T7558.hs:8:18: error:
    • Couldn't match expected type ‘a’ with actual type ‘Maybe a’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          f :: forall a. T a a -> Bool
        at T7558.hs:7:1-18
    • In the expression: y
      In the first argument of ‘seq’, namely ‘[x, y]’
      In the expression: [x, y] `seq` True
    • Relevant bindings include
        y :: Maybe a (bound at T7558.hs:8:10)
        x :: a (bound at T7558.hs:8:8)
        f :: T a a -> Bool (bound at T7558.hs:8:1)