-O0 and pattern synonyms triggers panic in trimJoinCont
Running haddock on the `generics-mrsop` package results in
haddock: panic! (the 'impossible' happened) (GHC version 8.4.3 for x86_64-unknown-linux): completeCall fail_av9P Select nodup wild_00 Stop[BoringCtxt] Rep Singl (El FamRoseInt) (Lkup ix CodesRoseInt) Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable pprPanic, called at compiler/simplCore/Simplify.hs:1533:9 in ghc:Simplify
The same problem affects 8.6.1 and HEAD, but not 8.2.2.
The panic seems to stem from this catch-all in `trimJoinCont`.