cmd/go: go vet to use -mod=vendor by default

[ialidzhikov]

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

$ go version
go version go1.12.7 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

What did you do?

What did you expect to see?

$ go vet github.com/ialidzhikov/gomod-example
go: finding github.com/ghodss/yaml v0.0.0-20180820084758-c7ce16629ff4
go: finding github.com/kr/pretty v0.1.0
go: finding gopkg.in/yaml.v2 v2.2.2
go: finding gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127

go vet does module resolution and requites network connectivity. Running with GOPROXY=off:

$ GOPROXY=off go vet github.com/ialidzhikov/gomod-example
go: gopkg.in/yaml.v2@v2.2.2: module lookup disabled by GOPROXY=off
go: github.com/kr/pretty@v0.1.0: module lookup disabled by GOPROXY=off
go: github.com/ghodss/yaml@v0.0.0-20180820084758-c7ce16629ff4: module lookup disabled by GOPROXY=off
go: gopkg.in/check.v1@v1.0.0-20180628173108-788fd7840127: module lookup disabled by GOPROXY=off
go: error loading module requirements

I would expect from a tools such as go vet that performs static checks to do not require module resolution/network connectivity.

What did you see instead?

[mvdan]

Duplicate of #27227? I'm pretty sure that go vet loads packages like other go commands.

[bcmills]

Duplicate of #27227