diff --git a/src/class.cr b/src/class.cr
index 1ec95b97dd1f..0128e90be204 100644
--- a/src/class.cr
+++ b/src/class.cr
@@ -54,7 +54,7 @@ class Class
   # Number > Number # => false
   # Number > Object # => false
   # ```
-  def >(other : T.class) forall T
+  def >(other : T.class) : Bool forall T
     # This is so that the method is expanded differently for each type
     {% @type %}
     other._lt(self)
@@ -141,7 +141,7 @@ class Class
   # Int32.nilable? # => false
   # Nil.nilable?   # => true
   # ```
-  def nilable?
+  def nilable? : Bool
     self == ::Nil
   end
 
diff --git a/src/colorize.cr b/src/colorize.cr
index 792faa55e229..1523e8699ecc 100644
--- a/src/colorize.cr
+++ b/src/colorize.cr
@@ -145,13 +145,13 @@ module Colorize
   #   io << "green and bold if Colorize.enabled"
   # end
   # ```
-  def self.with
+  def self.with : Colorize::Object(String)
     "".colorize
   end
 end
 
 module Colorize::ObjectExtensions
-  def colorize
+  def colorize : Colorize::Object
     Colorize::Object.new(self)
   end
 
@@ -278,7 +278,7 @@ struct Colorize::Object(T)
     end
   {% end %}
 
-  def fore(color : Symbol)
+  def fore(color : Symbol) : self
     {% for name in COLORS %}
       if color == :{{name.id}}
         @fore = ColorANSI::{{name.camelcase.id}}
@@ -289,11 +289,11 @@ struct Colorize::Object(T)
     raise ArgumentError.new "Unknown color: #{color}"
   end
 
-  def fore(@fore : Color)
+  def fore(@fore : Color) : self
     self
   end
 
-  def back(color : Symbol)
+  def back(color : Symbol) : self
     {% for name in COLORS %}
       if color == :{{name.id}}
         @back = ColorANSI::{{name.camelcase.id}}
@@ -304,11 +304,11 @@ struct Colorize::Object(T)
     raise ArgumentError.new "Unknown color: #{color}"
   end
 
-  def back(@back : Color)
+  def back(@back : Color) : self
     self
   end
 
-  def mode(mode : Symbol)
+  def mode(mode : Symbol) : self
     {% for name in MODES %}
       if mode == :{{name.id}}
         @mode |= MODE_{{name.upcase.id}}_FLAG
diff --git a/src/comparable.cr b/src/comparable.cr
index 8ea7c057dcad..7f7e0cc7fb8d 100644
--- a/src/comparable.cr
+++ b/src/comparable.cr
@@ -21,7 +21,7 @@
 module Comparable(T)
   # Compares this object to *other* based on the receiver’s `<=>` method,
   # returning `true` if it returns a negative number.
-  def <(other : T)
+  def <(other : T) : Bool
     cmp = self <=> other
     cmp ? cmp < 0 : false
   end
@@ -52,7 +52,7 @@ module Comparable(T)
 
   # Compares this object to *other* based on the receiver’s `<=>` method,
   # returning `true` if it returns a value greater then `0`.
-  def >(other : T)
+  def >(other : T) : Bool
     cmp = self <=> other
     cmp ? cmp > 0 : false
   end
diff --git a/src/csv.cr b/src/csv.cr
index adc099e9bc67..1b8e89e07a1f 100644
--- a/src/csv.cr
+++ b/src/csv.cr
@@ -218,7 +218,7 @@ class CSV
   # Advanced the cursor to the next row. Must be called once to position
   # the cursor in the first row. Returns `true` if a next row was found,
   # `false` otherwise.
-  def next
+  def next : Bool
     return false if @traversed
 
     row = @row ||= [] of String
@@ -318,7 +318,7 @@ class CSV
   end
 
   # :nodoc:
-  def indices
+  def indices : Hash(String, Int32)
     @indices || raise(Error.new("Headers not requested"))
   end
 
@@ -326,7 +326,7 @@ class CSV
   getter? strip
 
   # :nodoc:
-  def headers?
+  def headers? : Array(String)?
     @headers
   end
 
@@ -405,7 +405,7 @@ class CSV
 
     # Returns the number of columns in this row, regardless of the number
     # of headers (if requested).
-    def size
+    def size : Int32
       @row.size
     end
 
diff --git a/src/csv/lexer.cr b/src/csv/lexer.cr
index df1a514f3138..2656d53656b1 100644
--- a/src/csv/lexer.cr
+++ b/src/csv/lexer.cr
@@ -50,7 +50,7 @@ abstract class CSV::Lexer
   private abstract def current_char
 
