Bisected this to #17863 (https://golang.org/cl/33158/).

CC @shurcooL

Thanks for the report, I'll be able to investigate this fully tomorrow (and happy to send a CL).

From a brief look at this issue, it looks like cmd/go is still relying on the previous behavior of go/build, before #17863 was fixed, during the process of go geting relative import paths. It needs to be updated to take into account the post-#17863-fixed behavior of go/build.

Why not just roll back the incompatible change made for #17863?

CL https://golang.org/cl/42350 mentions this issue.

Why not just roll back the incompatible change made for #17863?

It's possible to do that, but it seems it would take more steps. That change was merged 2 months ago. If rolled back now, it would break x/tools/cmd/godoc tests. Since it's clear where the problem is, it's possible to fix this issue directly. But I can act differently if advised so.

The problem is that cmd/go relies on previous behavior of build.Import that was not tested. Specifically:

// If the path is a local import path naming a package that can be imported
// using a standard import path, the returned package will set p.ImportPath
// to that path.

In combination with:

// If an error occurs, Import returns a non-nil error and a non-nil
// *Package containing partial information.

In the case of local import path pointing to a directory that doesn't exist, that behavior was broken by the change in CL 33158.

A fix I've come up with is to bring back that previous behavior of build.Import and add tests for it. This can be done without re-introducing #17863 issue, so tests for that continue to pass.

I've sent CL 42350, @rsc, can you comment on how it looks?

For reference, here are all uses of FindOnly flag in GOROOT.

if build.IsLocalImport(arg) {
	bp, _ := cfg.BuildContext.ImportDir(filepath.Join(base.Cwd, arg), build.FindOnly)
	if bp.ImportPath != "" && bp.ImportPath != "." {
		arg = bp.ImportPath
	}
}

if build.IsLocalImport(arg) {
	cwd, _ := os.Getwd()
	bp, _ := cfg.BuildContext.ImportDir(filepath.Join(cwd, arg), build.FindOnly)
	if bp.ImportPath != "" && bp.ImportPath != "." {
		arg = bp.ImportPath
	}
}

@rsc, did you see my comment above? This is related to #19769 and CL https://go-review.googlesource.com/c/33158/#message-93b2fc6b74efcb9936bc119782ee3485ca897450.