(From submission #776.)

Following up on the discussion of issue 2555, this example is ill-formed because the two declarations correspond:

  struct K {
   void f(this K);
   void f();
  };

However, this rather similar situation is not ill-formed; the two declarations are considered not to correspond:

  struct OK {
   void f(this OK);
   void f() &;
  };

Both situations should be treated the same (and should be ill-formed).