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The following source code causes GHC 8.4.3 on 64bit Linux to panic:

{-# LANGUAGE TypeInType #-}

module Lib where

import Data.Kind

class Contravariant f where
    contramap :: (a -> b) -> f b -> f a

dimap :: (Contravariant (p :: * -> b -> p * b), Functor (p a)) => (z -> a) -> (b -> c) -> p a b -> p z c
dimap f g = contramap f . fmap g

I have no idea if it should compile or not, but it shouldn't do this:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for x86_64-unknown-linux):
	piResultTy
  k_a34u[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:947:35 in ghc:Type

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

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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