Allow Conditional Compilation Inside Enum Declaration

[IchorDev]

On Wednesday, 3 April 2024 at 17:57:31 UTC, Steven Schveighoffer 
wrote:
> I think what might be in order is to elevate enums to a more 
> structured type (like Swift does).
>
> Something like:
>
> ```d
> enum ??? foo {
>    case x;
>    case y;
>    case z;
>    // constructors?
>    this(int n) { this._value = cast(foo)n; }
>    // operators?
>    opBinary(...)
>    // static if works at a declaration scope!
>    static if(cond) {
>      case a;
>    }
> }
> ```
>
> The ??? is because I'm assuming it would be too ambiguous with 
> existing syntax. Maybe `enum struct`?
>
> But enums were basically copied from C (with duct-taped on 
> features), and other modern languages (such as swift) have 
> really considered how enums should be designed.
>
> -Steve

Swift's `enum` is a tagged union. I'd love if we got tagged 
unions, and the `enum struct` syntax isn't bad either. If D 
tagged unions could functionally replace enums (i.e. if they 
could have an arbitrary number of empty cases) then this would be 
a fine replacement in my opinion. Sadly, no such feature exists 
yet.