Why isn't this Typeable?

[David Feuer]

data Foo :: (forall a. a -> Maybe a) -> Type
Neither Foo nor Foo 'Just is Typeable. There seems to be a certain sense to excluding Foo proper, because it can't be decomposed with Fun. But why not Foo 'Just? Is there a fundamental reason, or is that largely an implementation artifact?
David FeuerWell-Typed, LLP
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