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