
     @exit_status & 0x7f
   end
@@ -412,7 +412,7 @@ class Process::Status
   # ```
   #
   # `ExitReason#description` provides the specific messages for non-signal exits.
-  def description
+  def description : String
     if exit_reason.signal? && (signal = exit_signal?)
       "Process received and didn't handle signal #{signal}"
     else
diff --git a/src/random/isaac.cr b/src/random/isaac.cr
index 294d439fb82d..c5c675fa8a36 100644
--- a/src/random/isaac.cr
+++ b/src/random/isaac.cr
@@ -12,11 +12,11 @@ class Random::ISAAC
   private getter bb
   private getter cc
 
-  private def random_seeds
+  private def random_seeds : StaticArray(UInt32, 8)
     Random::Secure.rand(StaticArray(UInt32, 8))
   end
 
-  def initialize(seeds = random_seeds)
+  def initialize(seeds : Array(Int32) | StaticArray(UInt32, 8) = random_seeds)
     @rsl = StaticArray(UInt32, 256).new { 0_u32 }
     @mm = StaticArray(UInt32, 256).new { 0_u32 }
     @counter = 0
@@ -37,7 +37,7 @@ class Random::ISAAC
     @rsl[counter]
   end
 
-  private def isaac
+  private def isaac : Nil
     @cc &+= 1
     @bb &+= cc
 
@@ -55,7 +55,7 @@ class Random::ISAAC
     end
   end
 
-  private def init_by_array(seeds)
+  private def init_by_array(seeds : Array(Int32) | StaticArray(UInt32, 8)) : Int32
     seed_len = seeds.size
     256.times { |i| @rsl[i] = i < seed_len ? seeds[i].to_u32 : 0_u32 }
 
diff --git a/src/random/pcg32.cr b/src/random/pcg32.cr
index bc34f736b282..d38d8f950581 100644
--- a/src/random/pcg32.cr
+++ b/src/random/pcg32.cr
@@ -39,11 +39,11 @@ class Random::PCG32
   @state : UInt64
   @inc : UInt64
 
-  def self.new
+  def self.new : Random::PCG32
     new(Random::Secure.rand(UInt64::MIN..UInt64::MAX), Random::Secure.rand(UInt64::MIN..UInt64::MAX))
   end
 
-  def initialize(initstate : UInt64, initseq = 0_u64)
+  def initialize(initstate : UInt64, initseq : UInt64 = 0_u64)
     # initialize to zeros to prevent compiler complains
     @state = 0_u64
     @inc = 0_u64
@@ -54,7 +54,7 @@ class Random::PCG32
     new_seed(Random::Secure.rand(UInt64::MIN..UInt64::MAX), Random::Secure.rand(UInt64::MIN..UInt64::MAX))
   end
 
-  def new_seed(initstate : UInt64, initseq = 0_u64) : UInt32
+  def new_seed(initstate : UInt64, initseq : UInt64 = 0_u64) : UInt32
     @state = 0_u64
     @inc = (initseq << 1) | 1
     next_u
@@ -70,7 +70,7 @@ class Random::PCG32
     UInt32.new!(xorshifted.rotate_right(rot))
   end
 
-  def jump(delta)
+  def jump(delta : Int32) : UInt64
     deltau64 = UInt64.new!(delta)
     acc_mult = 1u64
     acc_plus = 0u64
diff --git a/src/random/secure.cr b/src/random/secure.cr
index a6b9df03063f..36c0db191c08 100644
--- a/src/random/secure.cr
+++ b/src/random/secure.cr
@@ -19,7 +19,7 @@ module Random::Secure
   extend Random
   extend self
 
-  def next_u
+  def next_u : UInt8
     Crystal::System::Random.next_u
   end
 
diff --git a/src/reference.cr b/src/reference.cr
index 42bdcba2327a..eb035519959b 100644
--- a/src/reference.cr
+++ b/src/reference.cr
@@ -73,7 +73,7 @@ class Reference
   end
 
   # Returns `false`: a reference is never `nil`.
-  def same?(other : Nil)
+  def same?(other : Nil) : Bool
     false
   end
 
diff --git a/src/regex/lib_pcre2.cr b/src/regex/lib_pcre2.cr
index 6f45a4465219..65cb3b825f74 100644
--- a/src/regex/lib_pcre2.cr
+++ b/src/regex/lib_pcre2.cr
@@ -191,7 +191,7 @@ lib LibPCRE2
     INTERNAL_DUPMATCH = -65
     DFA_UINVALID_UTF  = -66
 
-    def utf8_validity?
+    def utf8_validity? : Bool
       in?(UTF8_ERR21..UTF8_ERR1)
     end
   end
diff --git a/src/regex/match_data.cr b/src/regex/match_data.cr
index 3fcbc5c424ac..141e788919ca 100644
--- a/src/regex/match_data.cr
+++ b/src/regex/match_data.cr
@@ -67,7 +67,7 @@ class Regex
     # "Crystal".match!(/r/).begin(1)     # IndexError: Invalid capture group index: 1
     # "Crystal".match!(/r(x)?/).begin(1) # IndexError: Capture group 1 was not matched
     # ```
-    def begin(n = 0) : Int32
+    def begin(n : Int32 = 0) : Int32
       @string.byte_index_to_char_index(byte_begin(n)).not_nil!
     end
 
@@ -86,7 +86,7 @@ class Regex
     # "Crystal".match!(/r/).end(1)     # IndexError: Invalid capture group index: 1
     # "Crystal".match!(/r(x)?/).end(1) # IndexError: Capture group 1 was not matched
     # ```
-    def end(n = 0) : Int32
+    def end(n : Int32 = 0) : Int32
       @string.byte_index_to_char_index(byte_end(n)).not_nil!
     end
 
@@ -105,7 +105,7 @@ class Regex
     # "Crystal".match!(/r/).byte_begin(1)     # IndexError: Invalid capture group index: 1
     # "Crystal".match!(/r(x)?/).byte_begin(1) # IndexError: Capture group 1 was not matched
     # ```
-    def byte_begin(n = 0) : Int32
+    def byte_begin(n : Int32 = 0) : Int32
       check_index_out_of_bounds n
       byte_range(n) { |normalized_n| raise_capture_group_was_not_matched(normalized_n) }.begin
     end
@@ -125,7 +125,7 @@ class Regex
     # "Crystal".match!(/r/).byte_end(1)     # IndexError: Invalid capture group index: 1
     # "Crystal".match!(/r(x)?/).byte_end(1) # IndexError: Capture group 1 was not matched
     # ```
-    def byte_end(n = 0) : Int32
+    def byte_end(n : Int32 = 0) : Int32
       check_index_out_of_bounds n
       byte_range(n) { |normalized_n| raise_capture_group_was_not_matched(normalized_n) }.end
     end
@@ -390,7 +390,7 @@ class Regex
       self
     end
 
-    def ==(other : Regex::MatchData)
+    def ==(other : Regex::MatchData) : Bool
       return false unless size == other.size
       return false unless regex == other.regex
       return false unless string == other.string
@@ -405,19 +405,19 @@ class Regex
       Slice.new(@ovector, size * 2).hash(hasher)
     end
 
-    private def check_index_out_of_bounds(index)
+    private def check_index_out_of_bounds(index : Int32) : Nil
       raise_invalid_group_index(index) unless valid_group?(index)
     end
 
-    private def valid_group?(index)
+    private def valid_group?(index : Int32) : Bool
       -size <= index < size
     end
 
-    private def raise_invalid_group_index(index)
+    private def raise_invalid_group_index(index : Int32) : Nil
       raise IndexError.new("Invalid capture group index: #{index}")
     end
 
-    private def raise_capture_group_was_not_matched(index)
+    private def raise_capture_group_was_not_matched(index : Int32) : Nil
       raise IndexError.new("Capture group #{index} was not matched")
     end
   end
diff --git a/src/regex/pcre2.cr b/src/regex/pcre2.cr
index b56a4ea68839..81adc7f33508 100644
--- a/src/regex/pcre2.cr
+++ b/src/regex/pcre2.cr
@@ -22,7 +22,7 @@ module Regex::PCRE2
   end
 
   # :nodoc:
-  def initialize(*, _source @source : String, _options @options)
+  def initialize(*, _source @source : String, _options @options : Regex::Options)
     options = pcre2_compile_options(options) | LibPCRE2::UTF | LibPCRE2::DUPNAMES | LibPCRE2::UCP
     @re = PCRE2.compile(source, options) do |error_message|
       raise ArgumentError.new(error_message)
@@ -54,14 +54,14 @@ module Regex::PCRE2
     end
   end
 
-  protected def self.get_error_message(errorcode)
+  protected def self.get_error_message(errorcode : Int32 | LibPCRE2::Error) : String
     String.new(256) do |buffer|
       bytesize = LibPCRE2.get_error_message(errorcode, buffer, 256)
       {bytesize, 0}
     end
   end
 
-  private def pcre2_compile_options(options)
+  private def pcre2_compile_options(options : Regex::Options) : Int32
     flag = 0
     Regex::CompileOptions.each do |option|
       if options.includes?(option)
@@ -92,11 +92,11 @@ module Regex::PCRE2
     flag
   end
 
-  def self.supports_compile_flag?(options)
+  def self.supports_compile_flag?(options : Regex::Options) : Bool
     true
   end
 
-  private def pcre2_match_options(options)
+  private def pcre2_match_options(options : Regex::Options) : Int32
     flag = 0
     Regex::Options.each do |option|
       if options.includes?(option)
@@ -125,7 +125,7 @@ module Regex::PCRE2
     flag
   end
 
-  private def pcre2_match_options(options : Regex::MatchOptions)
+  private def pcre2_match_options(options : Regex::MatchOptions) : Int32
     flag = 0
     Regex::MatchOptions.each do |option|
       if options.includes?(option)
@@ -146,11 +146,11 @@ module Regex::PCRE2
     flag
   end
 
-  def self.supports_match_flag?(options)
+  def self.supports_match_flag?(options : Regex::MatchOptions) : Bool
     true
   end
 
-  protected def self.error_impl(source)
+  protected def self.error_impl(source : String) : String?
     code = PCRE2.compile(source, LibPCRE2::UTF | LibPCRE2::DUPNAMES | LibPCRE2::UCP) do |error_message|
       return error_message
     end
@@ -160,20 +160,20 @@ module Regex::PCRE2
     nil
   end
 
-  private def pattern_info(what)
+  private def pattern_info(what : Int32) : UInt32
     value = uninitialized UInt32
     pattern_info(what, pointerof(value))
     value
   end
 
-  private def pattern_info(what, where)
+  private def pattern_info(what : Int32, where : Pointer(Pointer(UInt8)) | Pointer(UInt32)) : Nil
     ret = LibPCRE2.pattern_info(@re, what, where)
     if ret != 0
       raise "Error pattern_info #{what}: #{ret}"
     end
   end
 
-  private def name_table_impl
+  private def name_table_impl : Hash(Int32, String)
     lookup = Hash(Int32, String).new
 
     each_named_capture_group do |capture_number, name_entry|
@@ -200,11 +200,11 @@ module Regex::PCRE2
     end
   end
 
-  private def capture_count_impl
+  private def capture_count_impl : Int32
     pattern_info(LibPCRE2::INFO_CAPTURECOUNT).to_i32
   end
 
-  private def match_impl(str, byte_index, options)
+  private def match_impl(str : String, byte_index : Int32, options : Regex::MatchOptions | Regex::Options) : Regex::MatchData?
     match_data = match_data(str, byte_index, options) || return
 
     ovector_count = LibPCRE2.get_ovector_count(match_data)
@@ -218,7 +218,7 @@ module Regex::PCRE2
     ::Regex::MatchData.new(self, @re, str, byte_index, ovector.to_unsafe, ovector_count.to_i32 &- 1)
   end
 
-  private def matches_impl(str, byte_index, options)
+  private def matches_impl(str : String, byte_index : Int32, options : Regex::MatchOptions | Regex::Options) : Bool
     if match_data = match_data(str, byte_index, options)
       true
     else
@@ -238,7 +238,7 @@ module Regex::PCRE2
   # can't be any concurrent access to the JIT stack.
   @@jit_stack = Crystal::ThreadLocalValue(LibPCRE2::JITStack*).new
 
-  def self.jit_stack
+  def self.jit_stack : Pointer(LibPCRE2::JITStack)
     @@jit_stack.get do
       LibPCRE2.jit_stack_create(32_768, 1_048_576, nil) || raise "Error allocating JIT stack"
     end
@@ -250,20 +250,20 @@ module Regex::PCRE2
   # This buffer is heap-allocated and should be re-used for subsequent matches.
   @match_data = Crystal::ThreadLocalValue(LibPCRE2::MatchData*).new
 
-  private def match_data
+  private def match_data : Pointer(LibPCRE2::MatchData)
     @match_data.get do
       LibPCRE2.match_data_create_from_pattern(@re, nil)
     end
   end
 
-  def finalize
+  def finalize : Nil
     @match_data.consume_each do |match_data|
       LibPCRE2.match_data_free(match_data)
     end
     LibPCRE2.code_free @re
   end
 
-  private def match_data(str, byte_index, options)
+  private def match_data(str : String, byte_index : Int32, options : Regex::MatchOptions | Regex::Options) : Pointer(LibPCRE2::MatchData)?
     match_data = self.match_data
     match_count = LibPCRE2.match(@re, str, str.bytesize, byte_index, pcre2_match_options(options), match_data, PCRE2.match_context)
 
diff --git a/src/semantic_version.cr b/src/semantic_version.cr
index 476360b32a0e..5ea060e16bb6 100644
--- a/src/semantic_version.cr
+++ b/src/semantic_version.cr
@@ -92,7 +92,7 @@ struct SemanticVersion
   # current_version.copy_with(patch: 2)        # => SemanticVersion(@build=nil, @major=1, @minor=1, @patch=2, @prerelease=SemanticVersion::Prerelease(@identifiers=["rc"]))
   # current_version.copy_with(prerelease: nil) # => SemanticVersion(@build=nil, @major=1, @minor=1, @patch=1, @prerelease=SemanticVersion::Prerelease(@identifiers=[]))
   # ```
-  def copy_with(major : Int32 = @major, minor : Int32 = @minor, patch : Int32 = @patch, prerelease : String | Prerelease | Nil = @prerelease, build : String? = @build)
+  def copy_with(major : Int32 = @major, minor : Int32 = @minor, patch : Int32 = @patch, prerelease : String | Prerelease | Nil = @prerelease, build : String? = @build) : SemanticVersion
     SemanticVersion.new major, minor, patch, prerelease, build
   end
 
