First Draft: Nominal Sum Types via `enum union` and `switch` Expressions

[Dennis]

On Monday, 14 September 2026 at 06:35:13 UTC, Meta wrote:
> ## Rationale
>
> (...)
>
> Library solutions like `std.sumtype` implement tagged unions 
> via complex template metaprogramming. However, library 
> implementations suffer from slow compilation throughput, opaque 
> diagnostic errors, ...

Considering std.sumtype and your implementation both exist, can 
you give comparisons showcasing improved compile times and error 
messages?

> ... and the inability to exploit compiler memory layout 
> optimizations such as niche optimization.

If this is core rationale, why is "Niche Optimization" not 
implemented, and why couldn't a library sumtype do Niche 
Optimization? I feel like currently the opposite is true: library 
sum types are free to lay out the fields however they want, while 
this DIP's "In-Memory Layout" section bakes a strict layout 
scheme into the language.