Champion "Discriminated Unions"

[gafter]

• Proposal added
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed

See

• Proposal: Add completeness checking to pattern matching draft specification roslyn#188
• Discussion: Unions/closed hierarchies/ADTs, which one does C# 7.1+ need & what would the spec look like? #75
• https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-01-10.md
• Proposal: "Closed" type hierarchies #485

Design meetings

https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-08-31.md#discriminated-unions
https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-09-26.md#discriminated-unions
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-07-24.md#discriminated-unions

[gafter]

I wouldn't expect progress on this feature to precede records.

[DavidArno]

@gafter,

I've started writing a draft proposal around #75. Would you like me to continue with this, or did you have a proposal planned yourself?

[gafter]

@DavidArno I do not expect to invest any significant effort on this until we make progress on records.

[DavidArno]

@gafter,

OK, that's good to know. I'll carry on with my proposal then, but will take time over it as there's no rush.

[gafter]

/cc @agocke

[agocke]

I will be moving my proposal of "closed" type hierarchies from Roslyn to this repo shortly. I also think we should explore "real" discriminated unions and have some thoughts on that, but it's still much more of an exploratory phase for me.

However, I think I'm close to a reasonably complete proposal for closed and an analyzer-based prototype. I'll happily champion this feature, as well.