go/types: improve error messages for unexpected ListExprs
This CL is a mix of CL 312149 and CL 314409, adding the
Checker.singleIndex method to provide better error messages when an
unexpected ListExpr is encountered.
Change-Id: I45d6de9b4dfc299dc2d356ca14d05c9191de818d
Reviewed-on: https://go-review.googlesource.com/c/go/+/314869
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>