[Haskell-cafe] Rank-2 types and class constraints

Hi, Just out of curiosity (I cannot come up with a practical example): Why doesn't the following piece of code type check in GHC (with extensions)?

foo :: (forall a . (Eq a) => a) -> Integer foo = undefined

It seems like the type-class constraint is playing a decisive rôle here, since the following does check.

bar :: (forall a . a) -> Integer bar = undefined

TIA, Stefan