cmd/go: confusing error message 'post-v1 module path ".../v1"'

[hyangah]

$ export GO111MODULE=on
$ go mod tidy
go: github.com/hyangah/testmod1/[email protected]: red/go.mod has post-v1 module path "github.com/hyangah/testmod1/red/v1" at revision red/v1.0.0
go: error loading module requirements

I don't know how to interpret and act on the error message.
I didn't use the mentioned module path (ending with 'v1') anywhere in my go.mod files.

$ tree
.
├── blue
│   └── blue.go
├── go.mod
├── palette
│   └── main.go
├── purple
│   └── purple.go
└── red
    ├── go.mod
    └── red.go

$ cat go.mod
module github.com/hyangah/testmod1

require github.com/hyangah/testmod1/red v1.0.0

$ cat red/go.mod
module github.com/hyangah/testmod1/red

require github.com/hyangah/testmod1 v0.0.2

I was trying to move a package to a separate submodule in the same repo
following the package migration instruction found in #27858 (comment).

Started with the clone of upstream v0.0.1 that contains red and palette
packages in one single module, I created a single commit that moves the red
package to a separate module by creating a new red/go.mod file, and
updates the go.mod to require [email protected]. Then, tagged the commit with
red/v1.0.0 and also with v0.0.2.

The above error is what I got when I ran go tidy or go build before pushing
the commit and tags to origin.

I don't expect go tidy or go build to work anyway
because the tags are not yet pushed to upstream (that's another issue).
But the error message is not the kind of what I expected.

$ go version
go version devel +75f4aa86ba Thu Sep 27 22:02:08 2018 +0000 darwin/amd64
$ go env GOMOD
/Users/hakim/go/src/github.com/hyangah/testmod1/go.mod

[bcmills]

I'm not sure what caused this, but I'm unable to replicate it with go1.12beta2 (and there have been a lot of module-resolution fixes in the meantime).

Please do reopen (or start a new issue) if you see it again.