std.traits.ParameterIdentifierTuple producing empty results.

[Nick Treleaven]

On Monday, 1 April 2024 at 15:57:23 UTC, Carl Sturtivant wrote:
> As a practical matter right now, it would be good if parameter 
> names continue to be stored in the type of a function or 
> function pointer, as this is (so far as I have been able to 
> ascertain) the only way they are accessible in D. Right now I 
> am extracting them using CTFE!
>
> Again, as a practical matter, it would be good if he purist 
> view of function and function pointer types could be put aside 
> and `ParameterIdentifierTuple` be permitted to work on function 
> and function pointer types,

I don't think it's possible to make the `is(__parameters)` check 
work there without changing the language. Parameter names in a 
function type seem to be lost once that type is used to 
instantiate a template:

```d
void f(T)() {
     static if (is(T PT == __parameters))
         pragma(msg, PT);
}
void foo(int i, char c);
void main()
{
     static if (is(typeof(foo) PT == __parameters))
         pragma(msg, PT);

     f!(typeof(foo));
}
```
Output:
```
(int i, char c)
(int, char)
```
And that makes sense because otherwise a function type with 
different parameter names would need to instantiate a separate 
instance of the template in order to preserve the identifiers. So 
if that happened, function types with different identifiers would 
not always compare equal as types.