Parameterized inheritence issues, bug or ignorance?

[Kagamin via Digitalmars-d]

On Monday, 20 June 2016 at 00:01:58 UTC, Joerg Joergonson wrote:
> I have a gui based on the following classes:
>
> public class Widget { Widget Parent; }
> public class Item : Widget;
>
> public class Button(T : ButtonItem) : Widget { T[] Items; ... }
> public class ButtonItem : Item
> {
>  void Do() { auto parent = 
> (cast(Button!ButtonItem)this.Parent); ...}
>  ...
> }

What you ask for is called template covariance. D doesn't have it.

You can solve your problem by providing 
Button!ButtonItem-specific functionality in an intermediate 
ButtonBase class: https://dpaste.dzfl.pl/4c071b237aa0