gadtSyntaxFail001.hs:9:5: error: [GHC-25709]
    • Data constructor ‘C2’ has existential type variables, a context, or a specialised result type
        C2 :: forall a. a -> Char -> Foo a Int
    • In the definition of data constructor ‘C2’
      In the data type declaration for ‘Foo’
    Suggested fixes:
       Perhaps you intended to use ExistentialQuantification
       Perhaps you intended to use GADTs