{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Oops where

pattern What = True :: Maybe

Fails on 8.4.3 with:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for x86_64-unknown-linux):
	isUnliftedType
  (Maybe |> {co_a1Dg3}) :: TYPE t_a1Dg9[tau:1]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1939:10 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

I'm currently cloning from git, and will try to get around to check it with HEAD over the next days. BTW, it's not a show-stopper for me.