@@ -104,7 +104,7 @@ struct SemanticVersion
   # current_version = SemanticVersion.new 1, 1, 1, "rc"
   # current_version.bump_major # => SemanticVersion(@build=nil, @major=2, @minor=0, @patch=0, @prerelease=SemanticVersion::Prerelease(@identifiers=[]))
   # ```
-  def bump_major
+  def bump_major : SemanticVersion
     copy_with(major: major + 1, minor: 0, patch: 0, prerelease: nil, build: nil)
   end
 
@@ -116,7 +116,7 @@ struct SemanticVersion
   # current_version = SemanticVersion.new 1, 1, 1, "rc"
   # current_version.bump_minor # => SemanticVersion(@build=nil, @major=1, @minor=2, @patch=0, @prerelease=SemanticVersion::Prerelease(@identifiers=[]))
   # ```
-  def bump_minor
+  def bump_minor : SemanticVersion
     copy_with(minor: minor + 1, patch: 0, prerelease: nil, build: nil)
   end
 
@@ -130,7 +130,7 @@ struct SemanticVersion
   # next_patch = current_version.bump_patch # => SemanticVersion(@build=nil, @major=1, @minor=1, @patch=1, @prerelease=SemanticVersion::Prerelease(@identifiers=[]))
   # next_patch.bump_patch                   # => SemanticVersion(@build=nil, @major=1, @minor=1, @patch=2, @prerelease=SemanticVersion::Prerelease(@identifiers=[]))
   # ```
-  def bump_patch
+  def bump_patch : SemanticVersion
     if prerelease.identifiers.empty?
       copy_with(patch: patch + 1, prerelease: nil, build: nil)
     else
@@ -245,19 +245,19 @@ struct SemanticVersion
       0
     end
 
-    private def compare(x : Int32, y : String)
+    private def compare(x : Int32, y : String) : Int32
       -1
     end
 
-    private def compare(x : String, y : Int32)
+    private def compare(x : String, y : Int32) : Int32
       1
     end
 
-    private def compare(x : Int32, y : Int32)
+    private def compare(x : Int32, y : Int32) : Int32
       x <=> y
     end
 
-    private def compare(x : String, y : String)
+    private def compare(x : String, y : String) : Int32
       x <=> y
     end
   end
diff --git a/src/signal.cr b/src/signal.cr
index 37999c76b9e1..7093abcbd11e 100644
--- a/src/signal.cr
+++ b/src/signal.cr
@@ -128,7 +128,7 @@ enum Signal : Int32
   #   # ...
   # end
   # ```
-  def trap_handler?
+  def trap_handler? : Proc(Signal, Nil)?
     {% if @type.has_constant?("CHLD") %}
       return Crystal::System::Signal.child_handler if self == CHLD
     {% end %}
diff --git a/src/slice.cr b/src/slice.cr
index 266d7bb31249..791e78b18893 100644
--- a/src/slice.cr
+++ b/src/slice.cr
@@ -67,7 +67,7 @@ struct Slice(T)
   #
   # String.new(slice) # => "abc"
   # ```
-  def initialize(@pointer : Pointer(T), size : Int, *, @read_only = false)
+  def initialize(@pointer : Pointer(T), size : Int, *, @read_only : Bool = false)
     @size = size.to_i32
   end
 
@@ -118,7 +118,7 @@ struct Slice(T)
   # slice = Slice.new(3, 10)
   # slice # => Slice[10, 10, 10]
   # ```
-  def self.new(size : Int, value : T, *, read_only = false)
+  def self.new(size : Int, value : T, *, read_only : Bool = false) : Slice(Int32) | Slice(UInt8)
     new(size, read_only: read_only) { value }
   end
 
diff --git a/src/socket/address.cr b/src/socket/address.cr
index c07505ad43ab..aaa0190bacf0 100644
--- a/src/socket/address.cr
+++ b/src/socket/address.cr
@@ -8,7 +8,7 @@ class Socket
 
     # Returns either an `IPAddress` or `UNIXAddress` from the internal OS
     # representation. Only INET, INET6 and UNIX families are supported.
-    def self.from(sockaddr : LibC::Sockaddr*, addrlen) : Address
+    def self.from(sockaddr : LibC::Sockaddr*, addrlen : UInt32) : Address
       case family = Family.new(sockaddr.value.sa_family)
       when Family::INET6
         IPAddress.new(sockaddr.as(LibC::SockaddrIn6*), addrlen.to_i)
@@ -106,7 +106,7 @@ class Socket
     # Socket::IPAddress.new("127.0.0.1", 8080)                 # => Socket::IPAddress(127.0.0.1:8080)
     # Socket::IPAddress.new("fe80::2ab2:bdff:fe59:8e2c", 1234) # => Socket::IPAddress([fe80::2ab2:bdff:fe59:8e2c]:1234)
     # ```
-    def self.new(address : String, port : Int32)
+    def self.new(address : String, port : Int32) : Socket::IPAddress
       raise Error.new("Invalid port number: #{port}") unless IPAddress.valid_port?(port)
 
       if v4_fields = parse_v4_fields?(address)
@@ -122,7 +122,7 @@ class Socket
 
     # Creates an `IPAddress` from the internal OS representation. Supports both
     # INET and INET6 families.
-    def self.from(sockaddr : LibC::Sockaddr*, addrlen) : IPAddress
+    def self.from(sockaddr : LibC::Sockaddr*, addrlen : UInt32 | Int32) : IPAddress
       case family = Family.new(sockaddr.value.sa_family)
       when Family::INET6
         new(sockaddr.as(LibC::SockaddrIn6*), addrlen.to_i)
@@ -201,7 +201,7 @@ class Socket
       parse_v4_fields?(str.to_slice)
     end
 
-    private def self.parse_v4_fields?(bytes : Bytes)
+    private def self.parse_v4_fields?(bytes : Bytes) : StaticArray(UInt8, 4)?
       # port of https://git.musl-libc.org/cgit/musl/tree/src/network/inet_pton.c?id=7e13e5ae69a243b90b90d2f4b79b2a150f806335
       fields = StaticArray(UInt8, 4).new(0)
       ptr = bytes.to_unsafe
@@ -252,7 +252,7 @@ class Socket
       parse_v6_fields?(str.to_slice)
     end
 
-    private def self.parse_v6_fields?(bytes : Bytes)
+    private def self.parse_v6_fields?(bytes : Bytes) : StaticArray(UInt16, 8)?
       # port of https://git.musl-libc.org/cgit/musl/tree/src/network/inet_pton.c?id=7e13e5ae69a243b90b90d2f4b79b2a150f806335
       ptr = bytes.to_unsafe
       finish = ptr + bytes.size
@@ -323,7 +323,7 @@ class Socket
       fields
     end
 
-    private def self.from_hex(ch : UInt8)
+    private def self.from_hex(ch : UInt8) : UInt8
       if 0x30 <= ch <= 0x39
         ch &- 0x30
       elsif 0x41 <= ch <= 0x46
@@ -360,7 +360,7 @@ class Socket
       v4(fields, port)
     end
 
-    private def self.to_v4_field(field)
+    private def self.to_v4_field(field : Int32) : UInt8
       0 <= field <= 0xff ? field.to_u8! : raise Error.new("Invalid IPv4 field: #{field}")
     end
 
@@ -385,7 +385,7 @@ class Socket
       v6(fields, port)
     end
 
-    private def self.to_v6_field(field)
+    private def self.to_v6_field(field : Int32) : UInt16
       0 <= field <= 0xffff ? field.to_u16! : raise Error.new("Invalid IPv6 field: #{field}")
     end
 
@@ -409,7 +409,7 @@ class Socket
       v4_mapped_v6(v4_fields, port)
     end
 
-    private def self.ipv6_from_addr16(bytes : UInt16[8])
+    private def self.ipv6_from_addr16(bytes : UInt16[8]) : LibC::In6Addr
       addr = LibC::In6Addr.new
       {% if flag?(:darwin) || flag?(:bsd) %}
         addr.__u6_addr.__u6_addr16 = bytes
@@ -432,13 +432,13 @@ class Socket
       addr
     end
 
-    protected def initialize(sockaddr : LibC::SockaddrIn6*, @size)
+    protected def initialize(sockaddr : LibC::SockaddrIn6*, @size : Int32)
       @family = Family::INET6
       @addr = sockaddr.value.sin6_addr
       @port = IPAddress.endian_swap(sockaddr.value.sin6_port).to_i
     end
 
-    protected def initialize(sockaddr : LibC::SockaddrIn*, @size)
+    protected def initialize(sockaddr : LibC::SockaddrIn*, @size : Int32)
       @family = Family::INET
       @addr = sockaddr.value.sin_addr
       @port = IPAddress.endian_swap(sockaddr.value.sin_port).to_i
@@ -548,7 +548,7 @@ class Socket
     #
     # IPv4 addresses in `169.254.0.0/16` reserved by [RFC 3927](https://www.rfc-editor.org/rfc/rfc3927) and Link-Local IPv6
     # Unicast Addresses in `fe80::/10` reserved by [RFC 4291](https://tools.ietf.org/html/rfc4291) are considered link-local.
-    def link_local?
+    def link_local? : Bool
       case addr = @addr
       in LibC::InAddr
         addr.s_addr & 0x000000ffff_u32 == 0x0000fea9_u32 # 169.254.0.0/16
@@ -557,7 +557,7 @@ class Socket
       end
     end
 
-    private def ipv6_addr8(addr : LibC::In6Addr)
+    private def ipv6_addr8(addr : LibC::In6Addr) : StaticArray(UInt8, 16)
       {% if flag?(:darwin) || flag?(:bsd) %}
         addr.__u6_addr.__u6_addr8
       {% elsif flag?(:linux) && flag?(:musl) %}
@@ -577,7 +577,7 @@ class Socket
 
     def_equals_and_hash family, port, address_value
 
-    protected def address_value
+    protected def address_value : UInt128 | UInt32
       case addr = @addr
       in LibC::InAddr
         addr.s_addr
@@ -609,14 +609,14 @@ class Socket
       end
     end
 
-    private def address_to_s(io : IO, addr : LibC::InAddr)
+    private def address_to_s(io : IO, addr : LibC::InAddr) : String::Builder | IO::Memory
       io << (addr.s_addr & 0xFF)
       io << '.' << (addr.s_addr >> 8 & 0xFF)
       io << '.' << (addr.s_addr >> 16 & 0xFF)
       io << '.' << (addr.s_addr >> 24)
     end
 
-    private def address_to_s(io : IO, addr : LibC::In6Addr)
+    private def address_to_s(io : IO, addr : LibC::In6Addr) : Nil | String::Builder | IO::Memory
       bytes = ipv6_addr8(addr)
       if Slice.new(bytes.to_unsafe, 10).all?(&.zero?) && bytes[10] == 0xFF && bytes[11] == 0xFF
         io << "::ffff:" << bytes[12] << '.' << bytes[13] << '.' << bytes[14] << '.' << bytes[15]
@@ -712,7 +712,7 @@ class Socket
       end
     end
 
-    private def to_sockaddr_in6(addr)
+    private def to_sockaddr_in6(addr : LibC::In6Addr) : Pointer(LibC::Sockaddr)
       sockaddr = Pointer(LibC::SockaddrIn6).malloc
       sockaddr.value.sin6_family = family
       sockaddr.value.sin6_port = IPAddress.endian_swap(port.to_u16!)
@@ -720,7 +720,7 @@ class Socket
       sockaddr.as(LibC::Sockaddr*)
     end
 
-    private def to_sockaddr_in(addr)
+    private def to_sockaddr_in(addr : LibC::InAddr) : Pointer(LibC::Sockaddr)
       sockaddr = Pointer(LibC::SockaddrIn).malloc
       sockaddr.value.sin_family = family
       sockaddr.value.sin_port = IPAddress.endian_swap(port.to_u16!)
@@ -779,7 +779,7 @@ class Socket
     end
 
     # Creates an `UNIXSocket` from the internal OS representation.
-    def self.from(sockaddr : LibC::Sockaddr*, addrlen) : UNIXAddress
+    def self.from(sockaddr : LibC::Sockaddr*, addrlen : Int32) : UNIXAddress
       {% if flag?(:wasm32) %}
         raise NotImplementedError.new "Socket::UNIXAddress.from"
       {% else %}
@@ -865,7 +865,7 @@ class Socket
 
   # Returns `true` if the string represents a valid IPv4 or IPv6 address.
   @[Deprecated("Use `IPAddress.valid?` instead")]
-  def self.ip?(string : String)
+  def self.ip?(string : String) : Bool
     IPAddress.valid?(string)
   end
 end
diff --git a/src/socket/addrinfo.cr b/src/socket/addrinfo.cr
index 411c09143411..62b11d237b5d 100644
--- a/src/socket/addrinfo.cr
+++ b/src/socket/addrinfo.cr
@@ -46,7 +46,7 @@ class Socket
     #
     # addrinfos = Socket::Addrinfo.resolve("example.org", "http", type: Socket::Type::STREAM, protocol: Socket::Protocol::TCP)
     # ```
-    def self.resolve(domain : String, service, family : Family? = nil, type : Type = nil, protocol : Protocol = Protocol::IP, timeout = nil) : Array(Addrinfo)
+    def self.resolve(domain : String, service : Int32, family : Family? = nil, type : Type = nil, protocol : Protocol = Protocol::IP, timeout : Nil = nil) : Array(Addrinfo)
       addrinfos = [] of Addrinfo
 
       getaddrinfo(domain, service, family, type, protocol, timeout) do |addrinfo|
@@ -102,7 +102,7 @@ class Socket
       end
 
       @[Deprecated("Use `.from_os_error` instead")]
-      def self.new(error_code : Int32, message, domain)
+      def self.new(error_code : Int32, message : String, domain : String) : Socket::Addrinfo::Error
         from_os_error(message, Errno.new(error_code), domain: domain, type: nil, service: nil, protocol: nil)
       end
 
@@ -111,19 +111,19 @@ class Socket
         new error_code, nil, domain: domain
       end
 
-      protected def self.new_from_os_error(message : String?, os_error, *, domain, type, service, protocol, **opts)
+      protected def self.new_from_os_error(message : String?, os_error : Errno, *, domain : String, type : Socket::Type?, service : Int32?, protocol : Socket::Protocol?, **opts) : Socket::Addrinfo::Error
         new(message, **opts)
       end
 
-      protected def self.new_from_os_error(message : String?, os_error, *, domain, **opts)
+      protected def self.new_from_os_error(message : String?, os_error : Errno, *, domain : String, **opts) : Socket::Addrinfo::Error
         new(message, **opts)
       end
 
