tcfail065.hs:29:20:
    Couldn't match expected type ‘x’ with actual type ‘x1’
      ‘x1’ is a rigid type variable bound by
           the type signature for setX :: x1 -> X x -> X x
           at tcfail065.hs:29:3
      ‘x’ is a rigid type variable bound by
          the instance declaration at tcfail065.hs:28:10
    Relevant bindings include
      x :: x1 (bound at tcfail065.hs:29:8)
      setX :: x1 -> X x -> X x (bound at tcfail065.hs:29:3)
    In the first argument of ‘X’, namely ‘x’
    In the expression: X x