[GHC] #8566: Panic with kindFunResult

[GHC]

#8566: Panic with kindFunResult
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       Reporter:  dreixel           |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 The following program:
 {{{
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}

 module Bug where

 data U (s :: *) = forall k. AA k [U s]

 data I (u :: U *) (r :: [*]) :: * where
   A :: I (AA t as) r

 -- fs unused, but needs to be present for the bug
 class C (u :: U *) (r :: [*]) (fs :: [*]) where
   c :: I u r -> I u r

 instance (C (AA (t (I a ps)) as) ps fs) => C (AA t (a ': as)) ps fs where
   c A = c undefined
 }}}

 crashes a fresh copy of GHC HEAD with the following:
 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.7.20131126 for i386-unknown-linux):
         kindFunResult k1{tv a24f} [ssk]
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8566>
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