gadtSyntaxFail001.hs:8:5:
    Data constructor ‘C2’ has existential type variables, a context, or a specialised result type
      C2 :: forall a. a -> Char -> Foo a Int
      (Use ExistentialQuantification or GADTs to allow this)
    In the definition of data constructor ‘C2’
    In the data declaration for ‘Foo’