path/filepath: Split on a path with more than 1 separator at the start doesn't work on Windows

[Blackmage89]

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

$ go version
go version go1.11.4 windows/amd64

Does this issue reproduce with the latest release?

I don't have a working installation of it at the moment.

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

go env Output

$ go env
set GOARCH=amd64

set GOBIN=

set GOCACHE=C:\Users\vittorio.parrella\AppData\Local\go-build

set GOEXE=.exe

set GOFLAGS=

set GOHOSTARCH=amd64

set GOHOSTOS=windows

set GOOS=windows

set GOPATH=c:\GOPATH\trunk\ETS2

set GOPROXY=

set GORACE=

set GOROOT=C:\Go

set GOTMPDIR=

set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64

set GCCGO=gccgo

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\VITTOR~1.PAR\AppData\Local\Temp\2\go-build213636594=/tmp/go-build -gno-record-gcc-switches

What did you do?

import (
   "fmt"
   "path"
   "path/filepath"
)

func main() {
	dir,file := filepath.Split(`C:\test\test.jpg`)
	fmt.Println("path: ", dir, " - file: ", file)

        // this is the problematic scenario
        // seems that if there is more than separator at the start, the split does not work
	dir,file = filepath.Split(`\\test/test.jpg`)
	fmt.Println("path: ", dir, " - file: ", file)
	dir,file = filepath.Split(`\test/test.jpg`)
	fmt.Println("path: ", dir, " - file: ", file)

	dir,file = filepath.Split(`//test/test.jpg`)
	fmt.Println("path: ", dir, " - file: ", file)

       // in fact, using path.Clean eliminates the problem
	dir,file = filepath.Split(path.Clean(`//test/test.jpg`))
	fmt.Println("path: ", dir, " - file: ", file)
}

What did you expect to see?

path:  C:\test\  - file:  test.jpg
path:  \\test/  - file:  test.jpg
path:  \test/  - file:  test.jpg
path:  ////test/  - file:  test.jpg
path:  /test/  - file:  test.jpg

What did you see instead?

path:  C:\test\  - file:  test.jpg
path:  \\test/test.jpg  - file:  
path:  \test/  - file:  test.jpg
path:  //test/  - file:  test.jpg
path:  /test/  - file:  test.jpg

Note:
On linux it seems that the issue does not manifest, the output is as expected (minus the absolute windows path which is ok): https://play.golang.org/p/9PlmyANLFAL

[Blackmage89]

Just checked again and i wrongly said that path.Clean works around the issue on the last scenario, it does not using "\\test/test.jpg", the result is the same as without path.Clean