Panic with zonkQuantifiedTyVar in GHC 7.6.1

In GHC 7.6.1, the following code:

{-# LANGUAGE TypeFamilies, GADTs, FlexibleContexts, MultiParamTypeClasses #-}

type family T :: * -> *

data E :: * -> * where
  E :: C p => T (T p) -> E p

class C' b a where c :: T b -> a

class C' (T a) a => C a

-- f :: C' (T p) a => E p -> a
f (E d) = c d

produces the following error message:

[1 of 1] Compiling Main             ( Test.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.6.1 for i386-apple-darwin):
	zonkQuantifiedTyVar
<<details unavailable>>

Uncommented the type signature allows it to compile.

The code above does not panic in HEAD, so I guess it's fixed. But I thought it would be good to report it anyway.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	leather@cs.uu.nl
Operating system
Architecture