cannot cast

[Namespace]

Can anyone tell me, why the this code

[code]
module RefTest.Ref;

import std.stdio : writeln;
import std.conv : to, toImpl;

T const_cast(T : Object)(const T obj) {
	return cast(T) obj;
}

struct Ref(T : Object) {
private:
	 T _obj;

public:
	@disable
	this();// { }

	@disable
	this(typeof(null));// { }

	this(T obj) {
		assert(obj !is null, "Object is null!");

		this._obj = obj;
	}

	@property
	inout(T) access() inout {
		assert(this._obj !is null, "Access: Object is null!");

		return this._obj;
	}

	//alias access this; // dann kommt "Stackoverflow" oder 
"recursive expansion"
}

mixin template TRef(T : Object) {
	final Ref!(T) getRef(string file = __FILE__, size_t line = 
__LINE__) in {
		assert(this !is null, "Object is null! @ " ~ file ~ " in Line " 
~ to!(string)(line) ~ ".");
	} body {
		return Ref!(T)(this);
	}
	
	final Ref!(const T) getRef(string file = __FILE__, size_t line = 
__LINE__) const in {
		assert(this !is null, "Object is null! @ " ~ file ~ " in Line " 
~ to!(string)(line) ~ ".");
	} body {
		return Ref!(const T)(this);
	}

	U opCast(U : Object)() {
		return *(cast(U*) &this);
	}

	alias getRef this;
}

unittest {
	bool instanceof(T : Object, U : Object)(const Ref!U obj) {
		//return const_cast(obj.access).toString() == 
typeid(T).toString();
		const U o = obj.access;

		return const_cast(o).toString() == typeid(T).toString();
	}

	class A {
		mixin TRef!(A);
	}

	class B : A { }

	class C : B { }

	A a1 = new B();
	A a2 = new C();

	assert(instanceof!(A)(a1) == false);
	assert(instanceof!(B)(a1));
	assert(instanceof!(C)(a1) == false);

	writeln(a1);

	B b1 = cast(B) a1;

	writeln(b1);

	writeln();
}
[/code]

fails with:

[quote]
Fehler	3	instantiated from here: 
instanceof!(A,A)	D:\D\VisualD\Visual Studio 
2010\Projects\RefTest\RefTest\Ref.d	76
	
Fehler	2	Error: template instance RefTest.Ref.const_cast!(A) 
error instantiating	D:\D\VisualD\Visual Studio 
2010\Projects\RefTest\RefTest\Ref.d	62
	
Fehler	4	Error: template instance 
RefTest.Ref.__unittest1.instanceof!(A,A) error 
instantiating	D:\D\VisualD\Visual Studio 
2010\Projects\RefTest\RefTest\Ref.d	76	

Fehler	1	Error: function 
RefTest.Ref.__unittest1.A.TRef!(A).opCast!(A).opCast () is not 
callable using argument types ()	D:\D\VisualD\Visual Studio 
2010\Projects\RefTest\RefTest\Ref.d	7	
[/quote]

? Sounds like a bug.