[GHC] #11232: Panic whilst compiling syb due to OptCoercion

[GHC]

#11232: Panic whilst compiling syb due to OptCoercion
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Here is the minimised test case with most definitions inlined.

 {{{

 module Data.Generics.Aliases where

 import Control.Monad
 import Data.Data

 mkMp :: ( MonadPlus m
         , Typeable a
         , Typeable b
         )
      => (b -> m b)
      -> a
      -> m a
 mkMp ext = unM (maybe (M (const mzero)) id (gcast (M ext)))

 newtype M m x = M { unM :: x -> m x }
 }}}

 Panics with `-O2` as follows,

 {{{
 ~/Documents/haskell/syb-0.6:ghc-7.11.20151214 Aliases.hs  -O2
 [1 of 1] Compiling Data.Generics.Aliases ( Aliases.hs, Aliases.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.11.20151214 for x86_64-apple-darwin):
         ASSERT failed! file compiler/types/OptCoercion.hs, line 234

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11232>
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