scoped type variables in instance?

[Ross Paterson]

GHC (even without -fglasgow-exts) rejects the following:

	newtype Foo a = Foo a

	instance Eq a => Eq (Foo a) where
		Foo x == Foo y = bar
			where   bar :: a
				bar = undefined

It seems to treat the inner a as bound to the a in the instance header,
and thus not universally quantified.