Panic whilst compiling syb due to OptCoercion

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

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire
Operating system
Architecture