Comment by Metasyntactic

Hi there. One of the C# lang designers here.

You're correct. The unions we're working on right now are 'type unions'. So the type is inherent in the union distinction, and you would not be able to distinguish that case. That said, we're also looking at full blown discriminated unions (you can look at one of my proposals for that here: https://github.com/dotnet/csharplang/blob/main/meetings/work...), which would allow for that. Syntax entirely tbd, but you'd do something like:

  enum struct Either<T1, T2> // or enum class
  {
     First(T1 value),
     Second(T2 value)
  }

We view these features as complimentary. Indeed, if you look at the extended enum proposal, you'll see it builds on top of unions and closed types (another proposal coming in the next version of the lang).