It's missing the killer feature though, which is the compiler warning you when you haven't done an exhaustive match. That is the magic which makes adding new values easy instead of hunting through code to find if you missed the new case anywhere.

I used to think that.... but actually, in practice, and I've been using them for a long time now, it doesn't make much difference, good tooling will generate the cases and help you find all instances pretty quick (I use Rider).

This is the kind of OG way to do it even before we had patterns. There are a couple of problems with it, least of which is that I can't then use one or both of those in a wider union elsewhere, because their definitions are bound to that parent class. Ideally I'd like to see something very similar to the linked OneOf library. That allows you to do both in-line definitions OR subclass from OneOf<T1,T2,T3...> to reify the union as a class as well. If that is done and integrated properly into the pattern matching system, I believe it will yield very powerful expressiveness.

My guess is roles will land first in C#13.

Is is kind of a pain particularly when working across Typescript projects. OneOf is cool, but it DOES NOT work well with null and thus optional parameters.

It's just like in Kotlin !