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

$ go version
go version devel +da4d58587e Sat Dec 7 15:57:30 2019 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/myitcv/.cache/go-build"
GOENV="/home/myitcv/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/myitcv/gostuff"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/myitcv/gos"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/myitcv/gos/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build492256999=/tmp/go-build -gno-record-gcc-switches"

What did you do?

The docs for GOMOD state:

$ go help environment
...
        GOMOD
                The absolute path to the go.mod of the main module,
                or the empty string if not using modules.

For some time I have (incorrectly) been advising people that the way to determine if you are in a module context or not (i.e. modules not off and you are in the context of a go.mod) is by testing whether the output from go env GOMOD is the empty string or not.

However this fails when you consider:

$ cd $(mktemp -d)
$ GO111MODULE=on go env GOMOD
/dev/null

It's also not clear the docs are quite correct when it comes to modules being not off (i.e. auto or on):

$ cd $(mktemp -d)
$ go env GOMOD
 

(i.e. empty string, but we aren't not using modules, we are in the auto mode. Granted this is a bit less clearly incorrect)

I realise that this way go env GOMOD can be used to determine whether modules are off/not off, and whether you are in a module context or not assuming that they are not off. But documenting this a bit more clearly would, I think, help (selfishly me at least).

Because the actual logic for correctly determining that modules are not off and you are in a module context is:

cmd := exec.Command("go", "env", "GOMOD")
out, _ := cmd.CombinedOutput()
gomod := strings.TrimSpace(string(out))
if gomod != "" and gomod != os.DevNull {
   // ....

Thoughts?

cc @bcmills @jayconrod

FYI @mvdan @kortschak