T7558.hs:8:4: error:
    Couldn't match type ‘a’ with ‘Maybe a’
    ‘a’ is a rigid type variable bound by
    the type signature for:
      f :: forall a. T a a -> Bool
    at T7558.hs:7:6
    Inaccessible code in
      a pattern with constructor:
        MkT :: forall a b. (a ~ Maybe b) => a -> Maybe b -> T a b,
      in an equation for ‘f’
    In the pattern: MkT x y
    In an equation for ‘f’: f (MkT x y) = [x, y] `seq` True
    Relevant bindings include
      f :: T a a -> Bool (bound at T7558.hs:8:1)