Cannot implicitly convert derived type

[Jesse Phillips]

On Saturday, 22 February 2014 at 20:17:37 UTC, Frustrated wrote:
> I do this:
>
> @property WindowsButton button(WindowsButton b)
> {
>
> }
>
> The compiler turns this into
>
> @property WindowsButton button(iButton _b)
> {
>     if (is(_b : WindowsButton)) assert(0, "Rogue button used");
>     auto b = cast(WindowsButton)_b;
>
> }

Why does your WindowsGui violate the iGui contract?