   # Returns the next `Token` in this CSV.
-  def next_token
+  def next_token : CSV::Token
     if @last_empty_column
       @last_empty_column = false
       @token.kind = Token::Kind::Cell
diff --git a/src/ecr/lexer.cr b/src/ecr/lexer.cr
index acb5740b1260..b5a30cae8e84 100644
--- a/src/ecr/lexer.cr
+++ b/src/ecr/lexer.cr
@@ -32,7 +32,7 @@ class ECR::Lexer
     @column_number = 1
   end
 
-  def next_token
+  def next_token : Token
     copy_location_info_to_token
 
     case current_char
diff --git a/src/ecr/processor.cr b/src/ecr/processor.cr
index 7806c53417b2..786879e94273 100644
--- a/src/ecr/processor.cr
+++ b/src/ecr/processor.cr
@@ -6,12 +6,12 @@ module ECR
   DefaultBufferName = "__str__"
 
   # :nodoc:
-  def process_file(filename, buffer_name = DefaultBufferName)
+  def process_file(filename, buffer_name = DefaultBufferName) : String
     process_string File.read(filename), filename, buffer_name
   end
 
   # :nodoc:
-  def process_string(string, filename, buffer_name = DefaultBufferName)
+  def process_string(string, filename, buffer_name = DefaultBufferName) : String
     lexer = Lexer.new string
     token = lexer.next_token
 
diff --git a/src/enum.cr b/src/enum.cr
index 726cc8acd85c..39eeb79d533f 100644
--- a/src/enum.cr
+++ b/src/enum.cr
@@ -185,7 +185,7 @@ struct Enum
   # Color::Red + 2 # => Color::Blue
   # Color::Red + 3 # => Color.new(3)
   # ```
-  def +(other : Int)
+  def +(other : Int) : self
     self.class.new(value + other)
   end
 
@@ -197,7 +197,7 @@ struct Enum
   # Color::Blue - 2 # => Color::Red
   # Color::Blue - 3 # => Color.new(-1)
   # ```
-  def -(other : Int)
+  def -(other : Int) : self
     self.class.new(value - other)
   end
 
@@ -208,7 +208,7 @@ struct Enum
   # ```
   # (IOMode::Read | IOMode::Async) # => IOMode::Read | IOMode::Async
   # ```
-  def |(other : self)
+  def |(other : self) : self
     self.class.new(value | other.value)
   end
 
@@ -219,20 +219,20 @@ struct Enum
   # ```
   # (IOMode::Read | IOMode::Async) & IOMode::Read # => IOMode::Read
   # ```
-  def &(other : self)
+  def &(other : self) : self
     self.class.new(value & other.value)
   end
 
   # Returns the enum member that results from applying a logical
   # "xor" operation between this enum member's value and *other*.
   # This is mostly useful with flag enums.
-  def ^(other : self)
+  def ^(other : self) : self
     self.class.new(value ^ other.value)
   end
 
   # Returns the enum member that results from applying a logical
   # "not" operation of this enum member's value.
-  def ~
+  def ~ : self
     self.class.new(~value)
   end
 
@@ -276,7 +276,7 @@ struct Enum
   # mode.includes?(IOMode::Read)  # => true
   # mode.includes?(IOMode::Async) # => false
   # ```
-  def includes?(other : self)
+  def includes?(other : self) : Bool
     (value & other.value) != 0
   end
 
diff --git a/src/float/printer/cached_powers.cr b/src/float/printer/cached_powers.cr
index 7a00dcac019c..19cc1f09457c 100644
--- a/src/float/printer/cached_powers.cr
+++ b/src/float/printer/cached_powers.cr
@@ -140,7 +140,7 @@ module Float::Printer::CachedPowers
 
