go/build: wrong logic to avoid setting implicit GOPATH to GOROOT

[neild]

The go/build package will implicitly set $GOPATH to $HOME. It skips setting an implicit GOPATH if it would set GOPATH/go to GOROOT.

		def := filepath.Join(home, "go")
		if def == runtime.GOROOT() {
			// Don't set the default GOPATH to GOROOT,
			// as that will trigger warnings from the go tool.
			return ""
		}

https://go.googlesource.com/go/+/master/src/go/build/build.go#272

Note that the comment and the code disagree. I suspect the comment describes the intended behavior (don't set the default GOPATH to GOROOT), not the code.

Discovered when testing something with a GOROOT of ~/go:

# /usr/bin/go: All is good.
$ go test .
?       github.com/neild/foo    [no test files]

# ~/go/bin/go: Unhappy. Huh?
$ ~/go/bin/go test .
missing $GOPATH

# Explicitly set GOPATH=$HOME: All is good.
$ GOPATH=$HOME ~/go/bin/go test .
?       github.com/neild/foo    [no test files]

# This is presumably the case the code is trying to avoid: Set GOPATH=GOROOT.
$ GOPATH=$HOME/go ~/go/bin/go test .
warning: GOPATH set to GOROOT (/usr/local/google/home/dneil/go) has no effect
?       github.com/neild/foo    [no test files]

[ianlancetaylor]

The default GOPATH isn't HOME, it's HOME/go (as documented at https://golang.org/cmd/go/#hdr-GOPATH_environment_variable). We avoid that default if HOME/go is already GOROOT. I think the code and comment are correct.

[neild]

My mistake; I misread what was being returned for GOPATH. You're absolutely right.