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

$ go version
go version go1.11.2 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/deklerk/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/deklerk/workspace/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="/tmp/module-testing-2/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/lk/zs4m7sv12mq2vzk_wfn2tfvm00h16k/T/go-build157331356=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I have a repo https://github.com/jadekler/module-testing-2 with a v1 and a v3. v3 lives in a separate branch.

I place https://github.com/jadekler/module-testing-2 at $GOPATH/src/github.com/jadekler/module-testing-2 and make v1 depend on v3 (see below), run go mod tidy, and go build .

My altered v1 lib.go:

import "github.com/jadekler/module-testing-2/v3"

const LibV1 = "LibV1"
var _ = v3.LibV3

My altered v1 go.mod:

module github.com/jadekler/module-testing-2

require github.com/jadekler/module-testing-2/v3 v3.0.0

What did you expect to see?

• go mod tidy works, and downloads v3.
• go build . works.

What did you see instead?

• go mod tidy works, and downloads v3.
• go build . gives error:

$ go build .
lib.go:3:8: cannot find package "github.com/jadekler/module-testing-2/v3" in any of:
	/usr/local/go/src/github.com/jadekler/module-testing-2/v3 (from $GOROOT)
	/Users/deklerk/workspace/go/src/github.com/jadekler/module-testing-2/v3 (from $GOPATH)

However, when I place https://github.com/jadekler/module-testing-2 in /tmp and repeat those steps, everything works. This makes me believe it's looking for v3 in my GOPATH (which it will never find, since it's in a branch). I suspect the subdirectory approach of modules does not have this problem.