Why does D not have generics?

[jmh530]

On Tuesday, 12 January 2021 at 14:45:36 UTC, Adam D. Ruppe wrote:
> [snip]
>
> Yeah, in the D context since we obviously have generic 
> templates, I'd take it to mean the Java style.
>
> A Java generic doesn't generate new code for other types at 
> all. It is just a runtime class that works in terms of 
> interfaces, and when the compiler parameterizes it, it 
> basically just inserts static casts at the interface boundary 
> for you.
>
> The benefit of this is you have almost zero compile time cost 
> and runtime cost comparable to any other class. It avoids 
> template bloat in codegen that can be very significant in D.
>
> I'd love to have it as an option in D as well. There's a lot of 
> types that can use identical runtime code and just change 
> types. Not just classes, but like integer types too can be 
> identical and merged, const/immutable/shared/etc can be 
> identical and merged, and even other things with cast(void*) 
> can do it.
> [snip]

Not just classes? Could you explain a bit more about what you are 
thinking in that paragraph? If it's not a class, then are you 
talking about a struct? As in something like below. Does the same 
approach work when it's not a class?

generic Number(T)
     if (isNumber!T)
{
     struct Number {
         T x;
     }
}