-      def self.build_message(message, *, domain, **opts)
+      def self.build_message(message : String?, *, domain : String, **opts) : String
         "Hostname lookup for #{domain} failed"
       end
 
-      def self.os_error_message(os_error : Errno | WinError, *, type, service, protocol, **opts)
+      def self.os_error_message(os_error : Errno | WinError, *, type : Socket::Type?, service : Int32?, protocol : Socket::Protocol?, **opts) : String
         # when `EAI_NONAME` etc. is an integer then only `os_error.value` can
         # match; when `EAI_NONAME` is a `WinError` then `os_error` itself can
         # match
@@ -168,7 +168,7 @@ class Socket
     #
     # addrinfos = Socket::Addrinfo.tcp("example.org", 80)
     # ```
-    def self.tcp(domain : String, service, family = Family::UNSPEC, timeout = nil) : Array(Addrinfo)
+    def self.tcp(domain : String, service : Int32, family : Socket::Family = Family::UNSPEC, timeout : Nil = nil) : Array(Addrinfo)
       resolve(domain, service, family, Type::STREAM, Protocol::TCP, timeout)
     end
 
@@ -187,7 +187,7 @@ class Socket
     #
     # addrinfos = Socket::Addrinfo.udp("example.org", 53)
     # ```
-    def self.udp(domain : String, service, family = Family::UNSPEC, timeout = nil) : Array(Addrinfo)
+    def self.udp(domain : String, service : Int32, family : Socket::Family = Family::UNSPEC, timeout : Nil = nil) : Array(Addrinfo)
       resolve(domain, service, family, Type::DGRAM, Protocol::UDP, timeout)
     end
 
diff --git a/src/socket/common.cr b/src/socket/common.cr
index 19f1700a2cbd..8d937c4104f3 100644
--- a/src/socket/common.cr
+++ b/src/socket/common.cr
@@ -50,7 +50,7 @@ class Socket < IO
   end
 
   class Error < IO::Error
-    private def self.new_from_os_error(message, os_error, **opts)
+    private def self.new_from_os_error(message : String?, os_error : Errno | WasiError | WinError | Nil, **opts) : Socket::Error
       case os_error
       when Errno::ECONNREFUSED
         Socket::ConnectError.new(message, **opts)
diff --git a/src/socket/ip_socket.cr b/src/socket/ip_socket.cr
index 918b4d2b8591..a961fc64931a 100644
--- a/src/socket/ip_socket.cr
+++ b/src/socket/ip_socket.cr
@@ -20,7 +20,7 @@ class IPSocket < Socket
     @remote_address = nil
   end
 
-  def bind(addr)
+  def bind(addr : Socket::IPAddress | Int32) : Nil
     super(addr)
   ensure
     @local_address = nil
diff --git a/src/socket/tcp_server.cr b/src/socket/tcp_server.cr
index c5cf3e1fcef0..a4ca0b5eb388 100644
--- a/src/socket/tcp_server.cr
+++ b/src/socket/tcp_server.cr
@@ -33,7 +33,7 @@ class TCPServer < TCPSocket
   end
 
   # Binds a socket to the *host* and *port* combination.
-  def initialize(host : String, port : Int, backlog : Int = SOMAXCONN, dns_timeout = nil, reuse_port : Bool = false)
+  def initialize(host : String, port : Int, backlog : Int = SOMAXCONN, dns_timeout : Nil = nil, reuse_port : Bool = false)
     Addrinfo.tcp(host, port, timeout: dns_timeout) do |addrinfo|
       super(addrinfo.family, addrinfo.type, addrinfo.protocol)
 
diff --git a/src/socket/tcp_socket.cr b/src/socket/tcp_socket.cr
index 4edcb3d08e5f..9425b16c5cbc 100644
--- a/src/socket/tcp_socket.cr
+++ b/src/socket/tcp_socket.cr
@@ -15,7 +15,7 @@ require "./ip_socket"
 # ```
 class TCPSocket < IPSocket
   # Creates a new `TCPSocket`, waiting to be connected.
-  def self.new(family : Family = Family::INET, blocking = false)
+  def self.new(family : Family = Family::INET, blocking : Bool = false) : TCPSocket
     super(family, Type::STREAM, Protocol::TCP, blocking)
   end
 
@@ -26,7 +26,7 @@ class TCPSocket < IPSocket
   # must be in seconds (integers or floats).
   #
   # NOTE: *dns_timeout* is currently only supported on Windows.
-  def initialize(host : String, port, dns_timeout = nil, connect_timeout = nil, blocking = false)
+  def initialize(host : String, port : Int32, dns_timeout : Time::Span? = nil, connect_timeout : Time::Span? = nil, blocking : Bool = false)
     Addrinfo.tcp(host, port, timeout: dns_timeout) do |addrinfo|
       super(addrinfo.family, addrinfo.type, addrinfo.protocol, blocking)
       connect(addrinfo, timeout: connect_timeout) do |error|
@@ -36,11 +36,11 @@ class TCPSocket < IPSocket
     end
   end
 
-  protected def initialize(family : Family, type : Type, protocol : Protocol = Protocol::IP, blocking = false)
+  protected def initialize(family : Family, type : Type, protocol : Protocol = Protocol::IP, blocking : Bool = false)
     super family, type, protocol, blocking
   end
 
-  protected def initialize(fd : Handle, family : Family, type : Type, protocol : Protocol = Protocol::IP, blocking = false)
+  protected def initialize(fd : Handle, family : Family, type : Type, protocol : Protocol = Protocol::IP, blocking : Bool = false)
     super fd, family, type, protocol, blocking
   end
 
@@ -68,35 +68,35 @@ class TCPSocket < IPSocket
   end
 
   # Disables the Nagle algorithm when set to `true`, otherwise enables it.
-  def tcp_nodelay=(val : Bool)
+  def tcp_nodelay=(val : Bool) : Bool
     setsockopt_bool LibC::TCP_NODELAY, val, level: Protocol::TCP
   end
 
   # The amount of time in seconds the connection must be idle before sending keepalive probes.
-  def tcp_keepalive_idle
+  def tcp_keepalive_idle : Int32
     system_tcp_keepalive_idle
   end
 
-  def tcp_keepalive_idle=(val : Int)
+  def tcp_keepalive_idle=(val : Int) : Int32
     self.system_tcp_keepalive_idle = val
   end
 
   # The amount of time in seconds between keepalive probes.
-  def tcp_keepalive_interval
+  def tcp_keepalive_interval : Int32
     system_tcp_keepalive_interval
   end
 
-  def tcp_keepalive_interval=(val : Int)
+  def tcp_keepalive_interval=(val : Int) : Int32
     self.system_tcp_keepalive_interval = val
     val
   end
 
   # The number of probes sent, without response before dropping the connection.
-  def tcp_keepalive_count
+  def tcp_keepalive_count : Int32
     system_tcp_keepalive_count
   end
 
-  def tcp_keepalive_count=(val : Int)
+  def tcp_keepalive_count=(val : Int) : Int32
     self.system_tcp_keepalive_count = val
   end
 end
diff --git a/src/socket/udp_socket.cr b/src/socket/udp_socket.cr
index bba9d1aea39a..782e7fe74cfc 100644
--- a/src/socket/udp_socket.cr
+++ b/src/socket/udp_socket.cr
@@ -107,7 +107,7 @@ class UDPSocket < IPSocket
 
   # Sets whether transmitted multicast packets should be copied and sent back
   # to the originator, if the host has joined the multicast group.
-  def multicast_loopback=(val : Bool)
+  def multicast_loopback=(val : Bool) : Bool
     case @family
     when Family::INET
       setsockopt_bool LibC::IP_MULTICAST_LOOP, val, LibC::IPPROTO_IP
@@ -142,7 +142,7 @@ class UDPSocket < IPSocket
   # Multicast datagrams with a `hoplimit` of `0` will not be transmitted on any
   # network, but may be delivered locally if the sending host belongs to the
   # destination group and multicast loopback is enabled.
-  def multicast_hops=(val : Int)
+  def multicast_hops=(val : Int) : Int32
     case @family
     when Family::INET
       setsockopt LibC::IP_MULTICAST_TTL, val, LibC::IPPROTO_IP
@@ -159,7 +159,7 @@ class UDPSocket < IPSocket
   # IPv4 interface address for subsequent transmissions. Setting the interface
   # to `0.0.0.0` will select the default interface.
   # Raises `Socket::Error` unless the socket is IPv4 and an IPv4 address is provided.
-  def multicast_interface(address : IPAddress)
+  def multicast_interface(address : IPAddress) : Int32
     if @family == Family::INET
       addr = address.@addr
       if addr.is_a?(LibC::InAddr)
@@ -176,7 +176,7 @@ class UDPSocket < IPSocket
   # from the primary network interface. This function overrides the default
   # IPv6 interface for subsequent transmissions. Setting the interface to
   # index `0` will select the default interface.
-  def multicast_interface(index : UInt32)
+  def multicast_interface(index : UInt32) : Int32
     if @family == Family::INET6
       setsockopt LibC::IPV6_MULTICAST_IF, index, LibC::IPPROTO_IPV6
     else
@@ -187,7 +187,7 @@ class UDPSocket < IPSocket
   # A host must become a member of a multicast group before it can receive
   # datagrams sent to the group.
   # Raises `Socket::Error` if an incompatible address is provided.
-  def join_group(address : IPAddress)
+  def join_group(address : IPAddress) : Int32
     case @family
     when Family::INET
       group_modify(address, LibC::IP_ADD_MEMBERSHIP)
@@ -201,7 +201,7 @@ class UDPSocket < IPSocket
   # Drops membership to the specified group. Memberships are automatically
   # dropped when the socket is closed or the process exits.
   # Raises `Socket::Error` if an incompatible address is provided.
-  def leave_group(address : IPAddress)
+  def leave_group(address : IPAddress) : Int32
     case @family
     when Family::INET
       group_modify(address, LibC::IP_DROP_MEMBERSHIP)
@@ -212,7 +212,7 @@ class UDPSocket < IPSocket
     end
   end
 
-  private def group_modify(ip, operation)
+  private def group_modify(ip : Socket::IPAddress, operation : Int32) : Int32
     ip_addr = ip.@addr
 
     case @family
diff --git a/src/spec.cr b/src/spec.cr
index 474aa7c5a0dc..0c87a71de7cc 100644
--- a/src/spec.cr
+++ b/src/spec.cr
@@ -96,7 +96,7 @@ module Spec
     # :nodoc:
     #
     # Implement formatter configuration.
-    def configure_formatter(formatter, output_path = nil)
+    def configure_formatter(formatter : String, output_path : String? = nil) : Array(Spec::Formatter) | Spec::DotFormatter | Spec::VerboseFormatter | Spec::JUnitFormatter | Spec::TAPFormatter | Nil
       case formatter
       when "junit"
         junit_formatter = Spec::JUnitFormatter.file(Path.new(output_path.not_nil!))
@@ -108,7 +108,7 @@ module Spec
       end
     end
 
-    def main(args)
+    def main(args : Array(String)) : Nil
       Colorize.on_tty_only!
 
       begin
diff --git a/src/steppable.cr b/src/steppable.cr
index b7301b7e1414..c8b6a7e97233 100644
--- a/src/steppable.cr
+++ b/src/steppable.cr
@@ -70,7 +70,7 @@ module Steppable
   end
 
   # :ditto:
-  def step(*, to limit = nil, by step, exclusive : Bool = false)
+  def step(*, to limit : Char | Int8 | Nil | Int32 | UInt8 | Int16 | Float64 | Time | BigInt | Time::Span = nil, by step : Int32 | Float64 | Time::Span, exclusive : Bool = false) : Steppable::StepIterator(Char, Char, Int32) | Steppable::StepIterator(Int8, Int8, Int32) | Steppable::StepIterator(Int8, Nil, Int32) | Steppable::StepIterator(UInt8, Int32, Int32) | Steppable::StepIterator(UInt8, UInt8, Int32) | Steppable::StepIterator(Int16, Int16, Int32) | Steppable::StepIterator(Int32, Int32, Int32) | Steppable::StepIterator(Float64, Int32, Float64) | Steppable::StepIterator(Int32, Float64, Int32) | Steppable::StepIterator(Int32, Nil, Int32) | Steppable::StepIterator(Float64, Float64, Int32) | Steppable::StepIterator(Float64, Float64, Float64) | Steppable::StepIterator(Float64, Int32, Int32) | Steppable::StepIterator(Time, Time, Time::Span) | Steppable::StepIterator(BigInt, BigInt, Int32) | Steppable::StepIterator(Time::Span, Time::Span, Time::Span)
     raise ArgumentError.new("Zero step size") if step.zero? && limit != self
 
     StepIterator.new(self + (step - step), limit, step, exclusive: exclusive)
diff --git a/src/string_pool.cr b/src/string_pool.cr
index a4f2553a8d2e..546a5669a4b1 100644
--- a/src/string_pool.cr
+++ b/src/string_pool.cr
@@ -46,7 +46,7 @@ class StringPool
   # pool = StringPool.new(256)
   # pool.size # => 0
   # ```
-  def initialize(initial_capacity = 8)
+  def initialize(initial_capacity : Int32 = 8)
     @capacity = initial_capacity >= 8 ? Math.pw2ceil(initial_capacity) : 8
     @hashes = Pointer(UInt64).malloc(@capacity, 0_u64)
     @values = Pointer(String).malloc(@capacity, "")
@@ -116,7 +116,7 @@ class StringPool
   # pool.get("hey".to_unsafe, 3)
   # pool.size # => 1
   # ```
-  def get(str : UInt8*, len) : String
+  def get(str : UInt8*, len : Int32) : String
     hash = hash(str, len)
     get(hash, str, len) do |index|
       rehash if @size >= @capacity // 4 * 3
@@ -141,7 +141,7 @@ class StringPool
   # pool.empty?                   # => false
   # pool.get?("hey".to_unsafe, 3) # => "hey"
   # ```
-  def get?(str : UInt8*, len) : String?
+  def get?(str : UInt8*, len : Int32) : String?
     hash = hash(str, len)
     get(hash, str, len) { nil }
   end
@@ -163,7 +163,7 @@ class StringPool
     yield index
   end
 
-  private def put_on_rehash(hash : UInt64, entry : String)
+  private def put_on_rehash(hash : UInt64, entry : String) : String
     mask = (@capacity - 1).to_u64
     index = hash & mask
     next_probe_offset = 1_u64
@@ -272,7 +272,7 @@ class StringPool
     end
   end
 
