C++ User-defined class qualifiers in D

[Basile B.]

I've just read [this 
article](http://bannalia.blogspot.com/2023/08/user-defined-class-qualifiers-in-c23.html) about C++23 user defined typequals.

Thanks to `alias this` D can do the same since a long time and 
without new language feature:

```d
import std;

enum Qual;

template Qualified(Q,T)
{
     @Q struct Qualified
     {
         T t;
         alias t this;
     }
}

struct S
{}

alias SQual = Qualified!(Qual,S);

void filterQual(T : S)(T t)
if (hasUDA!(T, Qual))
{}

void both(T : S)(T t)
{}

void main()
{
     S s;
     SQual sq;

     both(sq);
     both(s);
     filterQual(sq); // OK
     filterQual(s);  // NG
}
```

Not sure how (and if) the C++ version will gain more usability, 
however.
As note the author

> A more interesting challenge is the following: As laid out, 
> this technique implements syntactic qualifier subtyping, but 
> does not do anything towards enforcing the semantics associated 
> to each qualifier