Fix #31368: joinpath works on collections of paths

base/path.jl

@@ -248,16 +248,17 @@ function splitpath(p::String)
     return out
 end
 
-joinpath(path::AbstractString)::String = path
-
 if Sys.iswindows()
 
-function joinpath(path::AbstractString, paths::AbstractString...)::String
-    result_drive, result_path = splitdrive(path)
+function joinpath(paths::Union{Tuple, AbstractVector{<:AbstractString}})::String
+    isempty(paths) && return path_separator
+    typeassert(paths[1], AbstractString)
+    result_drive, result_path = splitdrive(paths[1])
 
-    local p_drive, p_path
-    for p in paths
-        p_drive, p_path = splitdrive(p)
+    p_path = ""
+    for i in firstindex(paths)+1:lastindex(paths)
+        typeassert(paths[i], AbstractString)
+        p_drive, p_path = splitdrive(paths[i])
 
         if startswith(p_path, ('\\', '/'))
             # second path is absolute
@@ -293,8 +294,13 @@ end
 
 else
 
-function joinpath(path::AbstractString, paths::AbstractString...)::String
-    for p in paths
+function joinpath(paths::Union{Tuple, AbstractVector{<:AbstractString}})::String
+    isempty(paths) && return path_separator
+    typeassert(paths[1], AbstractString)
+    path = paths[1]
+    for i in firstindex(paths)+1:lastindex(paths)
+        p = paths[i]
+        typeassert(p, AbstractString)
         if isabspath(p)
             path = p
         elseif isempty(path) || path[end] == '/'
@@ -308,6 +314,28 @@ end
 
 end # os-test
 
+"""
+    joinpath(parts) -> String
+
+Join collection of path components into a full path. If some argument is an absolute path or
+(on Windows) has a drive specification that doesn't match the drive computed for
+the join of the preceding paths, then prior components are dropped.
+
+Note on Windows since there is a current directory for each drive, `joinpath("c:", "foo")`
+represents a path relative to the current directory on drive "c:" so this is equal to "c:foo",
+not "c:\\foo". Furthermore, `joinpath` treats this as a non-absolute path and ignores the drive
+letter casing, hence `joinpath("C:\\A","c:b") = "C:\\A\\b"`.
+
+# Examples
+```jldoctest
+julia> joinpath(["/home/myuser", "example.jl"])
+"/home/myuser/example.jl"
+julia> joinpath(())
+"$(path_separator)"
+```
+"""
+joinpath
+
 """
     joinpath(parts::AbstractString...) -> String
 
@@ -326,7 +354,8 @@ julia> joinpath("/home/myuser", "example.jl")
 "/home/myuser/example.jl"
 ```
 """
-joinpath
+joinpath(path::AbstractString)::String = path
+joinpath(path::AbstractString, paths::AbstractString...)::String = joinpath((path, paths...))
 
 """
     normpath(path::AbstractString) -> String

test/path.jl

@@ -42,6 +42,11 @@
         @test joinpath(S("foo"), S(homedir())) == homedir()
         @test joinpath(S(abspath("foo")), S(homedir())) == homedir()
 
+        @test joinpath(()) == sep
+        for str in map(S, [sep, "a$(sep)b", "a$(sep)b$(sep)c", "a$(sep)b$(sep)c$(sep)d"])
+            @test str == joinpath(splitpath(str))
+        end
+
         if Sys.iswindows()
             @test joinpath(S("foo"),S("bar:baz")) == "bar:baz"
             @test joinpath(S("C:"),S("foo"),S("D:"),S("bar")) == "D:bar"
@@ -58,6 +63,10 @@
             @test joinpath(S("\\\\server\\share"),S("a")) == "\\\\server\\share\\a"
             @test joinpath(S("\\\\server\\share\\"), S("a")) == "\\\\server\\share\\a"
 
+            for str in map(S, ["c:\\", "c:\\a", "c:\\a\\b", "c:\\a\\b\\c", "c:\\a\\b\\c\\d"])
+                @test str == joinpath(splitpath(str))
+            end
+
         elseif Sys.isunix()
             @test joinpath(S("foo"),S("bar:baz")) == "foo$(sep)bar:baz"
             @test joinpath(S("C:"),S("foo"),S("D:"),S("bar")) == "C:$(sep)foo$(sep)D:$(sep)bar"