Propose Pack Destructuring & Pack Splitting

[OguzYuuksel]

No description provided.

[PsychoH13]

Suggested change

Swift’s variadic generics story (SE‑0393 “Parameter Packs” and SE‑0404 “Pack Iteration”) unlocks powerful abstractions over arbitrary arities. However, two missing features—recursive decomposition of packs in patterns and split‑expansion of packs at call sites—force library authors into brittle workarounds like fixed‑arity overloads or type erasure. This proposal completes the variadic‑generic toolbox by introducing:

Swift’s variadic generics story ([SE‑0393 “Parameter Packs”](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0393-parameter-packs.md) and [SE‑0408 “Pack Iteration”](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0408-pack-iteration.md)) unlocks powerful abstractions over arbitrary arities. However, two missing features—recursive decomposition of packs in patterns and split‑expansion of packs at call sites—force library authors into brittle workarounds like fixed‑arity overloads or type erasure. This proposal completes the variadic‑generic toolbox by introducing:

[OguzYuuksel]

Apologies, @PsychoH13 . I unintentionally performed a force update without reviewing your suggestions. Could you please resubmit them?

[rjmccall]

Hi, Oguz. This is an interesting proposal. It's likely to have quite a complex implementation, though. Are you planning to work on that?

[OguzYuuksel]

Hi @rjmccall,
I’m not planning to work on the implementation right now, but if no one else is interested, I can take it on. That said, it would take quite a while (8-12 months), so I’m not sure if it’s worth waiting that long.

[rjmccall]

Well, I don't mind leaving this PR open if you're really planning to work on an implementation. I just want to make sure you aren't surprised if the proposal doesn't move forward in the meantime; we can't review it if there isn't at least a passable implementation.