gadtSyntaxFail003.hs:7:5: error:
    Data constructor ‘C1’ has existential type variables, a context, or a specialised result type
      C1 :: forall b a c. a -> Int -> c -> Foo b a
      (Use ExistentialQuantification or GADTs to allow this)
    In the definition of data constructor ‘C1’
    In the data type declaration for ‘Foo’