Dynamic templated virtuals - I still often want them

[surlymoor]

On Thursday, 23 July 2020 at 02:38:27 UTC, Adam D. Ruppe wrote:
> You might know about D's template this parameters, which adapt 
> to the static type of a `this` reference on a call, or with the 
> curiously recurring template pattern, which passes a derived 
> class to the base class so the base class can inspect the 
> derived class.
>
> Both of these are useful at times, but neither quite do what I 
> have in mind here.
>
> Imagine this:
>
> ---
> class Base {
>      virtual string serialize(this This)() {
>            return This.stringof; // just for example
>      }
> }
>
> class Derived : Base {
>
> }
>
> void main() {
>     Base b = new Derived();
>     assert(b.serialize() == "Derived");
> }
> ---
>
> That does NOT work today. For one, of course, D has no 
> `virtual` keyword, but if you left that out, it would compile, 
> but fail the assert because the static type of `b` passed to 
> the template This is actually still `Base`.
>
> But imagine if `serialize` actually got a virtual table entry, 
> just like if it wasn't a template at all, but each child class 
> automatically got an instance of it made for itself.
>
> So it would be as if I wrote
>
> class Base {
>      string serialize() {
>            return Base.stringof;
>      }
> }
>
> class Derived : Base {
>      override string serialize() {
>            return Derived.stringof;
>      }
> }
>
> by hand. Of course, it is possible to do this kind of thing 
> with mixin templates or the CRTP, but in both cases, the 
> Derived class must actually write it in the child class, and if 
> you don't there's no way to really tell; it will still compile 
> and just use the base class implementation. Which might be OK 
> but it isn't perfect.
>
> It would just be cool if the base class template was 
> automatically instantiated again for the child class, while 
> still working like a normal virtual call.

Man, I'd really kill for this right about now. I was designing 
something that depended on the proposed behavior existing. My 
ignorance is my own fault, of course, but still this would be a 
very cool feature for D.