Problem with receiveOnly and classes

[Ghislain]

Hello,

I need to pass objects of a hierarchy between threads and I have some 
troubles.

The sample code below displays:
Unknown
B.fun()

I do not understand why an object of type A is fetched as a Variant, while 
a object of type B is received correctly.

Instead of 
        (A a){a.fun();}
I also tried 
        (immutable(A) a){a.fun();}
which resulted in 
core.exception.AssertError@/usr/include/d/dmd/phobos/std/variant.d(286): 
immutable(A)

Any idea?
I am using dmd 2.057 with linux.

Thanks

import std.stdio;
import std.concurrency;

class A{
        public:
                void fun()const{
                        writeln("A.fun()");
                }
};
class B : A{
        public:
                override void fun()const{
                        writeln("B.fun()");
                }
};

void producer(Tid t){   
        auto a = new immutable(A);
        auto b = new immutable(B);
        t.send(a);
        t.send(b);
}

void main(){
        auto t = spawn(&producer, thisTid);
        while(1){
                receive( 
                        (A a){a.fun();},
                        (Variant v){writeln("Unknown");}
                        );
        }
}