this compare not using opCmp?

[Era Scarecrow]

 In some code I'm working on, my asserted when I've confirmed it 
was correct. With the opCmp() overridden.  'this' refers to the 
current object, so why doesn't the first one succeed?

class X {
  int z;
  this(int xx) {
        z = xx;
  }

  override int opCmp(Object y){
        X x = cast(X) y;
        return z - x.z;
  }

  void something(X rhs) {
    assert(this.opCmp(rhs) == 0); //works fine.
    assert(this == rhs);          //fails?
  }
}

int main(){
        X a = new X(10);
        X b = new X(10); //different objects, same compare value
        a.something(b);
        return 0;
}