Improvement to struct inheritance possible?

[Steven Schveighoffer]

On Thursday, 18 January 2024 at 07:10:49 UTC, Danilo wrote:
> The following code does not work, and I think it may be a bug 
> or could need an enhancement:
>
> ```d
> module app;
> import std;
>
> void main() {
>     auto x = new Derived(10, 20);
> }
>
> struct Base {
>     int a;
> }
>
> struct Derived
> {
>     Base base;
>     int b;
>     alias this = base;
>
>     @disable this();
>
>     // `a` is not available for use with constructor parameters.
>     //
>     this( typeof(a) _a, typeof(b) _b ) {
>         //
>         // inside the body of the constructor, `a` is available!
>         //
>         a = _a;
>         b = _b;
>
>         writeln( a, ", ", b );
>     }
> }
> ```
>
> The use of `typeof(a)` as a constructor parameter gives:
> Error: undefined identifier `a`
>
> Instead I have to explicitely use:
> ```d
> this( typeof(this.a) _a, typeof(b) _b )
> // or
> this( typeof(base.a) _a, typeof(b) _b )
> ```

Just FYI, it's kind of a special case that `typeof(this)` works 
in this context -- there is no "this" at declaration level, it 
only exists as a parameter to a function. D allows using 
typeof(this) to mean "the type of an instance" during 
declarations because it would be awful otherwise.

So I think you are playing around the edge of this "special case" 
and that's why it's not working.

I'm actually surprised `typeof(this.a)` works.

-Steve