T14845_fail1.hs:10:13: error:
    • Data constructor ‘S’ cannot be used here
        (it has an unpromotable context ‘cls a’)
    • In the first argument of ‘Foo’, namely ‘(S :: Struct Eq a)’
      In the type ‘Foo (S :: Struct Eq a)’
      In the definition of data constructor ‘F’