   Pow10Cache = {0, 1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000}
 
-  def self.largest_pow10(n, n_bits)
+  def self.largest_pow10(n, n_bits) : {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
diff --git a/src/float/printer/diy_fp.cr b/src/float/printer/diy_fp.cr
index 9b046d76b791..c41bc41d9a20 100644
--- a/src/float/printer/diy_fp.cr
+++ b/src/float/printer/diy_fp.cr
@@ -62,7 +62,7 @@ struct Float::Printer::DiyFP
   # must be greater than the *other*.
   #
   # NOTE: This result is not normalized.
-  def -(other : DiyFP)
+  def -(other : DiyFP) : self
     self.class.new(frac - other.frac, exp)
   end
 
@@ -76,7 +76,7 @@ struct Float::Printer::DiyFP
   # bits.
   #
   # NOTE: This result is not normalized.
-  def *(other : DiyFP)
+  def *(other : DiyFP) : self
     a = frac >> 32
     b = frac & MASK32
     c = other.frac >> 32
@@ -95,7 +95,7 @@ struct Float::Printer::DiyFP
     self.class.new(f, e)
   end
 
-  def normalize
+  def normalize : DiyFP
     f = frac
     e = exp
 
@@ -118,13 +118,13 @@ struct Float::Printer::DiyFP
     DiyFP.new(f, e)
   end
 
-  def self.from_f(d : Float64 | Float32)
+  def self.from_f(d : Float64 | Float32) : self
     frac, exp = IEEE.frac_and_exp(d)
     new(frac, exp)
   end
 
   # Normalize such that the most significant bit of `frac` is set.
-  def self.from_f_normalized(v : Float64 | Float32)
+  def self.from_f_normalized(v : Float64 | Float32) : self
     pre_normalized = from_f(v)
     f = pre_normalized.frac
     e = pre_normalized.exp
diff --git a/src/float/printer/grisu3.cr b/src/float/printer/grisu3.cr
index d4eab572129f..cc01a0df7ab1 100644
--- a/src/float/printer/grisu3.cr
+++ b/src/float/printer/grisu3.cr
@@ -53,7 +53,7 @@ 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)
+  def round_weed(buffer_ptr, length, distance_too_high_w, unsafe_interval, rest, ten_kappa, unit) : Bool
     buffer = buffer_ptr.to_slice(128)
     small_distance = distance_too_high_w - unit
     big_distance = distance_too_high_w + unit
diff --git a/src/float/printer/ieee.cr b/src/float/printer/ieee.cr
index cfe7bb15e454..db408d3e7913 100644
--- a/src/float/printer/ieee.cr
+++ b/src/float/printer/ieee.cr
@@ -50,35 +50,35 @@ module Float::Printer::IEEE
   DENORMAL_EXPONENT_32         = -EXPONENT_BIAS_32 + 1
   SIGN_MASK_32                 = 0x80000000_u32
 
-  def to_uint(v : Float64)
+  def to_uint(v : Float64) : UInt64
     v.unsafe_as(UInt64)
   end
 
-  def to_uint(v : Float32)
+  def to_uint(v : Float32) : UInt32
     v.unsafe_as(UInt32)
   end
 
-  def sign(d64 : UInt64)
+  def sign(d64 : UInt64) : Int32
     (d64 & SIGN_MASK_64) == 0 ? 1 : -1
   end
 
-  def sign(d32 : UInt32)
+  def sign(d32 : UInt32) : Int32
     (d32 & SIGN_MASK_32) == 0 ? 1 : -1
   end
 
-  def special?(d64 : UInt64)
+  def special?(d64 : UInt64) : Bool
     (d64 & EXPONENT_MASK_64) == EXPONENT_MASK_64
   end
 
-  def special?(d32 : UInt32)
+  def special?(d32 : UInt32) : Bool
     (d32 & EXPONENT_MASK_32) == EXPONENT_MASK_32
   end
 
-  def inf?(d64 : UInt64)
+  def inf?(d64 : UInt64) : Bool
     special?(d64) && (d64 & SIGNIFICAND_MASK_64 == 0)
   end
 
-  def inf?(d32 : UInt32)
+  def inf?(d32 : UInt32) : Bool
     special?(d32) && (d32 & SIGNIFICAND_MASK_32 == 0)
   end
 
@@ -96,7 +96,7 @@ module Float::Printer::IEEE
   # exponent as *m_plus*.
   #
   # Precondition: the value encoded by this `Float` must be greater than 0.
-  def normalized_boundaries(v : Float64)
+  def normalized_boundaries(v : Float64) : {minus: DiyFP, plus: DiyFP}
     w = DiyFP.from_f(v)
     m_plus = DiyFP.new((w.frac << 1) + 1, w.exp - 1).normalize
 
