gadtSyntaxFail001.hs:8:5: error:
    • 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 type declaration for ‘Foo’