First Draft: Nominal Sum Types via `enum union` and `switch` Expressions

[Dukc]

On Wednesday, 23 September 2026 at 09:14:43 UTC, Dukc wrote:
>> I'm fine with that. If you think it's worthwhile, you will 
>> need to show me a very convincing example of why it's useful.
>
> ```d
> enum union DynamicNumber
> {   case long, case float, case double, case error(string);
> }
>
> @safe sum(DynamicNumber a, DunamicNumber b) => switch(a)
> {   case error(s) => a,
>     case numA => switch(b)
>     {   case error(s) => b,
>         case numB => numA + numB,
>     },
> };
> ```

Actually, even better, assuming tuple pattern matching will one 
day work with switch expressions:
```d
@safe sum(DynamicNumber a, DynamicNumber b) => switch(tuple(a, b))
{   case (error(s), _) => a,
     case (_, error(s)) => b,
     case (float aNum, float bNum) => float(aNum+bNum),
     case (aNum, bNum) => aNum+bNum,
};
```