• Basile B. (36/40) Aug 19 2023 I've just read [this
• Tejas (12/17) Aug 21 2023 The only way to enforce semantics for `C++` and `D` would be via

Basile B. <b2.temp gmx.com> writes:

I've just read [this 
article](http://bannalia.blogspot.com/2023/08/user-defined-class-qua
ifiers-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

Tejas <notrealemail gmail.com> writes:

On Saturday, 19 August 2023 at 19:56:54 UTC, Basile B. wrote:

 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


The only way to enforce semantics for `C++` and `D` would be via 
creating a compiler plugin that enforces the semantics, I think.

The compiler can't perform the necessary semantics analysis if 
the logic for that is not present in it. The languages aren't 
even interpreted, such that the code to modify the behavior 
towards certain types/qualifiers could be presented on-the-fly 
and the interpreter/VM could adapt dynamically.


One can write C++ plugins, atleast for clang, using 
https://clang.llvm.org/docs/Tooling.html#clang-plugins

For D plugins, atleast for LDC, we most likely now have 
https://forum.dlang.org/post/clelngkqvihzybpxgldq forum.dlang.org