-  private def hash(str, len)
+  private def hash(str : Pointer(UInt8), len : Int32) : UInt64
     hasher = Crystal::Hasher.new
     hasher = str.to_slice(len).hash(hasher)
     # hash should be non-zero, so `or` it with high bit
diff --git a/src/string_scanner.cr b/src/string_scanner.cr
index 5e0654bb60b2..7d78833ec3e2 100644
--- a/src/string_scanner.cr
+++ b/src/string_scanner.cr
@@ -68,7 +68,7 @@ class StringScanner
   end
 
   # Sets the *position* of the scan offset.
-  def offset=(position : Int)
+  def offset=(position : Int) : Int32
     raise IndexError.new unless position >= 0
     @byte_offset = @str.char_index_to_byte_index(position) || @str.bytesize
   end
@@ -134,7 +134,7 @@ class StringScanner
     match(pattern, advance: true, anchored: false)
   end
 
-  private def match(pattern : Regex, advance = true, options = Regex::MatchOptions::ANCHORED)
+  private def match(pattern : Regex, advance : Bool = true, options : Regex::MatchOptions = Regex::MatchOptions::ANCHORED) : String?
     match = pattern.match_at_byte_index(@str, @byte_offset, options)
     @last_match = match
     if match
@@ -148,7 +148,7 @@ class StringScanner
     end
   end
 
-  private def match(pattern : String | Char, advance = true, anchored = true)
+  private def match(pattern : String | Char, advance : Bool = true, anchored : Bool = true) : String?
     @last_match = nil
     if pattern.bytesize > @str.bytesize - @byte_offset
       nil
@@ -295,7 +295,7 @@ class StringScanner
   # s["month"]    # => "Dec"
   # s["day"]      # => "12"
   # ```
-  def [](n) : String
+  def [](n : Int32 | String) : String
     @last_match.not_nil![n]
   end
 
@@ -321,7 +321,7 @@ class StringScanner
   # s.scan(/more/) # => nil
   # s[0]?          # => nil
   # ```
-  def []?(n) : String?
+  def []?(n : Int32 | String) : String?
     @last_match.try(&.[n]?)
   end
 
@@ -340,13 +340,13 @@ class StringScanner
   end
 
   # Resets the scan offset to the beginning and clears the last match.
-  def reset
+  def reset : Int32
     @last_match = nil
     @byte_offset = 0
   end
 
   # Moves the scan offset to the end of the string and clears the last match.
-  def terminate
+  def terminate : Int32
     @last_match = nil
     @byte_offset = @str.bytesize
   end
@@ -358,7 +358,7 @@ class StringScanner
 
   # Extracts a string corresponding to string[offset,*len*], without advancing
   # the scan offset.
-  def peek(len) : String
+  def peek(len : Int32) : String
     @str[offset, len]
   end
 
diff --git a/src/symbol.cr b/src/symbol.cr
index 88b9d0db904e..6a6e40b066bb 100644
--- a/src/symbol.cr
+++ b/src/symbol.cr
@@ -62,7 +62,7 @@ struct Symbol
   # Symbol.needs_quotes? "string"      # => false
   # Symbol.needs_quotes? "long string" # => true
   # ```
-  def self.needs_quotes?(string) : Bool
+  def self.needs_quotes?(string : String) : Bool
     case string
     when "+", "-", "*", "&+", "&-", "&*", "/", "//", "==", "<", "<=", ">", ">=", "!", "!=", "=~", "!~"
       false
@@ -78,7 +78,7 @@ struct Symbol
   # :nodoc:
   # Determines if a string needs to be quoted to be used for an external
   # parameter name or a named argument's key.
-  def self.needs_quotes_for_named_argument?(string) : Bool
+  def self.needs_quotes_for_named_argument?(string : String) : Bool
     case string
     when "", "_"
       true
@@ -119,7 +119,7 @@ struct Symbol
     end
   end
 
-  def clone
+  def clone : Symbol
     self
   end
 end
diff --git a/src/system/group.cr b/src/system/group.cr
index 47b9768cca52..f7a38829b665 100644
--- a/src/system/group.cr
+++ b/src/system/group.cr
@@ -59,7 +59,7 @@ class System::Group
     Crystal::System::Group.from_id?(id)
   end
 
-  def to_s(io)
+  def to_s(io : String::Builder) : String::Builder
     io << name << " (" << id << ')'
   end
 end
diff --git a/src/system/user.cr b/src/system/user.cr
index 01c8d11d9e1c..2af3b8592a35 100644
--- a/src/system/user.cr
+++ b/src/system/user.cr
@@ -82,7 +82,7 @@ class System::User
     Crystal::System::User.from_id?(id)
   end
 
-  def to_s(io)
+  def to_s(io : String::Builder) : String::Builder
     io << username << " (" << id << ')'
   end
 end
diff --git a/src/time.cr b/src/time.cr
index 4b29114dd190..2fb19e59db7c 100644
--- a/src/time.cr
+++ b/src/time.cr
@@ -714,7 +714,7 @@ struct Time
   # If the resulting date-time is ambiguous due to time zone transitions,
   # a correct time will be returned, but it does not guarantee which.
   def shift(*, years : Int = 0, months : Int = 0, weeks : Int = 0, days : Int = 0,
-            hours : Int = 0, minutes : Int = 0, seconds : Int = 0, nanoseconds : Int = 0)
+            hours : Int = 0, minutes : Int = 0, seconds : Int = 0, nanoseconds : Int = 0) : Time
     seconds = seconds.to_i64
 
     # Skip the entire month-based calculations if year and month are zero
@@ -1122,7 +1122,7 @@ struct Time
   #
   # Number of seconds decimals can be selected with *fraction_digits*.
   # Values accepted are 0 (the default, no decimals), 3 (milliseconds), 6 (microseconds) or 9 (nanoseconds).
-  def to_rfc3339(*, fraction_digits : Int = 0)
+  def to_rfc3339(*, fraction_digits : Int = 0) : String
     Format::RFC_3339.format(to_utc, fraction_digits)
   end
 
@@ -1174,7 +1174,7 @@ struct Time
   # into the given *io*.
   #
   # This is also compatible to [RFC 882](https://tools.ietf.org/html/rfc882) and [RFC 1123](https://tools.ietf.org/html/rfc1123#page-55).
-  def to_rfc2822(io : IO)
+  def to_rfc2822(io : IO) : String::Builder | IO::Memory
     Format::RFC_2822.format(to_utc, io)
   end
 
@@ -1473,7 +1473,7 @@ struct Time
   # The valid range for *year* is `1..9999` and for *month* `1..12`. The value
   # of *day*  is not validated and can exceed the number of days in the specified
   # month or even a year.
-  protected def self.absolute_days(year, month, day) : Int32
+  protected def self.absolute_days(year : Int32, month : Int32, day : Int32) : Int32
     days_per_month = leap_year?(year) ? DAYS_MONTH_LEAP : DAYS_MONTH
 
     days_in_year = day - 1
@@ -1488,11 +1488,11 @@ struct Time
     year * 365 + year // 4 - year // 100 + year // 400 + days_in_year
   end
 
-  protected def total_seconds
+  protected def total_seconds : Int64
     @seconds
   end
 
-  protected def offset_seconds
+  protected def offset_seconds : Int64
     @seconds + offset
   end
 
@@ -1545,7 +1545,7 @@ struct Time
     {year, month, day, ordinal_day_in_year}
   end
 
-  protected def self.zone_offset_at(seconds, location)
+  protected def self.zone_offset_at(seconds : Int64, location : Time::Location) : Int32
     unix = seconds - UNIX_EPOCH.total_seconds
     zone, range = location.lookup_with_boundaries(unix)
 
diff --git a/src/time/format.cr b/src/time/format.cr
index 6f09a1c90229..825a09e63e0d 100644
--- a/src/time/format.cr
+++ b/src/time/format.cr
@@ -80,7 +80,7 @@ struct Time::Format
   end
 
   # Parses a string into a `Time`.
-  def parse(string, location = @location) : Time
+  def parse(string : String, location : Time::Location? = @location) : Time
     parser = Parser.new(string)
     parser.visit(pattern)
     parser.time(location)
diff --git a/src/time/format/custom/http_date.cr b/src/time/format/custom/http_date.cr
index 25847b21aa00..e01b01a758e8 100644
--- a/src/time/format/custom/http_date.cr
+++ b/src/time/format/custom/http_date.cr
@@ -19,7 +19,7 @@ struct Time::Format
   # ```
   module HTTP_DATE
     # Parses a string into a `Time`.
-    def self.parse(string, location = Time::Location::UTC) : Time
+    def self.parse(string : String, location : Time::Location = Time::Location::UTC) : Time
       parser = Parser.new(string)
       parser.http_date
       parser.time(location)
@@ -28,7 +28,7 @@ struct Time::Format
     # Formats a `Time` into the given *io*.
     #
     # *time* is always converted to UTC.
-    def self.format(time : Time, io : IO)
+    def self.format(time : Time, io : IO) : String::Builder
       formatter = Formatter.new(time.to_utc, io)
       formatter.rfc_2822(time_zone_gmt: true, two_digit_day: true)
       io
diff --git a/src/time/format/custom/iso_8601.cr b/src/time/format/custom/iso_8601.cr
index b6e634c7a4de..36af3808c9c9 100644
--- a/src/time/format/custom/iso_8601.cr
+++ b/src/time/format/custom/iso_8601.cr
@@ -123,14 +123,14 @@ struct Time::Format
   # ```
   module ISO_8601_DATE
     # Parses a string into a `Time`.
-    def self.parse(string, location : Time::Location? = Time::Location::UTC) : Time
+    def self.parse(string : String, location : Time::Location? = Time::Location::UTC) : Time
       parser = Parser.new(string)
       parser.year_month_day_iso_8601
       parser.time(location)
     end
 
     # Formats a `Time` into the given *io*.
-    def self.format(time : Time, io : IO)
+    def self.format(time : Time, io : IO) : String::Builder
       formatter = Formatter.new(time, io)
       formatter.year_month_day_iso_8601
       io
@@ -152,14 +152,14 @@ struct Time::Format
   # ```
   module ISO_8601_DATE_TIME
     # Parses a string into a `Time`.
-    def self.parse(string, location : Time::Location? = Time::Location::UTC) : Time
+    def self.parse(string : String, location : Time::Location? = Time::Location::UTC) : Time
       parser = Parser.new(string)
       parser.date_time_iso_8601
       parser.time(location)
     end
 
     # Formats a `Time` into the given *io*.
-    def self.format(time : Time, io : IO)
+    def self.format(time : Time, io : IO) : String::Builder
       formatter = Formatter.new(time, io)
       formatter.rfc_3339
       io
diff --git a/src/time/format/custom/rfc_2822.cr b/src/time/format/custom/rfc_2822.cr
index 3397cce8e471..6c436b642984 100644
--- a/src/time/format/custom/rfc_2822.cr
+++ b/src/time/format/custom/rfc_2822.cr
@@ -11,14 +11,14 @@ struct Time::Format
   # ```
   module RFC_2822
     # Parses a string into a `Time`.
-    def self.parse(string, kind = Time::Location::UTC) : Time
+    def self.parse(string : String, kind : Time::Location = Time::Location::UTC) : Time
       parser = Parser.new(string)
       parser.rfc_2822
       parser.time(kind)
     end
 
     # Formats a `Time` into the given *io*.
-    def self.format(time : Time, io : IO)
+    def self.format(time : Time, io : IO) : String::Builder | IO::Memory
       formatter = Formatter.new(time, io)
       formatter.rfc_2822
       io
@@ -33,7 +33,7 @@ struct Time::Format
   end
 
   module Pattern
-    def rfc_2822(time_zone_gmt = false, two_digit_day = false)
+    def rfc_2822(time_zone_gmt : Bool = false, two_digit_day : Bool = false) : Bool?
       cfws?
       short_day_name_with_comma?
       if two_digit_day
@@ -70,7 +70,7 @@ struct Time::Format
   end
 
   struct Parser
-    def short_day_name_with_comma?
+    def short_day_name_with_comma? : Bool?
       return unless current_char.ascii_letter?
 
       short_day_name
@@ -79,7 +79,7 @@ struct Time::Format
       cfws
     end
 
-    def seconds_with_colon?
+    def seconds_with_colon? : Int32?
       if current_char == ':'
         next_char
         cfws?
@@ -88,7 +88,7 @@ struct Time::Format
     end
 
     # comment or folding whitespace
-    def cfws?
+    def cfws? : Bool
       in_comment = false
       seen_whitespace = false
       loop do
@@ -108,29 +108,29 @@ struct Time::Format
       seen_whitespace
     end
 
-    def cfws
+    def cfws : Bool
       cfws? || raise "Invalid format"
     end
 
-    def folding_white_space
+    def folding_white_space : Char?
       skip_space
     end
   end
 
   struct Formatter
-    def seconds_with_colon?
+    def seconds_with_colon? : Nil
       char ':'
       second
     end
 
-    def cfws?
+    def cfws? : Nil
     end
 
-    def cfws
+    def cfws : IO
       folding_white_space
     end
 
-    def folding_white_space
+    def folding_white_space : IO
       io << ' '
     end
   end
diff --git a/src/time/format/custom/yaml_date.cr b/src/time/format/custom/yaml_date.cr
index ea5e081ecb1d..6e8606db9792 100644
--- a/src/time/format/custom/yaml_date.cr
+++ b/src/time/format/custom/yaml_date.cr
@@ -11,7 +11,7 @@ struct Time::Format
   # custom time parser, probably for this same reason.
   module YAML_DATE
     # Parses a string into a `Time`.
-    def self.parse?(string) : Time?
+    def self.parse?(string : String) : Time?
       parser = Parser.new(string)
       if parser.yaml_date_time?
         parser.time(Time::Location::UTC) rescue nil
@@ -21,7 +21,7 @@ struct Time::Format
     end
 
     # Formats a `Time` into the given *io*.
-    def self.format(time : Time, io : IO)
+    def self.format(time : Time, io : IO) : Nil
       formatter = Formatter.new(time, io)
 
       if time.hour == 0 && time.minute == 0 && time.second == 0 && time.nanosecond == 0
@@ -40,7 +40,7 @@ struct Time::Format
   end
 
   struct Parser
-    def yaml_date_time?
+    def yaml_date_time? : Bool
       if (year = consume_number?(4)) && char?('-')
         @year = year
       else
@@ -78,7 +78,7 @@ struct Time::Format
       true
     end
 
-    def yaml_time?
+    def yaml_time? : Bool
       if (hour = consume_number?(2)) && char?(':')
         @hour = hour
       else
@@ -116,7 +116,7 @@ struct Time::Format
   end
 
   struct Formatter
-    def yaml_date_time
+    def yaml_date_time : Nil
       year_month_day
       char ' '
       twenty_four_hour_time_with_seconds
diff --git a/src/time/format/parser.cr b/src/time/format/parser.cr
index 95bac1a5799d..02765aba429d 100644
--- a/src/time/format/parser.cr
+++ b/src/time/format/parser.cr
@@ -24,7 +24,7 @@ struct Time::Format
     @day_of_week : Time::DayOfWeek?
     @day_of_year : Int32?
 
