[GHC] #8893: -XPolyKinds causes "*** Exception: Prelude.(!!): index too large"

[GHC]

#8893: -XPolyKinds causes "*** Exception: Prelude.(!!): index too large"
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       Reporter:  ghorn            |             Owner:
           Type:  bug              |            Status:  new
       Priority:  normal           |         Milestone:
      Component:  Compiler         |           Version:  7.8.1-rc2
       Keywords:  PolyKinds        |  Operating System:  Unknown/Multiple
   Architecture:                   |   Type of failure:  Compile-time crash
  Unknown/Multiple                 |         Test Case:
     Difficulty:  Unknown          |          Blocking:
     Blocked By:                   |
Related Tickets:                   |
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 The following program will compile fine:
 {{{
 {-# OPTIONS_GHC -Wall #-}
 {-# Language DeriveFunctor #-}
 --{-# Language PolyKinds #-}

 module Bug where

 data V a = V [a] deriving Functor

 data C x a = C (V (P x a)) deriving Functor

 data P x a = P (x a) deriving Functor
 }}}

 But when PolyKinds is enabled, GHC crashes with

 {{{
 $ ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:9:37:ghc: panic! (the 'impossible' happened)
   (GHC version 7.8.0.20140228 for x86_64-unknown-linux):
         Prelude.(!!): index too large
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8893>
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