ghc panics on accessing a labeled field

GHC panics about a following code:

class Bar a where
	bar :: Foo a -> Foo a
	bar x = x { foo = [] }
	
data Foo a = Bar a => Foo { foo :: [a] }

GHC says:

Chasing modules from: Main.hs Compiling Main ( Main.hs, Main.o ) ghc: panic! (the `impossible' happened, GHC version 6.4.2): types/Type.lhs:(1107,0)-(1108,77): Non-exhaustive patterns in function zip_ty_env

Trac metadata

Trac field	Value
Version	6.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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