@@ -124,7 +124,7 @@ module Float::Printer::IEEE
     return {minus: m_minus, plus: m_plus}
   end
 
-  def normalized_boundaries(v : Float32)
+  def normalized_boundaries(v : Float32) : {minus: DiyFP, plus: DiyFP}
     w = DiyFP.from_f(v)
     m_plus = DiyFP.new((w.frac << 1) + 1, w.exp - 1).normalize
 
@@ -144,7 +144,7 @@ module Float::Printer::IEEE
     return {minus: m_minus, plus: m_plus}
   end
 
-  def frac_and_exp(v : Float64)
+  def frac_and_exp(v : Float64) : {UInt64, Int32}
     d64 = to_uint(v)
 
     if (d64 & EXPONENT_MASK_64) == 0 # denormal float
@@ -158,7 +158,7 @@ module Float::Printer::IEEE
     {frac, exp}
   end
 
-  def frac_and_exp(v : Float32)
+  def frac_and_exp(v : Float32) : {UInt64, Int32}
     d32 = to_uint(v)
 
     if (d32 & EXPONENT_MASK_32) == 0 # denormal float
diff --git a/src/humanize.cr b/src/humanize.cr
index 9f2d87640400..85c6adc51c25 100644
--- a/src/humanize.cr
+++ b/src/humanize.cr
@@ -82,7 +82,7 @@ struct Number
   end
 
   # :nodoc:
-  def self.prefix_index(i, group = 3)
+  def self.prefix_index(i, group = 3) : Int32
     ((i - (i > 0 ? 1 : 0)) // group) * group
   end
 
diff --git a/src/int.cr b/src/int.cr
index 3e3781370fae..66f4438b6953 100644
--- a/src/int.cr
+++ b/src/int.cr
@@ -401,7 +401,7 @@ struct Int
   # 0b1101.bits_set?(0b0111) # => false
   # 0b1101.bits_set?(0b1100) # => true
   # ```
-  def bits_set?(mask)
+  def bits_set?(mask) : Bool
     (self & mask) == mask
   end
 
diff --git a/src/nil.cr b/src/nil.cr
index eb3ec05179b6..1f2d8a8378fa 100644
--- a/src/nil.cr
+++ b/src/nil.cr
@@ -63,7 +63,7 @@ struct Nil
   end
 
   # Returns `false`.
-  def same?(other : Reference)
+  def same?(other : Reference) : Bool
     false
   end
 
@@ -102,7 +102,7 @@ struct Nil
   # Raises `NilAssertionError`.
   #
   # See also: `Object#not_nil!`.
-  def not_nil!
+  def not_nil! : NoReturn
     raise NilAssertionError.new
   end
 
diff --git a/src/range.cr b/src/range.cr
index 4644e8fb6593..1f9d59303676 100644
--- a/src/range.cr
+++ b/src/range.cr
@@ -275,7 +275,7 @@ struct Range(B, E)
   # (1..10).excludes_end?  # => false
   # (1...10).excludes_end? # => true
   # ```
-  def excludes_end?
+  def excludes_end? : Bool
     @exclusive
   end
 
@@ -289,7 +289,7 @@ struct Range(B, E)
   # (1...10).includes?(9)  # => true
   # (1...10).includes?(10) # => false
   # ```
-  def includes?(value)
+  def includes?(value) : Bool
     begin_value = @begin
     end_value = @end
 
diff --git a/src/reference.cr b/src/reference.cr
index efaa818432e4..f57923e107dd 100644
--- a/src/reference.cr
+++ b/src/reference.cr
@@ -25,7 +25,7 @@ class Reference
 
   # Returns `true` if this reference is the same as *other*. This is only
   # `true` if this reference's `object_id` is the same as *other*'s.
-  def same?(other : Reference)
+  def same?(other : Reference) : Bool
     object_id == other.object_id
   end
 
diff --git a/src/time.cr b/src/time.cr
index b36d8a4cef1e..2e672042178a 100644
--- a/src/time.cr
+++ b/src/time.cr
@@ -549,7 +549,7 @@ struct Time
   # tokyo.inspect    # => "2019-01-01 00:00:00.0 +09:00 Asia/Tokyo"
   # new_york.inspect # => "2019-01-01 00:00:00.0 -05:00 America/New_York"
   # ```
-  def to_local_in(location : Location)
+  def to_local_in(location : Location) : Time
     local_seconds = offset_seconds
     local_seconds -= Time.zone_offset_at(local_seconds, location)
 
