This case was uncovered by existing tests, so breakage went undetected.
Make sure it remains stable.

Extracted from a larger patch by
Author: David Rowley
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/CAKJS1f-aGCJ5H7_hiSs5PhWs6Obmj+vGARjGymqH1=o5PcrNnQ@mail.gmail.com