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

[Kapendev]

On Monday, 14 September 2026 at 06:35:13 UTC, Meta wrote:
> ```d
> enum union NetworkPacket
> {
>     case Data(const(ubyte)[]),
>     case Ping(ulong),
>     case Reset(ushort),
>     case Heartbeat(),
>     case EndOfStream(),
> }
> ```
> ```d
> enum union ConfigValue
> {
>     bool,
>     long,
>     double,
>     string,
>     string[],
> }
> ```

Looks pretty good!
I didn't know how to feel about the commas, but they make sense 
if you are going to define structs inside the enum union. The 
main reasons I see making this part of the language is the first 
example above, the switch syntax, and it's a type that you often 
see in projects (like slices).

This DIP also tries to make a lot of different people happy. For 
example, I usually use sumtypes like the second enum union. 
Someone that is more familiar with languages like Rust probably 
prefers the first enum union. Both options exists and everyone 
has what they want.