[Haskell] Strange type inference

Hello! I'm observing a strange type inference result in the program at the end of the message. term1 and term2 have the same definitions, except for a type annotation for term2, but GHCi infers the following types: *Main> :t term1 term1 :: Term () *Main> :t term2 term2 :: forall a. Term a Could anyone please explain this? Thank you, / Emil ----------------------------------- data Term a = X class Constructive term a | term -> a where construct :: term -> Term a instance Constructive (Term a) a where construct = id term1 = construct X term2 :: Term a term2 = construct X