[GHC] #7280: zonkQuantifiedTyVar panic

[GHC]

#7280: zonkQuantifiedTyVar panic
------------------------------+---------------------------------------------
 Reporter:  rl                |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.6.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
------------------------------+---------------------------------------------
 Small program (extracted from the dev version of vector):

 {{{
 {-# LANGUAGE Rank2Types, MultiParamTypeClasses, FlexibleContexts,
 TypeFamilies, ScopedTypeVariables #-}

 module T where

 type family Mutable (v :: * -> *) :: * -> * -> *
 class MVector (v :: * -> * -> *) a
 class MVector (Mutable v) a => Vector v a where
   copy :: Monad m => Mutable v s a -> v a -> m ()

 data Chunk v s a = Chunk (forall m. (Monad m, Vector v a) => Mutable v s a
 -> m ())

 vstep (v:vs) = Chunk (\mv -> copy mv v)
 }}}

 When I compile this, I get:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.6.1 for x86_64-unknown-linux):
         zonkQuantifiedTyVar f_afr{tv} [fsk]
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7280>
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