T12427a.hs:17:29: error:
    • Couldn't match expected type ‘p’
                  with actual type ‘(forall b. [b] -> [b]) -> Int’
      ‘p’ is a rigid type variable bound by
        the inferred type of h11 :: T -> p
        at T12427a.hs:17:1-29
    • In the expression: v
      In a case alternative: T1 _ v -> v
      In the expression: case y of { T1 _ v -> v }
    • Relevant bindings include
        h11 :: T -> p (bound at T12427a.hs:17:1)

T12427a.hs:28:6: error:
    • Cannot instantiate unification variable ‘p0’
      with a type involving polytypes: (forall b. [b] -> [b]) -> Int
        GHC doesn't yet support impredicative polymorphism
    • In the pattern: T1 _ x1
      In a pattern binding: T1 _ x1 = undefined