cmd/go: 'get xxx@latest' does not report mismatch between module path and major version

[XuHuaiyu]

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

$ go version
go version go1.12 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="/Users/xuhuaiyu/Development/GOPATH/bin"
GOCACHE="/Users/xuhuaiyu/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/xuhuaiyu/Desktop/gopath"
GOPROXY=""
GORACE=""
GOROOT="/Users/xuhuaiyu/.gvm/gos/go1.12"
GOTMPDIR=""
GOTOOLDIR="/Users/xuhuaiyu/.gvm/gos/go1.12/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/xuhuaiyu/Desktop/go.mod"
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/ck/7tvksj1s7cl6djwl1j6kd67m0000gn/T/go-build099338216=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

$ GO111MODULE=on go get github.com/pingcap/tidb@latest

What did you expect to see?

github.com/pingcap/tidb v2.1.5

What did you see instead?

go: extracting github.com/pingcap/tidb v2.0.11+incompatible

[agnivade]

/cc @bcmills

[justinruggles]

I believe the issue is that the tidb module name does not include the /v2 to indicate major version 2

[bcmills]

@justinruggles has the right root cause, but we could certainly do a better job of diagnosing the problem.

CC @jayconrod

[jayconrod]

go release (#26420) will warn module authors about this before tagging new versions. I think that's the main solution for this.

go get could print a warning here for the latest semantic version if it has a go.mod file with a module path that doesn't match the semantic version.

In most cases though, when authors add go.mod files after a v2 version, they'll use a /v2 suffix. We shouldn't print an error message for that when looking for the latest version of the module without a suffix.