This is a corner case but it is suggested we call it out.

Fixes #11798.

Change-Id: I2ddb5b363cd2921666dbf03bbf98107697ca40e5
Reviewed-on: https://go-review.googlesource.com/12460Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>