T7558.hs:8:4: error:
    • Occurs check: cannot construct the infinite type: a ~ Maybe a
      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)