#4160: Panic! at the HEAD
-------------------------------+--------------------------------------------
    Reporter:  LouisWasserman  |       Owner:                    
        Type:  bug             |      Status:  new               
    Priority:  normal          |   Component:  Compiler          
     Version:  6.13            |    Keywords:                    
          Os:  Linux           |    Testcase:                    
Architecture:  x86             |     Failure:  Compile-time crash
-------------------------------+--------------------------------------------
 HEAD panics, but 6.12.1 doesn't, at the following module:

 {{{
 {-# LANGUAGE FlexibleContexts, MultiParamTypeClasses, TypeFamilies #-}
 module Foo where

 data P f g r = f r :*: g r
 type family TrieMapT (f :: * -> *) :: * -> (* -> *) -> * -> *
 newtype PMap m1 (m2 :: * -> (* -> *) -> * -> *) k (a :: * -> *) ix = PMap
 (m1 k (m2 k a) ix)
 type instance TrieMapT (P f g) = PMap (TrieMapT f) (TrieMapT g)

 class TrieKeyT f m where
         unionT :: (TrieMapT f ~ m) => (f k -> a ix -> a ix -> a ix) ->
                 m k a ix -> m k a ix -> m k a ix
         sizeT :: (TrieMapT f ~ m) => m k a ix -> Int

 instance (TrieKeyT f m1, TrieKeyT g m2) => TrieKeyT (P f g) (PMap m1 m2)
 where
         unionT f (PMap m1) (PMap m2) = PMap (uT  (\ a -> unionT (\ b -> f
 (a :*: b))) m1 m2)
                 where uT = unionT
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4160>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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