path/filepath: Rel() fails for UNC volumes when trailing slash is omitted #41230
Description
What version of Go are you using (go version)?
$ go version go version go1.15.1 windows/amd64
What operating system and processor architecture are you using (go env)?
go env Output
$ go env set GO111MODULE= set GOARCH=amd64 set GOBIN= set GOCACHE=C:\Users\micro\AppData\Local\go-build set GOENV=C:\Users\micro\AppData\Roaming\go\env set GOEXE=.exe set GOFLAGS= set GOHOSTARCH=amd64 set GOHOSTOS=windows set GOINSECURE= set GOMODCACHE=C:\Users\micro\go\pkg\mod set GONOPROXY= set GONOSUMDB= set GOOS=windows set GOPATH=C:\Users\micro\go set GOPRIVATE= set GOPROXY=https://proxy.golang.org,direct set GOROOT=C:\Go set GOSUMDB=sum.golang.org set GOTMPDIR= set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64 set GCCGO=gccgo set AR=ar set CC=gcc set CXX=g++ set CGO_ENABLED=1 set GOMOD= set CGO_CFLAGS=-g -O2 set CGO_CPPFLAGS= set CGO_CXXFLAGS=-g -O2 set CGO_FFLAGS=-g -O2 set CGO_LDFLAGS=-g -O2 set PKG_CONFIG=pkg-config set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\micro\AppData\Local\Temp\go-build149210574=/tmp/go-build -gno-record-gcc-switches
What did you do?
package main
import (
"fmt"
"path/filepath"
)
func main() {
fmt.Println(filepath.VolumeName(`\\host\share`))
fmt.Println(filepath.VolumeName(`\\host\share\`))
rel, err := filepath.Rel(`\\host\share`, `\\host\share\file.txt`)
fmt.Printf("rel: %v\n", rel)
fmt.Printf("err: %v\n", err)
rel, err = filepath.Rel(`\\host\share\`, `\\host\share\file.txt`)
fmt.Printf("rel: %v\n", rel)
fmt.Printf("err: %v\n", err)
}\\host\share
\\host\share
rel:
err: Rel: can't make \\host\share\file.txt relative to \\host\share
rel: file.txt
err: <nil>
What did you expect to see?
Expected both invocations of filepath.Rel to return the same relative path
What did you see instead?
First invocation returns an error