SPECIALIZE causes panic

the following code snippet makes GHC (6.6) panic:

delta' :: Eq a => a -> a -> b -> b -> b
delta' x y e f = if (x==y) then f else e
{-# SPECIALIZE delta' :: Num b => Int -> Int -> b -> b -> b #-} 		

delta :: (Eq a, Num b) => a -> a -> b
delta x y = delta' x y 0 1
{-# SPECIALIZE delta :: Num b => Int -> Int -> b #-} 		

the reply from GHC:

ghc-6.6: panic! (the 'impossible' happened)
  (GHC version 6.6 for i386-apple-darwin):
        Template variable unbound in rewrite rule $dNum{v a1bL} [lid]

it does not panic without the SPECIALIZE pragmas.

the version used is GHC 6.6, i386-apple-darwin.

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Operating system	Unknown
Architecture	Unknown