-    def initialize(string)
+    def initialize(string : String)
       @reader = Char::Reader.new(string)
       @year = 1
       @month = 1
@@ -89,11 +89,11 @@ struct Time::Format
       time
     end
 
-    def year
+    def year : Int32
       @year = consume_number(4)
     end
 
-    def year_modulo_100
+    def year_modulo_100 : Int32
       year = consume_number(2)
       if 69 <= year <= 99
         @year = year + 1900
@@ -104,11 +104,11 @@ struct Time::Format
       end
     end
 
-    def year_divided_by_100
+    def year_divided_by_100 : Int32
       @year = consume_number(2) * 100
     end
 
-    def full_or_short_year
+    def full_or_short_year : Int32
       @year = case year = consume_number(4)
               when 0..49
                 year + 2000
@@ -119,27 +119,27 @@ struct Time::Format
               end
     end
 
-    def calendar_week_year
+    def calendar_week_year : Int32
       @calendar_week_year = consume_number(4)
     end
 
-    def calendar_week_year_modulo100
+    def calendar_week_year_modulo100 : Int32
       @calendar_week_year = consume_number(2)
     end
 
-    def month
+    def month : Int32
       @month = consume_number(2)
     end
 
-    def month_zero_padded
+    def month_zero_padded : Int32
       month
     end
 
-    def month_blank_padded
+    def month_blank_padded : Int32
       @month = consume_number_blank_padded(2)
     end
 
-    def month_name
+    def month_name : Int32
       string = consume_string
       if string.size < 3
         raise "Invalid month"
@@ -154,11 +154,11 @@ struct Time::Format
       end
     end
 
-    def month_name_upcase
+    def month_name_upcase : Int32
       month_name
     end
 
-    def short_month_name
+    def short_month_name : Int32
       string = consume_string
       if string.size != 3
         raise "Invalid month"
@@ -173,27 +173,27 @@ struct Time::Format
       end
     end
 
-    def short_month_name_upcase
+    def short_month_name_upcase : Int32
       month_name
     end
 
-    def calendar_week_week
+    def calendar_week_week : Int32
       @calendar_week_week = consume_number(2)
     end
 
-    def day_of_month
+    def day_of_month : Int32
       @day = consume_number(2)
     end
 
-    def day_of_month_zero_padded
+    def day_of_month_zero_padded : Int32
       @day = consume_number(2)
     end
 
-    def day_of_month_blank_padded
+    def day_of_month_blank_padded : Int32
       @day = consume_number_blank_padded(2)
     end
 
-    def day_name
+    def day_name : Nil
       string = consume_string
       if string.size < 3
         raise "Invalid day name"
@@ -206,15 +206,15 @@ struct Time::Format
       end
     end
 
-    def day_name_upcase
+    def day_name_upcase : Nil
       day_name
     end
 
-    def short_day_name
+    def short_day_name : Nil
       day_name
     end
 
-    def short_day_name_upcase
+    def short_day_name_upcase : Nil
       day_name
     end
 
@@ -226,51 +226,51 @@ struct Time::Format
       whitespace
     end
 
-    def day_of_year_zero_padded
+    def day_of_year_zero_padded : Int32
       @day_of_year = consume_number(3)
     end
 
-    def hour_24_zero_padded
+    def hour_24_zero_padded : Int32
       @hour_is_12 = false
       @hour = consume_number(2)
     end
 
-    def hour_24_blank_padded
+    def hour_24_blank_padded : Int32
       @hour_is_12 = false
       @hour = consume_number_blank_padded(2)
     end
 
-    def hour_12_zero_padded
+    def hour_12_zero_padded : Bool
       hour_24_zero_padded
       @hour_is_12 = true
     end
 
-    def hour_12_blank_padded
+    def hour_12_blank_padded : Int32
       @hour_is_12 = true
       @hour = consume_number_blank_padded(2)
     end
 
-    def minute
+    def minute : Int32
       @minute = consume_number(2)
     end
 
-    def second
+    def second : Int32
       @second = consume_number(2)
     end
 
-    def milliseconds
+    def milliseconds : Int32
       second_decimals 3
     end
 
-    def microseconds
+    def microseconds : Int32
       second_decimals 6
     end
 
-    def nanoseconds
+    def nanoseconds : Int32
       second_decimals 9
     end
 
-    def second_fraction
+    def second_fraction : Nil
       second_decimals 9
       # consume trailing numbers
       while current_char.ascii_number?
@@ -278,7 +278,7 @@ struct Time::Format
       end
     end
 
-    private def second_decimals(precision)
+    private def second_decimals(precision : Int32) : Int32
       pos = @reader.pos
       # Consume at most *precision* digits as i64
       decimals = consume_number_i64(precision)
@@ -290,14 +290,14 @@ struct Time::Format
       @nanosecond = (decimals * 10 ** (precision_shift + nanoseconds_shift)).to_i
     end
 
-    def second_fraction?(fraction_digits = nil)
+    def second_fraction?(fraction_digits : Int32? = nil) : Int32?
       if current_char == '.'
         next_char
         nanoseconds
       end
     end
 
-    def am_pm
+    def am_pm : Bool
       string = consume_string
       case string.downcase
       when "am"
@@ -309,19 +309,19 @@ struct Time::Format
       end
     end
 
-    def am_pm_upcase
+    def am_pm_upcase : Bool
       am_pm
     end
 
-    def day_of_week_monday_1_7
+    def day_of_week_monday_1_7 : Time::DayOfWeek
       @day_of_week = Time::DayOfWeek.from_value(consume_number(1))
     end
 
-    def day_of_week_sunday_0_6
+    def day_of_week_sunday_0_6 : Time::DayOfWeek
       @day_of_week = Time::DayOfWeek.from_value(consume_number(1))
     end
 
-    def unix_seconds
+    def unix_seconds : Int64
       negative = false
       case current_char
       when '-'
@@ -336,7 +336,7 @@ struct Time::Format
       @unix_seconds = consume_number_i64(19) * (negative ? -1 : 1)
     end
 
-    def time_zone(with_seconds = false)
+    def time_zone(with_seconds : Bool = false) : Char | Time::Location
       case current_char
       when 'Z'
         time_zone_z
@@ -354,7 +354,7 @@ struct Time::Format
       end
     end
 
-    def time_zone_z_or_offset(**options)
+    def time_zone_z_or_offset(**options) : Char | Time::Location
       case current_char
       when 'Z', 'z'
         time_zone_z
@@ -365,14 +365,14 @@ struct Time::Format
       end
     end
 
-    def time_zone_z
+    def time_zone_z : Char
       raise "Invalid timezone" unless current_char.in?('Z', 'z')
 
       @location = Location::UTC
       next_char
     end
 
-    def time_zone_offset(force_colon = false, allow_colon = true, format_seconds = false, parse_seconds = true, force_zero_padding = true, force_minutes = true)
+    def time_zone_offset(force_colon : Bool = false, allow_colon : Bool = true, format_seconds : Bool = false, parse_seconds : Bool = true, force_zero_padding : Bool = true, force_minutes : Bool = true) : Time::Location
       case current_char
       when '-'
         sign = -1
@@ -445,11 +445,11 @@ struct Time::Format
       @location = Location.fixed(sign * (3600 * hours + 60 * minutes + seconds))
     end
 
-    def time_zone_colon
+    def time_zone_colon : Char | Time::Location
       time_zone
     end
 
-    def time_zone_colon_with_seconds
+    def time_zone_colon_with_seconds : Char | Time::Location
       time_zone(with_seconds: true)
     end
 
@@ -458,7 +458,7 @@ struct Time::Format
       @location = Location::UTC
     end
 
-    def time_zone_rfc2822
+    def time_zone_rfc2822 : Time::Location
       case current_char
       when '-', '+'
         time_zone_offset(allow_colon: false)
@@ -469,11 +469,11 @@ struct Time::Format
       end
     end
 
-    def time_zone_gmt_or_rfc2822(**options)
+    def time_zone_gmt_or_rfc2822(**options) : Time::Location
       time_zone_rfc2822
     end
 
-    def time_zone_name(zone = false)
+    def time_zone_name(zone : Bool = false) : Time::Location
       case current_char
       when '-', '+'
         time_zone_offset
@@ -492,7 +492,7 @@ struct Time::Format
       end
     end
 
-    def char?(char, *alternatives)
+    def char?(char : Char, *alternatives) : Bool
       if current_char == char || alternatives.includes?(current_char)
         next_char
         true
@@ -501,7 +501,7 @@ struct Time::Format
       end
     end
 
-    def char(char, *alternatives)
+    def char(char : Char, *alternatives) : Nil
       unless @reader.has_next?
         if alternatives.empty?
           raise "Expected #{char.inspect} but the end of the input was reached"
@@ -515,19 +515,19 @@ struct Time::Format
       end
     end
 
-    def consume_number(max_digits)
+    def consume_number(max_digits : Int32) : Int32
       consume_number_i64(max_digits).to_i
     end
 
-    def consume_number?(max_digits)
+    def consume_number?(max_digits : Int32) : Int32?
       consume_number_i64?(max_digits).try(&.to_i)
     end
 
-    def consume_number_i64(max_digits)
+    def consume_number_i64(max_digits : Int32) : Int64
       consume_number_i64?(max_digits) || raise "Invalid number"
     end
 
-    def consume_number_i64?(max_digits)
+    def consume_number_i64?(max_digits : Int32) : Int64?
       n = 0_i64
       char = current_char
 
@@ -549,7 +549,7 @@ struct Time::Format
       n
     end
 
-    def consume_number_blank_padded(max_digits)
+    def consume_number_blank_padded(max_digits : Int32) : Int32
       if current_char.ascii_whitespace?
         max_digits -= 1
         next_char
@@ -558,7 +558,7 @@ struct Time::Format
       consume_number(max_digits)
     end
 
-    def consume_string
+    def consume_string : String
       start_pos = @reader.pos
       while current_char.ascii_letter?
         next_char
@@ -566,32 +566,32 @@ struct Time::Format
       @reader.string.byte_slice(start_pos, @reader.pos - start_pos)
     end
 
-    def skip_space
+    def skip_space : Char?
       next_char if current_char.ascii_whitespace?
     end
 
-    def skip_spaces
+    def skip_spaces : Nil
       while current_char.ascii_whitespace?
         next_char
       end
     end
 
-    def whitespace
+    def whitespace : Char
       unless current_char.ascii_whitespace?
         ::raise "Unexpected char: #{current_char.inspect}"
       end
       next_char
     end
 
-    def current_char
+    def current_char : Char
       @reader.current_char
     end
 
-    def next_char
+    def next_char : Char
       @reader.next_char
     end
 
-    def raise(message, pos = @reader.pos)
+    def raise(message : String | ArgumentError, pos : Int32 | Bool = @reader.pos) : Nil
       string = @reader.string
       if pos.is_a?(Int)
         string = "#{string.byte_slice(0, pos)}>>#{string.byte_slice(pos, string.bytesize - pos)}"
diff --git a/src/time/format/pattern.cr b/src/time/format/pattern.cr
index 4e63e22f5e78..aaee06925b9a 100644
--- a/src/time/format/pattern.cr
+++ b/src/time/format/pattern.cr
@@ -1,7 +1,7 @@
 struct Time::Format
   # :nodoc:
   module Pattern
-    def visit(pattern)
+    def visit(pattern : String) : Nil
       reader = Char::Reader.new(pattern)
       while reader.has_next?
         char = reader.current_char
@@ -10,7 +10,7 @@ struct Time::Format
       end
     end
 
-    private def check_char(reader, char)
+    private def check_char(reader : Char::Reader, char : Char) : Char::Reader
       case char
       when '%'
         case char = reader.next_char
@@ -171,7 +171,7 @@ struct Time::Format
       reader
     end
 
-    def date_and_time
+    def date_and_time : Int32?
       short_day_name
       char ' '
       short_month_name
@@ -183,7 +183,7 @@ struct Time::Format
       year
     end
 
-    def date
+    def date : Int32?
       month_zero_padded
       char '/'
       day_of_month_zero_padded
@@ -191,7 +191,7 @@ struct Time::Format
       year_modulo_100
     end
 
-    def year_month_day
+    def year_month_day : Int32?
       year
       char '-'
       month_zero_padded
@@ -199,7 +199,7 @@ struct Time::Format
       day_of_month_zero_padded
     end
 
-    def twelve_hour_time
+    def twelve_hour_time : Bool?
       hour_12_zero_padded
       char ':'
       minute
@@ -209,13 +209,13 @@ struct Time::Format
       am_pm_upcase
     end
 
-    def twenty_four_hour_time
+    def twenty_four_hour_time : Int32?
       hour_24_zero_padded
       char ':'
       minute
     end
 
-    def twenty_four_hour_time_with_seconds
+    def twenty_four_hour_time_with_seconds : Int32?
       hour_24_zero_padded
       char ':'
       minute
diff --git a/src/time/location/loader.cr b/src/time/location/loader.cr
index 6a104101405c..a6357106085b 100644
--- a/src/time/location/loader.cr
+++ b/src/time/location/loader.cr
@@ -212,11 +212,11 @@ class Time::Location
     end
   end
 
-  private def self.read_int32(io : IO)
+  private def self.read_int32(io : IO) : Int32
     io.read_bytes(Int32, IO::ByteFormat::BigEndian)
   end
 
-  private def self.read_buffer(io : IO, size : Int)
+  private def self.read_buffer(io : IO, size : Int) : IO::Memory
     buffer = Bytes.new(size)
     io.read_fully(buffer)
     IO::Memory.new(buffer)
diff --git a/src/time/span.cr b/src/time/span.cr
index 34910a7ed92e..6c23df490c73 100644
--- a/src/time/span.cr
+++ b/src/time/span.cr
@@ -89,7 +89,7 @@ struct Time::Span
   # Time::Span.new(nanoseconds: 500_000_000)   # => 00:00:00.500000000
   # Time::Span.new(nanoseconds: 5_500_000_000) # => 00:00:05.500000000
   # ```
-  def self.new(*, nanoseconds : Int)
+  def self.new(*, nanoseconds : Int) : Time::Span
     new(seconds: 0, nanoseconds: nanoseconds)
   end
 
