T7220a.hs:17:6: error:
    • Could not deduce (C a b)
      from the context: (C a0 b, TF b ~ Y)
        bound by a type expected by the context:
                   forall b. (C a0 b, TF b ~ Y) => b
        at T7220a.hs:17:6-44
      Possible fix:
        add (C a b) to the context of
          a type expected by the context:
            forall b. (C a0 b, TF b ~ Y) => b
    • In the ambiguity check for ‘f’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the type signature: f :: (forall b. (C a b, TF b ~ Y) => b) -> X