cmd/go: non-top-level packages ending in "/vendor" are omitted from module zip files #37397

The following program fails in both go1.14rc1 and go1.13.7 darwin/amd64.

package main

import (
	"fmt"

	"github.com/bep/testmodlib"
	"github.com/bep/testmodlib/somepackage"
	"github.com/bep/testmodlib/somepackage/vendor"
	"github.com/bep/testmodlib/somepackage/vendors"
)

func main() {
	fmt.Println(testmodlib.Hello())
	fmt.Println(somepackage.Hello())
	fmt.Println(vendor.Hello())
	fmt.Println(vendors.Hello())
}

Fails with:

build github.com/bep/temp: cannot load github.com/bep/testmodlib/somepackage/vendor: module github.com/bep/testmodlib@latest found (v1.0.1), but does not contain package github.com/bep/testmodlib/somepackage/vendor

If I add a replace directive for that module:

replace github.com/bep/testmodlib => /Users/bep/dev/go/bep/testmodlib

It compiles/runs fine:

testmodlib
somepackage
vendor package
vendors package

I understand that the vendor top level folder in a module has a special meaning, but according to https://golang.org/ref/spec#Packages that package name should be fine -- and just deleting it sounds rather harsh.

I assume this is related to #31562 -- I have not read that one in detail, but I don't see how the conclusion can be correct.

• github.com/bep/testmodlib/somepackage/vendor seem to be a valid package with a replace, and I assume that the compiler works as expected here.
• Also, a module may contain non-Go folders (test resources etc. to make the test pass).