@@ -1118,7 +1118,7 @@ struct Time
   end
 
   # Parse time format specified by [RFC 3339](https://tools.ietf.org/html/rfc3339) ([ISO 8601](http://xml.coverpages.org/ISO-FDIS-8601.pdf) profile).
-  def self.parse_rfc3339(time : String)
+  def self.parse_rfc3339(time : String) : self
     Format::RFC_3339.parse(time)
   end
 
@@ -1139,7 +1139,7 @@ struct Time
   # ```
   #
   # 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
+  def to_rfc2822 : String
     Format::RFC_2822.format(to_utc)
   end
 
@@ -1154,7 +1154,7 @@ struct Time
   # Parse time format specified by [RFC 2822](https://www.ietf.org/rfc/rfc2822.txt).
   #
   # 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 self.parse_rfc2822(time : String)
+  def self.parse_rfc2822(time : String) : self
     Format::RFC_2822.parse(time)
   end
 
@@ -1393,12 +1393,12 @@ struct Time
   def_at_end(hour) { Time.local(year, month, day, hour, 59, 59, nanosecond: 999_999_999, location: location) }
 
   # Returns a copy of this `Time` representing the end of the minute.
-  def at_end_of_minute
+  def at_end_of_minute : Time
     Time.new(seconds: total_seconds - second + 59, nanoseconds: 999_999_999, location: location)
   end
 
   # Returns a copy of this `Time` representing the end of the second.
-  def at_end_of_second
+  def at_end_of_second : Time
     Time.new(seconds: total_seconds, nanoseconds: 999_999_999, location: location)
   end
 
diff --git a/src/time/format/custom/http_date.cr b/src/time/format/custom/http_date.cr
index 5482d4285e7c..70ca4dba7193 100644
--- a/src/time/format/custom/http_date.cr
+++ b/src/time/format/custom/http_date.cr
@@ -37,7 +37,7 @@ struct Time::Format
     # Formats a `Time` into a `String`.
     #
     # *time* is always converted to UTC.
-    def self.format(time : Time)
+    def self.format(time : Time) : String
       String.build do |io|
         format(time, io)
       end
@@ -45,7 +45,7 @@ struct Time::Format
   end
 
   struct Parser
-    def http_date
+    def http_date : Time::Location
       ansi_c_format = http_date_short_day_name_with_comma?
 
       if ansi_c_format
@@ -74,7 +74,7 @@ struct Time::Format
       time_zone_gmt_or_rfc2822
     end
 
-    def http_date_ansi_c
+    def http_date_ansi_c : Time::Location
       short_month_name
       whitespace
       day_of_month_blank_padded
@@ -94,7 +94,7 @@ struct Time::Format
       !ansi_c_format && current_char.ascii_whitespace?
     end
 
-    def http_date_short_day_name_with_comma?
+    def http_date_short_day_name_with_comma? : Bool?
       return unless current_char.ascii_letter?
 
       short_day_name
diff --git a/src/time/format/custom/iso_8601.cr b/src/time/format/custom/iso_8601.cr
index c015fd748b48..6d1331e2ee08 100644
--- a/src/time/format/custom/iso_8601.cr
+++ b/src/time/format/custom/iso_8601.cr
@@ -132,7 +132,7 @@ struct Time::Format
     end
 
     # Formats a `Time` into a `String`.
-    def self.format(time : Time)
+    def self.format(time : Time) : String
       String.build do |io|
         format(time, io)
       end
@@ -156,7 +156,7 @@ struct Time::Format
     end
 
     # Formats a `Time` into a `String`.
-    def self.format(time : Time)
+    def self.format(time : Time) : String
       String.build do |io|
         format(time, io)
       end
diff --git a/src/time/format/custom/rfc_2822.cr b/src/time/format/custom/rfc_2822.cr
index 24dc2ca26267..e2d81642f744 100644
--- a/src/time/format/custom/rfc_2822.cr
+++ b/src/time/format/custom/rfc_2822.cr
@@ -18,7 +18,7 @@ struct Time::Format
     end
 
