class context restrictions in GADT types not assumed

I think this should compile:

	class C a where
		f :: a -> Bool

	data T a where
		MkT :: (C a) => a -> T a

	tf1 :: T Int -> Bool
	tf1 (MkT aa) = f aa

	tf2 :: T a -> Bool
	tf2 (MkT aa) = f aa

tf1 does not give an error, but tf2 does. The type signature for tf2 
should not need a class context, just as there's no "C Int" instance.

ThingEncoding.hs:32:23:
    No instance for (C a)
      arising from use of `f' at ThingEncoding.hs:32:23
    Probable fix: Add (C a) to the type signature(s) for `tf2'
    In the definition of `tf2': tf2 (MkT aa) = f aa