Fix relpath when path and startpath are in the same drive (#40323)

* Fix relpath when path and startpath are in the same drive

When startpath == ".", assume the startpath is in the same drive.
This subsequently required tweaking the existing logic to then
canonicalize the drive casing in instances where the drive casing
differs.

(cherry picked from commit 89fff18)

Author: musm

base/path.jl

@@ -514,12 +514,16 @@ function relpath(path::String, startpath::String = ".")
     curdir = "."
     pardir = ".."
     path == startpath && return curdir
-    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)
+        path_drive, path_without_drive = splitdrive(path)
+        startpath_drive, startpath_without_drive = splitdrive(startpath)
+        isempty(startpath_drive) && (startpath_drive = path_drive) # by default assume same as path drive
+        uppercase(path_drive) == uppercase(startpath_drive) || return abspath(path) # if drives differ return first path
+        path_arr  = split(abspath(path_drive * path_without_drive),      path_separator_re)
+        start_arr = split(abspath(path_drive * startpath_without_drive), path_separator_re)
+    else
+        path_arr  = split(abspath(path),      path_separator_re)
+        start_arr = split(abspath(startpath), path_separator_re)
     end
     i = 0
     while i < min(length(path_arr), length(start_arr))

test/path.jl

@@ -290,6 +290,10 @@
             # Additional cases
             @test_throws ArgumentError relpath(S("$(sep)home$(sep)user$(sep)dir_withendsep$(sep)"), "")
             @test_throws ArgumentError relpath(S(""), S("$(sep)home$(sep)user$(sep)dir_withendsep$(sep)"))
+
+            # issue 40237
+            path = "..$(sep)a$(sep)b$(sep)c"
+            @test relpath(abspath(path)) == path
         end
         test_relpath()
     end