@@ -102,14 +102,14 @@ struct Time::Span
   # Time::Span.new(days: 1, hours: 2, minutes: 3)                             # => 1.02:03:00
   # Time::Span.new(days: 1, hours: 2, minutes: 3, seconds: 4, nanoseconds: 5) # => 1.02:03:04.000000005
   # ```
-  def self.new(*, days : Int = 0, hours : Int = 0, minutes : Int = 0, seconds : Int = 0, nanoseconds : Int = 0)
+  def self.new(*, days : Int = 0, hours : Int = 0, minutes : Int = 0, seconds : Int = 0, nanoseconds : Int = 0) : Time::Span
     new(
       seconds: compute_seconds(days, hours, minutes, seconds),
       nanoseconds: nanoseconds,
     )
   end
 
-  private def self.compute_seconds(days, hours, minutes, seconds)
+  private def self.compute_seconds(days : Int32 | Int64, hours : Int32, minutes : Int32, seconds : Int32 | Int64) : Int64
     days_to_seconds = SECONDS_PER_DAY.to_i64 * days
     hours_to_seconds = SECONDS_PER_HOUR.to_i64 * hours
     minutes_to_seconds = SECONDS_PER_MINUTE.to_i64 * minutes
@@ -322,7 +322,7 @@ struct Time::Span
     total_nanoseconds.to_f64 / other.total_nanoseconds.to_f64
   end
 
-  def <=>(other : self)
+  def <=>(other : self) : Int32
     cmp = to_i <=> other.to_i
     cmp = nanoseconds <=> other.nanoseconds if cmp == 0
     cmp
diff --git a/src/unicode/data.cr b/src/unicode/data.cr
index ccb7d702e892..e5082c0289eb 100644
--- a/src/unicode/data.cr
+++ b/src/unicode/data.cr
@@ -14993,7 +14993,7 @@ module Unicode
     hash[key] = value
   end
 
-  private def self.put(hash : Hash, key, *values) : Nil
+  private def self.put(hash : Hash, key : Int32, *values) : Nil
     hash[key] = values
   end
 end
diff --git a/src/unicode/unicode.cr b/src/unicode/unicode.cr
index ab49ea31368b..e3402865c395 100644
--- a/src/unicode/unicode.cr
+++ b/src/unicode/unicode.cr
@@ -141,7 +141,7 @@ module Unicode
     x
   end
 
-  private def self.put1(array : Array, value) : Nil
+  private def self.put1(array : Array, value : UInt64) : Nil
     array << value
   end
 
@@ -196,7 +196,7 @@ module Unicode
     yield check_upcase_ranges(char)
   end
 
-  private def self.check_upcase_ascii(char, options)
+  private def self.check_upcase_ascii(char : Char, options : Unicode::CaseOptions) : Char?
     if (char.ascii? && options.none?) || options.ascii?
       if char.ascii_lowercase?
         return (char.ord - 32).unsafe_chr
@@ -207,7 +207,7 @@ module Unicode
     nil
   end
 
-  private def self.check_upcase_turkic(char, options)
+  private def self.check_upcase_turkic(char : Char, options : Unicode::CaseOptions) : Char?
     if options.turkic?
       case char
       when 'ı' then 'I'
@@ -219,7 +219,7 @@ module Unicode
     end
   end
 
-  private def self.check_upcase_ranges(char)
+  private def self.check_upcase_ranges(char : Char) : Char
     result = search_ranges(upcase_ranges, char.ord)
     return char + result if result
 
@@ -263,7 +263,7 @@ module Unicode
     yield check_downcase_ranges(char)
   end
 
-  private def self.check_downcase_ascii(char, options)
+  private def self.check_downcase_ascii(char : Char, options : Unicode::CaseOptions) : Char?
     if (char.ascii? && options.none?) || options.ascii?
       if char.ascii_uppercase?
         return (char.ord + 32).unsafe_chr
@@ -275,7 +275,7 @@ module Unicode
     nil
   end
 
-  private def self.check_downcase_turkic(char, options)
+  private def self.check_downcase_turkic(char : Char, options : Unicode::CaseOptions) : Char?
     if options.turkic?
       case char
       when 'I' then 'ı'
@@ -287,7 +287,7 @@ module Unicode
     end
   end
 
-  private def self.check_downcase_ranges(char)
+  private def self.check_downcase_ranges(char : Char) : Char
     result = search_ranges(downcase_ranges, char.ord)
     return char + result if result
 
@@ -362,7 +362,7 @@ module Unicode
     yield char
   end
 
-  private def self.check_foldcase(char, options)
+  private def self.check_foldcase(char : Char, options : Unicode::CaseOptions) : Tuple(Int32) | Tuple(Int32, Int32, Int32) | Nil
     if options.fold?
       result = search_ranges(casefold_ranges, char.ord)
       return {char.ord + result} if result
@@ -638,19 +638,19 @@ module Unicode
     end
   end
 
-  private def self.search_ranges(haystack, needle)
+  private def self.search_ranges(haystack : Array(Tuple(Int32, Int32, Int32)), needle : Int32) : Int32?
     search_ranges(haystack, needle) { |value| value[2] }
   end
 
-  private def self.search_alternate(haystack, needle)
+  private def self.search_alternate(haystack : Array(Tuple(Int32, Int32)), needle : Int32) : Int32?
     search_ranges(haystack, needle) { |value| value[0] }
   end
 
-  private def self.in_category?(needle, haystack)
+  private def self.in_category?(needle : Int32, haystack : Array(Tuple(Int32, Int32, Int32))) : Bool
     !!search_ranges(haystack, needle) { |value| (needle - value[0]).divisible_by?(value[2]) }
   end
 
-  private def self.in_any_category?(needle, *haystacks) : Bool
+  private def self.in_any_category?(needle : Int32, *haystacks) : Bool
     haystacks.any? { |haystack| in_category?(needle, haystack) }
   end
 end
diff --git a/src/uri/encoding.cr b/src/uri/encoding.cr
index a7c5be7f1fb9..ea18bb43277d 100644
--- a/src/uri/encoding.cr
+++ b/src/uri/encoding.cr
@@ -220,19 +220,19 @@ class URI
   #
   # Reserved characters are ':', '/', '?', '#', '[', ']', '@', '!',
   # '$', '&', "'", '(', ')', '*', '+', ',', ';' and '='.
-  def self.reserved?(byte) : Bool
+  def self.reserved?(byte : UInt8) : Bool
     sub_delim?(byte) || gen_delim?(byte)
   end
 
   # :nodoc:
   # Returns `true` if the byte is URI gen-delims (https://datatracker.ietf.org/doc/html/rfc3986#section-2.2).
-  def self.gen_delim?(byte)
+  def self.gen_delim?(byte : UInt8) : Bool
     byte.unsafe_chr.in?('#', '/', ':', '?', '@', '[', ']')
   end
 
   # :nodoc:
   # Returns `true` if the byte is URI sub-delims (https://datatracker.ietf.org/doc/html/rfc3986#section-2.2).
-  def self.sub_delim?(byte) : Bool
+  def self.sub_delim?(byte : UInt8) : Bool
     char = byte.unsafe_chr
     '&' <= char <= ',' ||
       char.in?('!', '$', ';', '=')
@@ -242,7 +242,7 @@ class URI
   # [RFC 3986 §2.3](https://datatracker.ietf.org/doc/html/rfc3986#section-2.3).
   #
   # Unreserved characters are ASCII letters, ASCII digits, `_`, `.`, `-` and `~`.
-  def self.unreserved?(byte) : Bool
+  def self.unreserved?(byte : UInt8) : Bool
     char = byte.unsafe_chr
     char.ascii_alphanumeric? ||
       char.in?('_', '.', '-', '~')
@@ -324,7 +324,7 @@ class URI
   end
 
   # :nodoc:
-  def self.decode_one(string, bytesize, i, byte, char, io, plus_to_space = false)
+  def self.decode_one(string : String, bytesize : Int32, i : Int32, byte : UInt8, char : Char, io : IO::Memory, plus_to_space : Bool = false) : Int32
     self.decode_one(string, bytesize, i, byte, char, io, plus_to_space) { false }
   end
 
diff --git a/src/uri/json.cr b/src/uri/json.cr
index 00b58f419be5..73f58ebc4e30 100644
--- a/src/uri/json.cr
+++ b/src/uri/json.cr
@@ -11,7 +11,7 @@ class URI
   # uri.scheme                                        # => "http"
   # uri.host                                          # => "crystal-lang.org"
   # ```
-  def self.new(parser : JSON::PullParser)
+  def self.new(parser : JSON::PullParser) : URI
     parse parser.read_string
   end
 
@@ -22,7 +22,7 @@ class URI
   #
   # URI.parse("http://example.com").to_json # => %("http://example.com")
   # ```
-  def to_json(builder : JSON::Builder)
+  def to_json(builder : JSON::Builder) : Nil
     builder.string self
   end
 
diff --git a/src/uri/params.cr b/src/uri/params.cr
index 4670c2725681..2ef7f08e3e1f 100644
--- a/src/uri/params.cr
+++ b/src/uri/params.cr
@@ -240,7 +240,7 @@ class URI
     # params["email"]              # => "john@example.org"
     # params["non_existent_param"] # KeyError
     # ```
-    def [](name) : String
+    def [](name : String) : String
       fetch(name) { raise KeyError.new "Missing param name: #{name.inspect}" }
     end
 
@@ -250,7 +250,7 @@ class URI
     # params["email"]?              # => "john@example.org"
     # params["non_existent_param"]? # nil
     # ```
-    def []?(name) : String?
+    def []?(name : String) : String?
       fetch(name, nil)
     end
 
@@ -284,7 +284,7 @@ class URI
     # params["a"]           # => "e"
     # params.fetch_all("a") # => ["e", "f"]
     # ```
-    def []=(name, value : String | Array(String))
+    def []=(name : String, value : String | Array(String)) : Array(String)
       raw_params[name] =
         case value
         in String        then [value]
@@ -298,7 +298,7 @@ class URI
     # params.set_all("item", ["pencil", "book", "workbook"])
     # params.fetch_all("item") # => ["pencil", "book", "workbook"]
     # ```
-    def fetch_all(name) : Array(String)
+    def fetch_all(name : String) : Array(String)
       raw_params.fetch(name) { [] of String }
     end
 
@@ -310,7 +310,7 @@ class URI
     # params.fetch("email", "none@example.org")           # => "john@example.org"
     # params.fetch("non_existent_param", "default value") # => "default value"
     # ```
-    def fetch(name, default)
+    def fetch(name : String, default : String?) : String?
       fetch(name) { default }
     end
 
@@ -334,7 +334,7 @@ class URI
     # params.add("item", "keychain")
     # params.fetch_all("item") # => ["pencil", "book", "workbook", "keychain"]
     # ```
-    def add(name, value)
+    def add(name : String, value : String) : Array(String)
       params = raw_params.put_if_absent(name) { [] of String }
       params.clear if params.size == 1 && params[0] == ""
       params << value
@@ -346,7 +346,7 @@ class URI
     # params.set_all("item", ["keychain", "keynote"])
     # params.fetch_all("item") # => ["keychain", "keynote"]
     # ```
-    def set_all(name, values)
+    def set_all(name : String, values : Array(String)) : Array(String)
       raw_params[name] = values
     end
 
@@ -382,7 +382,7 @@ class URI
     #
     # params.delete("non_existent_param") # KeyError
     # ```
-    def delete(name) : String
+    def delete(name : String) : String
       value = raw_params[name].shift
       raw_params.delete(name) if raw_params[name].size == 0
       value
@@ -396,7 +396,7 @@ class URI
     # params.delete_all("comments") # => ["hello, world!", ":+1:"]
     # params.has_key?("comments")   # => false
     # ```
-    def delete_all(name) : Array(String)?
+    def delete_all(name : String) : Array(String)?
       raw_params.delete(name)
     end
 
@@ -456,7 +456,7 @@ class URI
     # params = URI::Params.parse("item=keychain&greeting=hello+world&email=john@example.org")
     # params.to_s(space_to_plus: false) # => "item=keychain&greeting=hello%20world&email=john%40example.org"
     # ```
-    def to_s(*, space_to_plus : Bool = true)
+    def to_s(*, space_to_plus : Bool = true) : String
       String.build do |io|
         to_s(io, space_to_plus: space_to_plus)
       end
@@ -476,7 +476,7 @@ class URI
     end
 
     # :nodoc:
-    def self.decode_one_www_form_component(query, bytesize, i, byte, char, buffer)
+    def self.decode_one_www_form_component(query : String, bytesize : Int32, i : Int32, byte : UInt8, char : Char, buffer : IO::Memory) : Int32
       URI.decode_one query, bytesize, i, byte, char, buffer, true
     end
 
@@ -494,7 +494,7 @@ class URI
       end
 
       # Adds a key-value pair to the params being built.
-      def add(key, value : String?)
+      def add(key : String, value : String?) : URI::Params::Builder
         @io << '&' unless @first
         @first = false
         URI.encode_www_form key, @io, space_to_plus: @space_to_plus
@@ -504,7 +504,7 @@ class URI
       end
 
       # Adds all of the given *values* as key-value pairs to the params being built.
-      def add(key, values : Array)
+      def add(key : String, values : Array) : URI::Params::Builder
         values.each { |value| add(key, value) }
         self
       end
diff --git a/src/uri/params/from_www_form.cr b/src/uri/params/from_www_form.cr
index 819c9fc9d82e..e4f176243794 100644
--- a/src/uri/params/from_www_form.cr
+++ b/src/uri/params/from_www_form.cr
@@ -1,5 +1,5 @@
 # :nodoc:
-def Object.from_www_form(params : URI::Params, name : String)
+def Object.from_www_form(params : URI::Params, name : String) : Nil | Bool | UInt8 | Int32 | Float64 | String
   return unless value = params[name]?
 
   self.from_www_form value
@@ -19,7 +19,7 @@ def Array.from_www_form(params : URI::Params, name : String)
 end
 
 # :nodoc:
-def Bool.from_www_form(value : String)
+def Bool.from_www_form(value : String) : Bool?
   case value
   when "true", "1", "yes", "on"  then true
   when "false", "0", "no", "off" then false
@@ -27,12 +27,12 @@ def Bool.from_www_form(value : String)
 end
 
