path/filepath: regression in filepath.VolumeName

[ncw]

What version of Go are you using (go version)?

go version go1.21.4 linux/amd64

Does this issue reproduce with the latest release?

Yes (with the latest stable release).

What operating system and processor architecture are you using (go env)?

I've been cross compiling on Linux and running on my Windows 10 VM. This also fails on Github CI.

What did you do?

I compiled and ran this program

package main

import (
	"fmt"
	"path/filepath"
)

func main() {
	fmt.Println(filepath.VolumeName(`\\?\C:`))
}

What did you expect to see?

I expected it to print \\?\C: like it does under go1.21.3

What did you see instead?

However it prints \\? under go1.21.4

Discussion

I noticed this regression when github CI upgraded from go1.21.3 to go1.21.4

I used git bisect to discover the commit that breaks the tests is this one by @neild

9e933c1 - path/filepath: fix various issues in parsing Windows paths

I think \\?\C: is not a valid path in Windows, at least I get this under CMD

Z:\>dir \\?\C:
"\\?\C:" is not a recognized device.
The filename, directory name, or volume label syntax is incorrect.

This issue is probably only of academic interest since \\?\C: is not a valid Windows path, but I'm reporting it as a FYI - feel free to close!

[bcmills]

Duplicate of #64028