alt Hacker NewsHmm, they seem to have chosen to avoid names to the choices in the union, joining C++ variants and (sort of) TypeScript unions: unions are effectively just defined by a collection of types.
Other languages have unions with named choices, where each name selects a type and the types are not necessarily all different. Rust, Haskell, Lean4, and even plain C unions are in this category (although plain C unions are not discriminated at all, so they’re not nearly as convenient).
I personally much prefer the latter design.
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Not sure, but I think C++ actually does allow std::variant with multiple choices using the same type. You might not be able to distinguish between them by type (using get<Type>()), but you can by position (get<0>(), get<1>(), ...)
The C and Rust union types are extremely sharp blades, enough so that I expect the average Rust beginner doesn't even know Rust has unions (and I assume you were thinking of Rust's enum not union)
I've seen exactly one Rust type which is actually a union, and it's a pretty good justification for the existence of this feature, but one isn't really enough. That type is MaybeUninit<T> which is a union of a T and the empty tuple. Very, very, clever and valuable, but I didn't run into any similarly good uses outside that.
Hi there! One of the C# language designers here, working on unions. We're interesting in both forms! We decided to go with this first as felt there was the most value here, and we could build the named form on top of this. In no way are we thinking the feature is done in C#15. But it's part of our ongoing evolution.
If you're interested, i can point you to specs i'm writing that address the area you care about :)