env GOVCS='private:all,public:none'
! go get github.com/google/go-cmp
stderr '^go: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'

# other patterns work (for more patterns, see TestGOVCS)
env GOPRIVATE=
env GOVCS='github.com:svn|hg'
! go get github.com/google/go-cmp
stderr '^go: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'