 # :nodoc:
-def Number.from_www_form(value : String)
+def Number.from_www_form(value : String) : UInt8 | Int16 | Int32 | Int64 | Float32 | Float64
   new value, whitespace: false
 end
 
 # :nodoc:
-def String.from_www_form(value : String)
+def String.from_www_form(value : String) : String
   value
 end
 
@@ -42,7 +42,7 @@ def Enum.from_www_form(value : String)
 end
 
 # :nodoc:
-def Time.from_www_form(value : String)
+def Time.from_www_form(value : String) : Time
   Time::Format::ISO_8601_DATE_TIME.parse value
 end
 
diff --git a/src/uri/params/serializable.cr b/src/uri/params/serializable.cr
index 54d3b970e53c..11dacf5d8d68 100644
--- a/src/uri/params/serializable.cr
+++ b/src/uri/params/serializable.cr
@@ -117,7 +117,7 @@ struct URI::Params
     end
 
     # :nodoc:
-    def to_www_form(builder : URI::Params::Builder, name : String)
+    def to_www_form(builder : URI::Params::Builder, name : String) : Nil
       {% for ivar in @type.instance_vars %}
         @{{ivar.name.id}}.to_www_form builder, "#{name}[#{{{ivar.name.stringify}}}]"
       {% end %}
diff --git a/src/uri/punycode.cr b/src/uri/punycode.cr
index b31574d2fbe2..397dbfeb7ee2 100644
--- a/src/uri/punycode.cr
+++ b/src/uri/punycode.cr
@@ -15,7 +15,7 @@ class URI
 
     private BASE36 = "abcdefghijklmnopqrstuvwxyz0123456789"
 
-    private def self.adapt(delta, numpoints, firsttime)
+    private def self.adapt(delta : UInt32 | Int32, numpoints : Int32, firsttime : Bool) : Int32
       delta //= firsttime ? DAMP : 2
       delta += delta // numpoints
       k = 0
@@ -26,11 +26,11 @@ class URI
       k + (((BASE - TMIN + 1) * delta) // (delta + SKEW))
     end
 
-    def self.encode(string) : String
+    def self.encode(string : String) : String
       String.build { |io| encode string, io }
     end
 
-    def self.encode(string, io) : Nil
+    def self.encode(string : String, io : String::Builder) : Nil
       others = [] of Char
 
       string.each_char do |c|
@@ -87,7 +87,7 @@ class URI
       end
     end
 
-    def self.decode(string) : String
+    def self.decode(string : String) : String
       output, _, rest = string.rpartition(DELIMITER)
       output = output.chars
 
@@ -138,7 +138,7 @@ class URI
       output.join
     end
 
-    def self.to_ascii(string) : String
+    def self.to_ascii(string : String) : String
       return string if string.ascii_only?
 
       String.build do |io|
diff --git a/src/uri/uri_parser.cr b/src/uri/uri_parser.cr
index 3c70aebff668..47be96c83f39 100644
--- a/src/uri/uri_parser.cr
+++ b/src/uri/uri_parser.cr
@@ -12,7 +12,7 @@ class URI
 
     @input : UInt8*
 
-    def initialize(input)
+    def initialize(input : String)
       @uri = URI.new
       @input = input.strip.to_unsafe
       @ptr = 0
@@ -32,7 +32,7 @@ class URI
       self
     end
 
-    private def parse_scheme_start
+    private def parse_scheme_start : Nil
       if alpha?
         parse_scheme
       elsif c === '/' && @input[@ptr + 1] === '/'
@@ -44,7 +44,7 @@ class URI
       end
     end
 
-    private def parse_scheme
+    private def parse_scheme : Nil
       start = @ptr
       loop do
         if alpha? || numeric? || c === '-' || c === '.' || c === '+'
@@ -65,7 +65,7 @@ class URI
       end
     end
 
-    private def parse_path_or_authority
+    private def parse_path_or_authority : Nil
       if c === '/'
         @uri.host = ""
         parse_authority
@@ -75,7 +75,7 @@ class URI
       end
     end
 
-    private def parse_no_scheme
+    private def parse_no_scheme : Nil
       case c
       when '#'
         parse_fragment
@@ -84,7 +84,7 @@ class URI
       end
     end
 
-    private def parse_authority
+    private def parse_authority : Nil
       @ptr += 1
       start = @ptr
       loop do
@@ -100,7 +100,7 @@ class URI
       end
     end
 
-    private def parse_userinfo
+    private def parse_userinfo : Nil
       start = @ptr
       password_flag = false
       loop do
@@ -123,7 +123,7 @@ class URI
       end
     end
 
-    private def parse_host
+    private def parse_host : Nil
       start = @ptr
       bracket_flag = false
       return parse_path if c === '/'
@@ -143,7 +143,7 @@ class URI
       end
     end
 
-    private def parse_port
+    private def parse_port : Nil
       start = @ptr
       loop do
         if numeric?
@@ -161,7 +161,7 @@ class URI
       end
     end
 
-    private def parse_relative
+    private def parse_relative : Nil
       case c
       when '\0'
         nil
@@ -174,7 +174,7 @@ class URI
       end
     end
 
-    private def parse_path
+    private def parse_path : Nil
       start = @ptr
       loop do
         case c
@@ -193,7 +193,7 @@ class URI
       end
     end
 
-    private def parse_query
+    private def parse_query : Nil
       @ptr += 1
       start = @ptr
       loop do
@@ -210,7 +210,7 @@ class URI
       end
     end
 
-    private def parse_fragment
+    private def parse_fragment : Nil
       @ptr += 1
       start = @ptr
       loop do
@@ -224,20 +224,20 @@ class URI
       end
     end
 
-    private def from_input(start)
+    private def from_input(start : Int32) : String
       String.new(@input + start, @ptr - start)
     end
 
-    private def alpha?
+    private def alpha? : Bool
       ('a'.ord <= c && c <= 'z'.ord) ||
         ('A'.ord <= c && c <= 'Z'.ord)
     end
 
-    private def numeric?
+    private def numeric? : Bool
       '0'.ord <= c && c <= '9'.ord
     end
 
-    private def end_of_host?
+    private def end_of_host? : Bool
       c === '\0' || c === '/' || c === '?' || c === '#'
     end
   end
diff --git a/src/uri/yaml.cr b/src/uri/yaml.cr
index c6b53ed0221c..1e76fb52c1b3 100644
--- a/src/uri/yaml.cr
+++ b/src/uri/yaml.cr
@@ -11,7 +11,7 @@ class URI
   # uri.scheme                                        # => "http"
   # uri.host                                          # => "crystal-lang.org"
   # ```
-  def self.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)
+  def self.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node) : URI
     parse String.new(ctx, node)
   end
 
@@ -22,7 +22,7 @@ class URI
   #
   # URI.parse("http://example.com").to_yaml # => "--- http://example.com\n"
   # ```
-  def to_yaml(yaml : YAML::Nodes::Builder)
+  def to_yaml(yaml : YAML::Nodes::Builder) : Nil
     yaml.scalar to_s
   end
 end
diff --git a/src/uuid.cr b/src/uuid.cr
index b1a043785472..1906ae4cdf55 100644
--- a/src/uuid.cr
+++ b/src/uuid.cr
@@ -102,7 +102,7 @@ struct UUID
 
   # Creates UUID from 16-bytes slice. Raises if *slice* isn't 16 bytes long. See
   # `#initialize` for *variant* and *version*.
-  def self.new(slice : Slice(UInt8), variant : Variant? = nil, version : Version? = nil)
+  def self.new(slice : Slice(UInt8), variant : Variant? = nil, version : Version? = nil) : UUID
     raise ArgumentError.new "Invalid bytes length #{slice.size}, expected 16" unless slice.size == 16
 
     bytes = uninitialized UInt8[16]
@@ -113,14 +113,14 @@ struct UUID
 
   # Creates another `UUID` which is a copy of *uuid*, but allows overriding
   # *variant* or *version*.
-  def self.new(uuid : UUID, variant : Variant? = nil, version : Version? = nil)
+  def self.new(uuid : UUID, variant : Variant? = nil, version : Version? = nil) : UUID
     new(uuid.bytes, variant, version)
   end
 
   # Creates new UUID by decoding `value` string from hyphenated (ie `ba714f86-cac6-42c7-8956-bcf5105e1b81`),
   # hexstring (ie `89370a4ab66440c8add39e06f2bb6af6`) or URN (ie `urn:uuid:3f9eaf9e-cdb0-45cc-8ecb-0e5b2bfb0c20`)
   # format, raising an `ArgumentError` if the string does not match any of these formats.
-  def self.new(value : String, variant : Variant? = nil, version : Version? = nil)
+  def self.new(value : String, variant : Variant? = nil, version : Version? = nil) : UUID
     bytes = uninitialized UInt8[16]
 
     case value.size
@@ -193,13 +193,13 @@ struct UUID
 
   # Raises `ArgumentError` if string `value` at index `i` doesn't contain hex
   # digit followed by another hex digit.
-  private def self.hex_pair_at(value : String, i) : UInt8
+  private def self.hex_pair_at(value : String, i : Int32) : UInt8
     hex_pair_at?(value, i) || raise ArgumentError.new "Invalid hex character at position #{i * 2} or #{i * 2 + 1}, expected '0' to '9', 'a' to 'f' or 'A' to 'F'"
   end
 
   # Parses 2 hex digits from `value` at index `i` and `i + 1`, returning `nil`
   # if one or both are not actually hex digits.
-  private def self.hex_pair_at?(value : String, i) : UInt8?
+  private def self.hex_pair_at?(value : String, i : Int32) : UInt8?
     if (ch1 = value[i].to_u8?(16)) && (ch2 = value[i + 1].to_u8?(16))
       ch1 * 16 + ch2
     end
@@ -328,7 +328,7 @@ struct UUID
   # Generates an RFC9562-compatible v7 UUID, allowing the values to be sorted
   # chronologically (with 1ms precision) by their raw or hexstring
   # representation.
-  def self.v7(random r : Random = Random::Secure)
+  def self.v7(random r : Random = Random::Secure) : UUID
     buffer = uninitialized UInt8[18]
     value = buffer.to_slice
 
diff --git a/src/uuid/json.cr b/src/uuid/json.cr
index 966ee0cabfc9..c131c2ad1cf9 100644
--- a/src/uuid/json.cr
+++ b/src/uuid/json.cr
@@ -20,7 +20,7 @@ struct UUID
   # example = Example.from_json(%({"id": "ba714f86-cac6-42c7-8956-bcf5105e1b81"}))
   # example.id # => UUID(ba714f86-cac6-42c7-8956-bcf5105e1b81)
   # ```
-  def self.new(pull : JSON::PullParser)
+  def self.new(pull : JSON::PullParser) : UUID
     new(pull.read_string)
   end
 
@@ -44,7 +44,7 @@ struct UUID
   # Deserializes the given JSON *key* into a `UUID`.
   #
   # NOTE: `require "uuid/json"` is required to opt-in to this feature.
-  def self.from_json_object_key?(key : String)
+  def self.from_json_object_key?(key : String) : UUID
     UUID.new(key)
   end
 end
diff --git a/src/uuid/yaml.cr b/src/uuid/yaml.cr
index 2c7eec1f5a45..f2fac6329a39 100644
--- a/src/uuid/yaml.cr
+++ b/src/uuid/yaml.cr
@@ -20,7 +20,7 @@ struct UUID
   # example = Example.from_yaml("id: 50a11da6-377b-4bdf-b9f0-076f9db61c93")
   # example.id # => UUID(50a11da6-377b-4bdf-b9f0-076f9db61c93)
   # ```
-  def self.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)
+  def self.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node) : UUID
     ctx.read_alias(node, String) do |obj|
       return UUID.new(obj)
     end
@@ -42,7 +42,7 @@ struct UUID
   # uuid = UUID.new("50a11da6-377b-4bdf-b9f0-076f9db61c93")
   # uuid.to_yaml # => "--- 50a11da6-377b-4bdf-b9f0-076f9db61c93\n"
   # ```
-  def to_yaml(yaml : YAML::Nodes::Builder)
+  def to_yaml(yaml : YAML::Nodes::Builder) : Nil
     yaml.scalar self.to_s
   end
 end
diff --git a/src/wait_group.cr b/src/wait_group.cr
index 003921bd9f46..da1713449cd1 100644
--- a/src/wait_group.cr
+++ b/src/wait_group.cr
@@ -135,7 +135,7 @@ class WaitGroup
     raise RuntimeError.new("Positive WaitGroup counter (early wake up?)")
   end
 
-  private def done?
+  private def done? : Bool
     counter = @counter.get(:acquire)
     raise RuntimeError.new("Negative WaitGroup counter") if counter < 0
     counter == 0
diff --git a/src/winerror.cr b/src/winerror.cr
index ae4eceb1f18e..8cf490950627 100644
--- a/src/winerror.cr
+++ b/src/winerror.cr
@@ -23,7 +23,7 @@ enum WinError : UInt32
   # which signifies the error code of the previously called win32 function.
   #
   # Raises `NotImplementedError` on non-win32 platforms.
-  def self.value=(winerror : self)
+  def self.value=(winerror : self) : Nil
     {% if flag?(:win32) %}
       LibC.SetLastError(winerror.value)
     {% else %}
@@ -35,7 +35,7 @@ enum WinError : UInt32
   # which is used to retrieve the error code of the previously called Windows Socket API function.
   #
   # Raises `NotImplementedError` on non-win32 platforms.
-  def self.wsa_value
+  def self.wsa_value : Nil
     {% if flag?(:win32) %}
       WinError.new LibC.WSAGetLastError.to_u32!
     {% else %}
@@ -47,7 +47,7 @@ enum WinError : UInt32
   # which signifies the error code of the previously called Windows Socket API function.
   #
   # Raises `NotImplementedError` on non-win32 platforms.
-  def self.wsa_value=(winerror : self)
+  def self.wsa_value=(winerror : self) : Nil
     {% if flag?(:win32) %}
       LibC.WSASetLastError(winerror.value)
     {% else %}
diff --git a/src/xml/attributes.cr b/src/xml/attributes.cr
index 1df5aea55483..257ae9276e93 100644
--- a/src/xml/attributes.cr
+++ b/src/xml/attributes.cr
@@ -38,7 +38,7 @@ class XML::Attributes
     find { |node| node.name == name }
   end
 
-  def []=(name : String, value)
+  def []=(name : String, value : String | Int32) : String | Int32
     LibXML.xmlSetProp(@node, name, value.to_s)
     value
   end
@@ -69,7 +69,7 @@ class XML::Attributes
     to_s(io)
   end
 
