cmd/go: run gives conflicting advice when using path@version syntax

[davecheney]

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

% go version
go version devel +97d5cb24b1 Sat Dec 22 09:37:04 2018 +0000 linux/amd64

Does this issue reproduce with the latest release?

unknown

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

go env Output

% go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/dfc/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH=""
GOPROXY=""
GORACE=""
GOROOT="/home/dfc/go"
GOTMPDIR=""
GOTOOLDIR="/home/dfc/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
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-build125128327=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Go 1.12beta1 (tip) does not permit @Version syntax unless modules explicitly enabled, even if GOPATH is unset

% unset GOPATH && go run -v github.com/davecheney/httpstat@1.0.0
package github.com/davecheney/httpstat@1.1.0: cannot use path@version syntax in GOPATH mode

What did you expect to see?

env GO111MODULE=on go run -v github.com/davecheney/httpstat@1.0.0

would download and build httpstat v1.0.0

What did you see instead?

% unset GOPATH && env GO111MODULE=on go run -v github.com/davecheney/httpstat@1.0.0
missing $GOPATH

Running without modules enabled tells me I must enable modules even thought I don't have GOPATH set. Running with modules enable tells me I must set GOPATH.

lucky(/tmp) % env GO111MODULE=on GOPATH=/tmp/ go run -v github.com/davecheney/httpstat@1.0.0
package github.com/davecheney/httpstat@1.0.0: can only use path@version syntax with 'go get'

Setting both tells me that this isn't supported.

a. Can go run path@version be supported? If not, why not?
b. Can the error messages be improved make it clearer sooner that the operation will not succeed once the correct permutation of environment variables is discovered.

[odeke-em]

Thank you for this report @davecheney!

Kindly paging @bcmills @rsc.

[jadekler]

Seems to not be working for me, either:

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

[ahmetb]

Hit this as well in #29575. Closing that as duplicate.

[rsc]

Only go get and module-manipulation commands take path@version.
go build, go run, etc do not.

[jadekler]

Go get does not take path@version for me. I've filed #29659, since that appears to be different from the go run case that this issue was describing. (happy to dedupe if you prefer the conversation to happen here)