[GHC] #9305: GHC panic when deriving Functor on a Fixed type

12 Jul 2014

      #9305: GHC panic when deriving Functor on a Fixed type
-----------------------------------+---------------------------------------
       Reporter:  andreyLevushkin  |             Owner:
           Type:  bug              |            Status:  new
       Priority:  normal           |         Milestone:
      Component:  Compiler         |           Version:  7.8.2
       Keywords:                   |  Operating System:  Linux
   Architecture:  x86_64 (amd64)   |   Type of failure:  Compile-time crash
     Difficulty:  Unknown          |         Test Case:
     Blocked By:                   |          Blocking:
Related Tickets:                   |
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 Building the following
 {{{
 #!haskell
 {-# LANGUAGE DeriveFunctor#-}
 module Main where

 data Event a b = Event a deriving (Functor)

 newtype F f = F (f (F f))

 data EventF a = EventF (F (Event a)) deriving (Functor)

 }}}

 results in
 {{{

 Main.hs:7:48:ghc: panic! (the 'impossible' happened)
   (GHC version 7.8.2 for x86_64-unknown-linux):
         Prelude.(!!): index too large

 }}}

 I am not sure if the above code should actually compile but it probably
 shouldn't panic.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9305
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