Fix #23646: relpath recognizes Windows drive letters (#38259)

Author: belamenso

base/path.jl

@@ -495,15 +495,23 @@ contractuser(path::AbstractString)
 Return a relative filepath to `path` either from the current directory or from an optional
 start directory. This is a path computation: the filesystem is not accessed to confirm the
 existence or nature of `path` or `startpath`.
+
+On Windows, case sensitivity is applied to every part of the path except drive letters. If
+`path` and `startpath` refer to different drives, the absolute path of `path` is returned.
 """
 function relpath(path::String, startpath::String = ".")
     isempty(path) && throw(ArgumentError("`path` must be specified"))
     isempty(startpath) && throw(ArgumentError("`startpath` must be specified"))
     curdir = "."
     pardir = ".."
     path == startpath && return curdir
-    path_arr  = split(abspath(path),      path_separator_re)
-    start_arr = split(abspath(startpath), path_separator_re)
+    path_drive, path_without_drive = splitdrive(path)
+    startpath_drive, startpath_without_drive = splitdrive(startpath)
+    path_arr  = split(abspath(path_without_drive),      path_separator_re)
+    start_arr = split(abspath(startpath_without_drive), path_separator_re)
+    if Sys.iswindows()
+        lowercase(path_drive) != lowercase(startpath_drive) && return abspath(path)
+    end
     i = 0
     while i < min(length(path_arr), length(start_arr))
         i += 1

test/path.jl

@@ -262,6 +262,12 @@
                     res = relpath(filep, startp)
                     idx += 1
                     @test res == relpath_expected_results[idx]
+                    if Sys.iswindows()
+                        @test relpath("e:$filep", "e:$startp") == relpath_expected_results[idx]
+                        @test relpath("e:$filep", "E:$startp") == relpath_expected_results[idx]
+                        @test relpath("E:$filep", "e:$startp") == relpath_expected_results[idx]
+                        @test relpath("E:$filep", "E:$startp") == relpath_expected_results[idx]
+                    end
                 end
             end
             # Additional cases
@@ -271,6 +277,20 @@
         test_relpath()
     end
 
+    if Sys.iswindows()
+        @testset "issue #23646" begin
+            @test lowercase(relpath("E:\\a\\b", "C:\\c")) == "e:\\a\\b"
+            @test lowercase(relpath("E:\\a\\b", "c:\\c")) == "e:\\a\\b"
+            @test lowercase(relpath("e:\\a\\b", "C:\\c")) == "e:\\a\\b"
+            @test lowercase(relpath("e:\\a\\b", "c:\\c")) == "e:\\a\\b"
+
+            @test relpath("C:\\a\\b", "c:\\a\\b") == "."
+            @test relpath("c:\\a\\b", "C:\\a\\b") == "."
+            @test lowercase(relpath("C:\\a\\b", "c:\\c\\d")) == "..\\..\\a\\b"
+            @test lowercase(relpath("c:\\a\\b", "C:\\c\\d")) == "..\\..\\a\\b"
+        end
+    end
+
     @testset "type stability" begin
         @test isa(joinpath(S("a"), S("b")), String)
         @test isa(joinpath(S(abspath("a")), S("b")), String)