const behaviour

[Namespace]

Based to the current const discussions (once again) I wanted to 
appease my curiosity and want to ask why the following code works 
as described in the comments:

[code]
import std.stdio;

class Bar { }

class Foo {
private:
        string _text;
        
        Bar _b;
        
public:
        this(string text, Bar b) {
                this._text = text;
                this._b = b;
        }
        
	// const_behaviour.d(18): Error: cannot implicitly convert 
expression (this._b) of type const(Bar) to const_behaviour.Bar
	Bar GetB() const pure nothrow { /// <- must be const(Bar) 
instead of Bar
                return this._b;
        }
        
	string GetText() const pure nothrow { /// <- no const(string) is 
neccessary. Why?
                return this._text;
        }
}

void main() {
        Bar b = new Bar();
        
        Foo f = new Foo("foobar", b);
}

[/code]