cannot cast

[Namespace]

I finished my Ref/NotNull struct, but i've got a problem:
If i try to cast the class, which should implicit convert to 
Ref!(Type) with alias this, i get the following error message:
"cannot cast a1.getRef("Ref.d",72u) of type Ref!(A) to type 
RefTest.Ref.__unittest1.B"

Can someone explain that to me or help me with it?
It seems that alias this replacement isn't smart enough to 
distinguish, if it has to convert.

[code]
import std.conv : to;

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; // print "Stackoverflow" or "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);
	}

	alias getRef this;
}

unittest {

	class A {
		mixin TRef!(A);
	}

	class B : A { }

	class C : B { }

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

	B b1 = cast(B) a1; // line 72
}