bug? alias this <-> opDispatch interaction

["Luís]

I'm experimenting with several designs and I think I might have 
hit a bug, but I wanted to confirm it. Is this a bug?

     class A
     {
         void opDispatch(string name, T...)(T msg) {}
     }

     class B : A
     {
         C obj;
         alias obj this;

         @disable void opDispatch(string name, T...)(T msg);
     }

     class C
     {
         void foo() {}
     }

     void main()
     {
         auto b = new B;
         b.foo();
     }

  Output:

     Error: no property 'foo' for type 'B'

Disabling the opDispatch that B inherits from A also seems to 
disable B's alias this to C. The @disable works well in isolation 
-- as does the alias this, of course.

My current workaround:

     class B : A
     {
         C obj;

         void opDispatch(string name, T...)(T msg)
         {
             mixin("obj." ~ name ~ "();");
         }
     }

(which now makes me wonder what actually are the differences 
between those two...)

BTW, I was asking in the IRC channel (I should have discovered 
that lovely place earlier!) about the following:

1:    T[string] foo;
2:    foo[""] = new T;
3:    foo[null] = new T;

Lines 2 and 3 seem to do the same, apparently. People seemed to 
agree that 3 should not be relied upon (should it?), but if I 
recall/understand it was not totally clear to people why 2 and 3 
*do* do the same thing, so feel free to clarify.