See regression test for explanation.
I audited the code for similar issues and found none.
Many thanks to Daniel Morsing for providing a small
reproducible test case, a rarity for PTA bugs!
Fixesgolang/go#9002
LGTM=crawshaw
R=crawshaw
CC=daniel.morsing, golang-codereviews
https://golang.org/cl/163350043
Alan Donovan