gadt7.hs:16:38: error:
    • Couldn't match expected type ‘r’ with actual type ‘r1’
        ‘r’ is untouchable
          inside the constraints: t ~ Int
          bound by a pattern with constructor: K :: T Int,
                   in a case alternative
          at gadt7.hs:16:33
      ‘r’ is a rigid type variable bound by
        the inferred type of i1b :: T t -> r1 -> r at gadt7.hs:16:1
      ‘r1’ is a rigid type variable bound by
        the inferred type of i1b :: T t -> r1 -> r at gadt7.hs:16:1
      Possible fix: add a type signature for ‘i1b’
    • In the expression: y1
      In a case alternative: K -> y1
      In the expression: case t1 of { K -> y1 }
    • Relevant bindings include
        y1 :: r1 (bound at gadt7.hs:16:16)
        y :: r1 (bound at gadt7.hs:16:7)
        i1b :: T t -> r1 -> r (bound at gadt7.hs:16:1)