optional has no effect(?)

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=2727

Reported by Martin Stjernholm mast@roxen.com

I didn't expect a type error in this program:

    class X
    {
      optional void foo();
      X x();
    }

class Y
{
  X x() {return Y();} // Line 9
}

int main() {}

In both 7.2 and 7.3, I get:

foo.pike:9:Wrong return type.
foo.pike:9:Expected: object(implements 65586)
foo.pike:9:Got     : object(is 65587)
foo.pike:9:Missing identifier "foo".
foo.pike:10:Class definition failed.
Pike: Failed to compile script:
Compilation failed.

I thought that the "optional" modifier was supposed to prevent exactly this situation.