Krzysztof Gogolewski authored Nov 21, 2021 and Marge Bot committed Nov 23, 2021

Previously, it was an error to pattern match on a GADT
without GADTs or TypeFamilies.
This is now allowed. Instead, we check the flag MonoLocalBinds;
if it is not enabled, we issue a warning, controlled by -Wgadt-mono-local-binds.

Also fixes #20485: pattern synonyms are now checked too.