enh(swift) add parameter pack keywords

[bradleymackey]

Parameter packs are supported in Swift 5.9

Their usage can be seen in SE-0393. This essentially just requires a promotion of each to a keyword and matching repeat and each as keywords in generic parameters.

Changes

• Support repeat and each parameter pack keywords.

Checklist

• Added markup tests, or they don't apply here because...
• Updated the changelog at CHANGES.md

[joshgoebel]

So what is repeat when used outside a generic?

[bradleymackey]

It's a general mechanism for destructuring and composing type/value packs, so it's supported in more contexts than just generic parameter lists. This is a good example from the proposal, the keyword can be used in the context of a parameter pack directly:

struct Pair<First, Second> {
  init(_ first: First, _ second: Second)
}

func makePairs<each First, each Second>(
  firsts first: repeat each First,
  seconds second: repeat each Second
) -> (repeat Pair<each First, each Second>) {
  return (repeat Pair(each first, each second))
}

let pairs = makePairs(firsts: 1, "hello", seconds: true, 1.0)
// 'pairs' is '(Pair(1, true), Pair("hello", 2.0))'

[joshgoebel]

Is it not a keyword in those other contexts?

[bradleymackey]

It is a keyword in those contexts as well, it's already in the "keywords" list though because it's already used for loops.

repeat {
    print(number)
    number += 1
} while number <= 20

[joshgoebel]

Ah that's what I was missing. 😀