zonkQuantifiedTyVar panic

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]

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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