cmd/go: go.mod cannot be in drive root folder on Windows

[cwmos]

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

go version go1.17.8 windows/amd64

Does this issue reproduce with the latest release?

I believe this is the latest stable release.

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

go env Output

set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\XXX\AppData\Local\go-build
set GOENV=C:\Users\XXX\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\XXX\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\XXX\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.17.8
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=C:\go.mod
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\XXX\AppData\Local\Temp\go-build3973278424=/tmp/go-build -gno-record-gcc-switches

What did you do?

I placed my go.mod file in the root folder of a drive and placed my main package in a subfolder. I then ran "go build" in the subfolder.

What did you expect to see?

I expected my program to compile

What did you see instead?

The go compiler outputed this error:

directory . outside available modules

[bcmills]

@cwmos, it's possible that this may have been fixed by the change to str.TrimFilePathPrefix in CL 396694. Could you try a go built from head (see https://go.dev/doc/install/source) or gotip and check whether it still reproduces?

[cwmos]

I tried with gotip and there is still the same issue. The error message has changed to:

directory . outside main module or its selected dependencies

[mattn]

I'm trying this with HEAD.