[swift-evolution] Variadic generics discussion

[Austin Zheng]

Hello swift-evolution,

I put together a draft proposal for the variadic generics feature described
in "Completing Generics" as a major objective for Swift 3.x. It can be
found here:

https://github.com/austinzheng/swift-evolution/blob/az-variadic-generics/proposals/XXXX-variadic-generics.md

It adopts the syntax and semantics that are described in Completing
Generics, and attempts to remain as simple as possible while still being
useful for certain use cases (although I think there is still room to
simplify). The proposal contains sample implementations for four use cases:

- Arbitrary-arity 'zip' sequence
- Arbitrary-arity tuple comparison for equality
- Tuple splat/function application
- Multiple-arity Clojure-style 'map' function

There is a lot of scope for design refinements, and even for alternative
designs. With enhanced existentials, there was already an informal
consensus that the feature would involve composing some protocols and class
requirements, and placing constraints on the associated types, and most
everything else was working out the various implications of doing so.
That's not true for this feature.

In particular, I'm interested to see if there are similarly expressive
designs that use exclusively tuple-based patterns and no parameter packs. I
think Rust once considered a similar approach, although their proposal
ended up introducing a parameter-pack like construct for use with fn
application: https://github.com/rust-lang/rfcs/issues/376

Feedback would be greatly appreciated. Again, be unsparing.

Best,
Austin
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