A proposal: Sumtypes

[IchorDev]

On Thursday, 8 February 2024 at 15:42:25 UTC, Richard (Rikki) 
Andrew Cattermole wrote:
> Yesterday I mentioned that I wasn't very happy with Walter's 
> design of sum types, at least as per his write-up in his DIP 
> repository.
> I have finally after two years written up an alternative to it, 
> that should cover everything you would expect from such a 
> language feature.
> There are also a couple of key differences with regards to the 
> tag and ABI that will make value type exceptions aka zero cost 
> exceptions work fairly fast.

I am pretty pleased with both of these DIPs.
The syntax of the sum types could be tweaked a little:

1. I think that the new keyword should be avoided:
`sumtype` => `enum union`/`case union` (or similar)

2. The `:none` in the sum type's declaration is really odd... it 
seems to reference itself from within its own declaration? Why 
not just use `void`?

3. Why comma-separation between members? These are a union-like 
type, use semicolons.

So, all in all my suggestions would look like this:
```d
case union Nullable(T){
	void none;
	T value;
}