It went out of its way to look for implicit assignments
but never checked explicit assignments.

This detects the root bug for #12099.

Change-Id: I6a6e774cc38749ea8be7cfd58ba6421247b67000
Reviewed-on: https://go-review.googlesource.com/13646Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>