Fix JuliaLang#23646: relpath recognizes Windows drive letters

base/path.jl

@@ -501,6 +501,13 @@ function relpath(path::String, startpath::String = ".")
     path == startpath && return curdir
     path_arr  = split(abspath(path),      path_separator_re)
     start_arr = split(abspath(startpath), path_separator_re)
+    if Sys.iswindows()
+        if lowercase(path_arr[1]) != lowercase(start_arr[1]) # no relative paths between drives
+            return abspath(path)
+        elseif path_arr[1] != start_arr[1] # paths are case-sensitive but cannot go above drive's root directory
+            path_arr[1] = start_arr[1]
+        end
+    end
     i = 0
     while i < min(length(path_arr), length(start_arr))
         i += 1

test/path.jl

@@ -271,6 +271,20 @@
         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)