Alternatively (in particular for subclassing), it can always be worked around with an intermediate object. For example:

  Constructor(A a, B b) { … }

  Constructor(C c, D d)
  {
      // A and B can be derived from C and D, and
      // we want to forward to the (A, B) constructor,
      // so we use an intermediate object to do the
      // conversion and provide A and B:
      
      this(new Intermediate(c, d));
  }

  private Constructor(Intermediate x)
  {
      this(x.a, x.b);
  }

Of course, with the JEP this will become much simpler.