cmd/go: go get path@version does not work

[jeanbza]

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

$ go version
go version go1.11.4 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/deklerk/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/deklerk/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lk/zs4m7sv12mq2vzk_wfn2tfvm00h16k/T/go-build633732581=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

cd /tmp/whatever && unset GOPATH && go get cloud.google.com/go@v0.30.0

What did you expect to see?

Get cloud.google.com/go@v0.30.0

What did you see instead?

$ cd /tmp/whatever && unset GOPATH && go get cloud.google.com/go@v0.30.0
go: cannot use path@version syntax in GOPATH mode

[bcmills]

Dup of #24250: should work in go1.12beta1 if you set GO111MODULE=on.

(If you unset GOPATH, it defaults to '$HOME/go`.)

[jeanbza]

Thanks!