Cannot implicitly convert derived type

[Eric Suen]

Generic?

"Frustrated" <c1514843 at drdrb.com> ...
> interface iGui
> {
> @property iButton button(ref iButton button);
> }
>
> class WindowsGui : iGui
> {
> WindowsButton _button;
>
> @property WindowsButton button(ref WindowsButton button)
> //@property iButton button(ref iButton button)
> {
> _button = button;
> return button;
> }
> }
>
> interface iButton { }
> class WindowsButton : iButton { }
>
>
> Should this not work?
>
> Error: class main.WindowsGui interface function 'iButton
> button(ref iButton
> button) @property' is not implemented
> Or by using the commented line:
>
> Error: cannot implicitly convert expression (button) of type
> main.iButton to main.WindowsButton
>
> 1. In the first case I override a property using a more derived
> type. This should work but doesn't. Seems D doens't support
> covariance properly?
>
> 2. In the second case, I can cast to make everything work. This
> seems wrong. Hence goto 1. WindowsGui is designed to only work
> with WindowsButton, say, and I should never have to use iButton
> in the WindowsGui class unless, maybe, I want to support
> non-windows buttons in the WindowsGui for some reason.
>
> Basically, because of the covariance issue I end up having to use
> a lot of casts. Hopefully theres some simple trick that won't
> pollute the code to make this work. I guess I could use a
> templated property with a generic type that is derivable from
> iButton to make it work?
>
> In some sense I can understand the error. If I'm using iGui then
> I have the option to use any button(since it is generic) but if
> iGui is a WindowsGui I'm not allowing this. The issue is, that I
> will have some type of dependency restrictions on the types.
>
> e.g.,
>
> iGui g = new WindowsGui;
>
> g.button = new LinuxButton; // ok but not ok!! (should result in
> an error in some way)
>
> Obviously I can cast and check the type and do all that. Just
> feels like the wrong way to go about it because it requires a lot
> of casting and polluting the code with checks that in general,
> should be unnecessary. Again: In the WindowsGui I want to use
> WindowsButton, not iButton because WindowsGui will never need any
> other type of button. iButton is too general to use in WindowsGui.
>
> (it would be cool if one could do something like
>
> class WindowsGui : iGui
>     iButton <= WindowsButton // constrains iButton to always be a
> WindowsButton. Checks/casts are automatically added by compiler
> when necessary
> {
>     // use WindowsButton here with proper covariance relations and
> checks/casts
> }
>
> Anyways, hopefully there is some single trick to get what I'm
> asking?