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Minimal example:

{-# LANGUAGE 
  MultiParamTypeClasses, DataKinds, FunctionalDependencies, TypeOperators,
  KindSignatures, PolyKinds, FlexibleInstances, FlexibleContexts, UndecidableInstances #-}

import GHC.TypeLits 

data Proxy (a :: k) = Proxy 

class FunctorN n f a fa | n f a -> fa where 
  fmapn :: Proxy n -> Proxy f -> (a -> a) -> fa -> fa

instance FunctorN 0 f a a where 
  fmapn _ _ a = a 

instance (Functor f, FunctorN (n - 1) f a fa) => FunctorN n f a (f fa) where 
  fmapn _ pf f = fmap (fmapn (Proxy :: Proxy (n-1)) pf f)

Crashes with ghc and ghci:

>ghc test
[1 of 1] Compiling Main             ( test.hs, test.o )
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 7.8.20140130 for x86_64-unknown-mingw32):
        kindFunResult k{tv azb} [sk]

>ghci test
GHCi, version 7.8.20140130: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( test.hs, interpreted )
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 7.8.20140130 for x86_64-unknown-mingw32):
        kindFunResult k{tv aPm} [sk]

Removing the functional dependency makes the code compile.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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