-  protected def props
+  protected def props : Pointer(LibXML::Attr)
     @node.to_unsafe.value.properties
   end
 end
diff --git a/src/xml/builder.cr b/src/xml/builder.cr
index 54e5f45adddc..089a9ee4f555 100644
--- a/src/xml/builder.cr
+++ b/src/xml/builder.cr
@@ -29,7 +29,7 @@ class XML::Builder
   end
 
   # Emits the start of the document.
-  def start_document(version = nil, encoding = nil) : Nil
+  def start_document(version : Nil = nil, encoding : Nil = nil) : Nil
     call StartDocument, string_to_unsafe(version), string_to_unsafe(encoding), nil
   end
 
@@ -84,7 +84,7 @@ class XML::Builder
   end
 
   # Emits an element with the given *attributes*.
-  def element(__name__ : String, **attributes)
+  def element(__name__ : String, **attributes) : Nil
     element(__name__, attributes)
   end
 
@@ -143,12 +143,12 @@ class XML::Builder
   end
 
   # Emits an attribute with a *value*.
-  def attribute(name : String, value) : Nil
+  def attribute(name : String, value : Int32 | String) : Nil
     call WriteAttribute, string_to_unsafe(name), string_to_unsafe(value.to_s)
   end
 
   # Emits an attribute with namespace info and a *value*.
-  def attribute(prefix : String?, name : String, namespace_uri : String?, value) : Nil
+  def attribute(prefix : String?, name : String, namespace_uri : String?, value : Int32 | String) : Nil
     call WriteAttributeNS, string_to_unsafe(prefix), string_to_unsafe(name), string_to_unsafe(namespace_uri), string_to_unsafe(value.to_s)
   end
 
@@ -241,7 +241,7 @@ class XML::Builder
   end
 
   # Emits a namespace.
-  def namespace(prefix, uri) : Nil
+  def namespace(prefix : String, uri : String) : Nil
     attribute "xmlns", prefix, nil, uri
   end
 
@@ -254,7 +254,7 @@ class XML::Builder
   end
 
   # Sets the indent string.
-  def indent=(str : String)
+  def indent=(str : String) : Nil
     if str.empty?
       call SetIndent, 0
     else
@@ -264,7 +264,7 @@ class XML::Builder
   end
 
   # Sets the indent *level* (number of spaces).
-  def indent=(level : Int)
+  def indent=(level : Int) : Nil
     if level <= 0
       call SetIndent, 0
     else
@@ -274,7 +274,7 @@ class XML::Builder
   end
 
   # Sets the quote char to use, either `'` or `"`.
-  def quote_char=(char : Char)
+  def quote_char=(char : Char) : Nil
     unless char.in?('\'', '"')
       raise ArgumentError.new("Quote char must be ' or \", not #{char}")
     end
@@ -287,7 +287,7 @@ class XML::Builder
     check ret, {{@def.name.stringify}}
   end
 
-  private def check(ret, msg)
+  private def check(ret : Int32, msg : String) : Nil
     raise XML::Error.new("Error in #{msg}", 0) if ret < 0
   end
 
@@ -295,12 +295,12 @@ class XML::Builder
     raise XML::Error.new("Invalid #{element_type}: '#{name}'", 0) if LibXML.xmlValidateNameValue(unsafe_name).zero?
   end
 
-  private def string_to_unsafe(string : String)
+  private def string_to_unsafe(string : String) : Pointer(UInt8)
     raise XML::Error.new("String cannot contain null character", 0) if string.includes? '\0'
     string.to_unsafe
   end
 
-  private def string_to_unsafe(string : Nil)
+  private def string_to_unsafe(string : Nil) : Pointer(UInt8)
     Pointer(UInt8).null
   end
 end
diff --git a/src/xml/error.cr b/src/xml/error.cr
index 389aa53910c2..44530a4d77c2 100644
--- a/src/xml/error.cr
+++ b/src/xml/error.cr
@@ -3,11 +3,11 @@ require "./libxml2"
 class XML::Error < Exception
   getter line_number : Int32
 
-  def self.new(error : LibXML::Error*)
+  def self.new(error : LibXML::Error*) : XML::Error
     new String.new(error.value.message).chomp, error.value.line
   end
 
-  def initialize(message, @line_number)
+  def initialize(message : String, @line_number : Int32)
     super(message)
   end
 
diff --git a/src/xml/node.cr b/src/xml/node.cr
index 45878e762b0f..296dc10abba5 100644
--- a/src/xml/node.cr
+++ b/src/xml/node.cr
@@ -29,7 +29,7 @@ class XML::Node
 
   # Sets *attribute* of this node to *value*.
   # Raises `XML::Error` if this node does not support attributes.
-  def []=(name : String, value)
+  def []=(name : String, value : String | Int32) : String | Int32
     raise XML::Error.new("Can't set attribute of #{type}", 0) unless element?
     attributes[name] = value
   end
@@ -41,7 +41,7 @@ class XML::Node
   end
 
   # Compares with *other*.
-  def ==(other : Node)
+  def ==(other : Node) : Bool
     @node == other.@node
   end
 
@@ -91,7 +91,7 @@ class XML::Node
 
   # Sets the Node's content to a Text node containing string.
   # The string gets XML escaped, not interpreted as markup.
-  def content=(content)
+  def content=(content : String) : Nil
     check_no_null_byte(content)
     LibXML.xmlNodeSetContent(self, content)
   end
@@ -265,7 +265,7 @@ class XML::Node
   end
 
   # Sets the name for this Node.
-  def name=(name)
+  def name=(name : String) : Nil
     if document? || text? || cdata? || fragment?
       raise XML::Error.new("Can't set name of XML #{type}", 0)
     end
@@ -406,7 +406,7 @@ class XML::Node
   end
 
   # Same as `#content=`.
-  def text=(text)
+  def text=(text : String) : Nil
     self.content = text
   end
 
@@ -425,7 +425,7 @@ class XML::Node
   # Serialize this Node as XML and return a `String` using default options.
   #
   # See `XML::SaveOptions.xml_default` for default options.
-  def to_xml(indent : Int = 2, indent_text = " ", options : SaveOptions = SaveOptions.xml_default) : String
+  def to_xml(indent : Int = 2, indent_text : String = " ", options : SaveOptions = SaveOptions.xml_default) : String
     String.build do |str|
       to_xml str, indent, indent_text, options
     end
@@ -437,7 +437,7 @@ class XML::Node
   # Serialize this Node as XML to *io* using default options.
   #
   # See `XML::SaveOptions.xml_default` for default options.
-  def to_xml(io : IO, indent = 2, indent_text = " ", options : SaveOptions = SaveOptions.xml_default)
+  def to_xml(io : IO, indent : Int32 = 2, indent_text : String = " ", options : SaveOptions = SaveOptions.xml_default) : String::Builder | IO::Memory
     # We need to use a mutex because we modify global libxml variables
     SAVE_MUTEX.synchronize do
       XML.with_indent_tree_output(true) do
@@ -464,7 +464,7 @@ class XML::Node
   end
 
   # Returns underlying `LibXML::Node*` instance.
-  def to_unsafe
+  def to_unsafe : Pointer(LibXML::Node)
     @node
   end
 
@@ -487,7 +487,7 @@ class XML::Node
   # (`Bool | Float64 | String | XML::NodeSet`).
   #
   # Raises `XML::Error` on evaluation error.
-  def xpath(path, namespaces = nil, variables = nil)
+  def xpath(path : String, namespaces : Nil | Hash(String, String) | Hash(String, String | Nil) = nil, variables : Hash(String, Int32)? = nil) : Bool | Float64 | String | XML::NodeSet
     ctx = XPathContext.new(self)
 
     if namespaces
@@ -511,7 +511,7 @@ class XML::Node
   #
   # doc.xpath_bool("count(//person) > 0") # => true
   # ```
-  def xpath_bool(path, namespaces = nil, variables = nil)
+  def xpath_bool(path : String, namespaces : Nil = nil, variables : Hash(String, Int32)? = nil) : Bool
     xpath(path, namespaces, variables).as(Bool)
   end
 
@@ -524,7 +524,7 @@ class XML::Node
   #
   # doc.xpath_float("count(//person)") # => 1.0
   # ```
-  def xpath_float(path, namespaces = nil, variables = nil)
+  def xpath_float(path : String, namespaces : Nil = nil, variables : Hash(String, Int32)? = nil) : Float64
     xpath(path, namespaces, variables).as(Float64)
   end
 
@@ -539,7 +539,7 @@ class XML::Node
   # nodes.class       # => XML::NodeSet
   # nodes.map(&.name) # => ["person"]
   # ```
-  def xpath_nodes(path, namespaces = nil, variables = nil)
+  def xpath_nodes(path : String, namespaces : Nil = nil, variables : Hash(String, Int32)? = nil) : XML::NodeSet
     xpath(path, namespaces, variables).as(NodeSet)
   end
 
@@ -554,7 +554,7 @@ class XML::Node
   # doc.xpath_node("//person")  # => #<XML::Node:0x2013e80 name="person">
   # doc.xpath_node("//invalid") # => nil
   # ```
-  def xpath_node(path, namespaces = nil, variables = nil)
+  def xpath_node(path : String, namespaces : Nil = nil, variables : Hash(String, Int32)? = nil) : XML::Node?
     xpath_nodes(path, namespaces, variables).first?
   end
 
@@ -567,7 +567,7 @@ class XML::Node
   #
   # doc.xpath_string("string(/persons/person[1])")
   # ```
-  def xpath_string(path, namespaces = nil, variables = nil)
+  def xpath_string(path : String, namespaces : Nil = nil, variables : Hash(String, Int32)? = nil) : String
     xpath(path, namespaces, variables).as(String)
   end
 
@@ -577,7 +577,7 @@ class XML::Node
     return @errors unless @errors.try &.empty?
   end
 
-  private def check_no_null_byte(string)
+  private def check_no_null_byte(string : String) : Nil
     if string.includes? Char::ZERO
       raise XML::Error.new("Cannot escape string containing null character", 0)
     end
diff --git a/src/xml/node_set.cr b/src/xml/node_set.cr
index 0263facbfd55..0ee4472c10bb 100644
--- a/src/xml/node_set.cr
+++ b/src/xml/node_set.cr
@@ -4,7 +4,7 @@ class XML::NodeSet
   def initialize(@doc : Node, @set : LibXML::NodeSet*)
   end
 
-  def self.new(doc : Node)
+  def self.new(doc : Node) : XML::NodeSet
     new doc, LibXML.xmlXPathNodeSetCreate(nil)
   end
 
@@ -53,7 +53,7 @@ class XML::NodeSet
     @set
   end
 
-  private def internal_at(index)
+  private def internal_at(index : Int32) : XML::Node
     Node.new(@set.value.node_tab[index])
   end
 end
diff --git a/src/xml/reader.cr b/src/xml/reader.cr
index d4dbe91f7eeb..abf8ce064114 100644
--- a/src/xml/reader.cr
+++ b/src/xml/reader.cr
@@ -193,7 +193,7 @@ class XML::Reader
   end
 
   # Returns a reference to the underlying `LibXML::XMLTextReader`.
-  def to_unsafe
+  def to_unsafe : Pointer(Void)
     @reader
   end
 
@@ -201,7 +201,7 @@ class XML::Reader
     Error.collect(@errors) { yield }
   end
 
-  private def check_no_null_byte(attribute)
+  private def check_no_null_byte(attribute : String) : Nil
     if attribute.byte_index(0)
       raise XML::Error.new("Invalid attribute name: #{attribute.inspect} (contains null character)", 0)
     end
diff --git a/src/xml/xpath_context.cr b/src/xml/xpath_context.cr
index 53ede8aeb929..6f3a87ae4bbf 100644
--- a/src/xml/xpath_context.cr
+++ b/src/xml/xpath_context.cr
@@ -6,7 +6,7 @@ class XML::XPathContext
     @ctx.value.node = node.to_unsafe
   end
 
-  def evaluate(search_path : String)
+  def evaluate(search_path : String) : Bool | Float64 | String | XML::NodeSet
     xpath = XML::Error.collect_generic(@errors) { LibXML.xmlXPathEvalExpression(search_path, self) }
 
     raise XML::Error.new("Error in '#{search_path}' expression", 0) unless xpath
@@ -29,24 +29,24 @@ class XML::XPathContext
     end
   end
 
-  def register_namespaces(namespaces) : Nil
+  def register_namespaces(namespaces : Hash(String, String) | Hash(String, String | Nil)) : Nil
     namespaces.each do |prefix, uri|
       register_namespace prefix, uri
     end
   end
 
-  def register_namespace(prefix : String, uri : String?)
+  def register_namespace(prefix : String, uri : String?) : Int32
     prefix = prefix.lchop("xmlns:")
     LibXML.xmlXPathRegisterNs(self, prefix, uri.to_s)
   end
 
-  def register_variables(variables) : Nil
+  def register_variables(variables : Hash(String, Int32)) : Nil
     variables.each do |name, value|
       register_variable name, value
     end
   end
 
-  def register_variable(name, value)
+  def register_variable(name : String, value : Int32) : Int32
     case value
     when Bool
       obj = LibXML.xmlXPathNewBoolean(value ? 1 : 0)
@@ -59,7 +59,7 @@ class XML::XPathContext
     LibXML.xmlXPathRegisterVariable(self, name, obj)
   end
 
-  def to_unsafe
+  def to_unsafe : Pointer(LibXML::XPathContext)
     @ctx
   end
 end
diff --git a/src/yaml.cr b/src/yaml.cr
index c9ecb746ee69..bf731ec69cc0 100644
--- a/src/yaml.cr
+++ b/src/yaml.cr
@@ -85,7 +85,7 @@ module YAML
     getter line_number : Int32
     getter column_number : Int32
 
-    def initialize(message, line_number, column_number, context_info = nil)
+    def initialize(message : String | ArgumentError, line_number : Int32 | UInt64, column_number : Int32 | UInt64, context_info : Tuple(String, UInt64, UInt64)? = nil)
       @line_number = line_number.to_i
       @column_number = column_number.to_i
       if context_info
@@ -152,12 +152,12 @@ module YAML
   end
 
   # Serializes an object to YAML, returning it as a `String`.
-  def self.dump(object) : String
+  def self.dump(object : Array(String)) : String
     object.to_yaml
   end
 
   # Serializes an object to YAML, writing it to *io*.
-  def self.dump(object, io : IO) : Nil
+  def self.dump(object : Array(String), io : IO) : Nil
     object.to_yaml(io)
   end