[GHC] #9106: GHC Panic related to functional dependencies - kindFunResult

[GHC]

#9106: GHC Panic related to functional dependencies - kindFunResult
----------------------------+---------------------------------------
       Reporter:  yuriy     |             Owner:
           Type:  bug       |            Status:  new
       Priority:  normal    |         Milestone:
      Component:  Compiler  |           Version:  7.8.2
       Keywords:            |  Operating System:  Windows
   Architecture:  x86       |   Type of failure:  Compile-time crash
     Difficulty:  Unknown   |         Test Case:
     Blocked By:            |          Blocking:
Related Tickets:            |
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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.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9106>
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