Equality constraint causes ghc panic

I have the following module:

{-# LANGUAGE TypeFamilies #-}
module GhcBug where

newtype (a ~ b) => S a b = S { unS :: a }

Loading this in ghci results in this:

> :l GhcBug
[1 of 1] Compiling GhcBug           ( GhcBug.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.4 for x86_64-unknown-linux):
	typePrimRep <pred>a{tv a1MH} [tv] ~ b{tv a1MI} [tv]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Trac metadata

Trac field	Value
Version	6.10.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	brad.larsen@gmail.com
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Equality constraint causes ghc panic