     # Formats a `Time` into a `String`.
-    def self.format(time : Time)
+    def self.format(time : Time) : String
       String.build do |io|
         format(time, io)
       end
diff --git a/src/time/format/custom/rfc_3339.cr b/src/time/format/custom/rfc_3339.cr
index 7c16669aab9a..cacde53b2da6 100644
--- a/src/time/format/custom/rfc_3339.cr
+++ b/src/time/format/custom/rfc_3339.cr
@@ -16,7 +16,7 @@ struct Time::Format
     end
 
     # Formats a `Time` into a `String`.
-    def self.format(time : Time, fraction_digits = 0)
+    def self.format(time : Time, fraction_digits = 0) : String
       String.build do |io|
         format(time, io, fraction_digits: fraction_digits)
       end
diff --git a/src/time/format/custom/yaml_date.cr b/src/time/format/custom/yaml_date.cr
index 9ef4808a7a1d..636778e5ea1f 100644
--- a/src/time/format/custom/yaml_date.cr
+++ b/src/time/format/custom/yaml_date.cr
@@ -32,7 +32,7 @@ struct Time::Format
     end
 
     # Formats a `Time` into a `String`.
-    def self.format(time : Time)
+    def self.format(time : Time) : String
       String.build do |io|
         format(time, io)
       end
diff --git a/src/time/format/parser.cr b/src/time/format/parser.cr
index b29e13bc96c5..8e7d6ae400a5 100644
--- a/src/time/format/parser.cr
+++ b/src/time/format/parser.cr
@@ -37,7 +37,7 @@ struct Time::Format
       @nanosecond_offset = 0_i64
     end
 
-    def time(location : Location? = nil)
+    def time(location : Location? = nil) : Time
       if @hour_is_12
         if @hour > 12
           raise ArgumentError.new("Invalid hour for 12-hour clock")
diff --git a/src/time/location.cr b/src/time/location.cr
index 8a0f559a00f3..7e8c2ee8671d 100644
--- a/src/time/location.cr
+++ b/src/time/location.cr
@@ -223,7 +223,7 @@ class Time::Location
   # Creates a `Location` instance with fixed *offset* in seconds from UTC.
   #
   # The formatted *offset* is used as name.
-  def self.fixed(offset : Int32)
+  def self.fixed(offset : Int32) : self
     zone = Zone.new(nil, offset, false)
     new zone.name, [zone]
   end
diff --git a/src/time/location/loader.cr b/src/time/location/loader.cr
index d25789ce3305..fc780927be6d 100644
--- a/src/time/location/loader.cr
+++ b/src/time/location/loader.cr
@@ -12,21 +12,21 @@ class Time::Location
   end
 
   # :nodoc:
-  def self.load?(name : String, sources : Enumerable(String))
+  def self.load?(name : String, sources : Enumerable(String)) : Time::Location?
     if source = find_zoneinfo_file(name, sources)
       load_from_dir_or_zip(name, source)
     end
   end
 
   # :nodoc:
-  def self.load(name : String, sources : Enumerable(String))
+  def self.load(name : String, sources : Enumerable(String)) : Time::Location?
     if source = find_zoneinfo_file(name, sources)
       load_from_dir_or_zip(name, source) || raise InvalidLocationNameError.new(name, source)
     end
   end
 
   # :nodoc:
-  def self.load_from_dir_or_zip(name : String, source : String)
+  def self.load_from_dir_or_zip(name : String, source : String) : Time::Location?
     if source.ends_with?(".zip")
       open_file_cached(name, source) do |file|
         read_zip_file(name, file) do |io|
@@ -60,7 +60,7 @@ class Time::Location
   end
 
   # :nodoc:
-  def self.find_zoneinfo_file(name : String, sources : Enumerable(String))
+  def self.find_zoneinfo_file(name : String, sources : Enumerable(String)) : String?
     sources.each do |source|
       if source.ends_with?(".zip")
         path = source
@@ -77,7 +77,7 @@ class Time::Location
   # See https://data.iana.org/time-zones/tz-link.html, https://github.com/eggert/tz, tzfile(5)
 
   # :nodoc:
-  def self.read_zoneinfo(location_name : String, io : IO)
+  def self.read_zoneinfo(location_name : String, io : IO) : Time::Location
     raise InvalidTZDataError.new unless io.read_string(4) == "TZif"
 
     # 1-byte version, then 15 bytes of padding