Sebastian Graf authored Sep 27, 2019 and Marge Bot committed Oct 08, 2019

The vanilla COMPLETE set is attached to the representation TyCon of a
data family instance, whereas the user-defined COMPLETE sets are
attached to the parent data family TyCon itself.

Previously, we weren't trying particularly hard to get back to the
representation TyCon to the parent data family TyCon, resulting in bugs
like #17207. Now we should do much better.

Fixes the original issue in #17207, but I found another related bug that
isn't so easy to fix.