Summary:
Commit b207b536 (#11714) happened to
fix #13603 as well. Let's add a regression test so that it stays fixed.

Test Plan: make test TEST=T13603

Reviewers: bgamari, austin, simonpj

Reviewed By: bgamari, simonpj

Subscribers: simonpj, rwbarton, thomie

GHC Trac Issues: #13603

Differential Revision: https://phabricator.haskell.org/D3489