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Opening existential arguments to optional parameters

Available from Swift 5.8

Paul Hudson      @twostraws

SE-0375 extends a Swift 5.7 feature that allowed us to call generic functions using a protocol, fixing a small but annoying inconsistency: Swift 5.7 would not allow this behavior with optionals, whereas Swift 5.8 does.

For example, this code worked great in Swift 5.7, because it uses a non-optional T parameter:

func double<T: Numeric>(_ number: T) -> T {
    number * 2

let first = 1
let second = 2.0
let third: Float = 3

let numbers: [any Numeric] = [first, second, third]

for number in numbers {

In Swift 5.8, that same parameter can now be optional, like this:

func optionalDouble<T: Numeric>(_ number: T?) -> T {
    let numberToDouble = number ?? 0
    return  numberToDouble * 2

for number in numbers {

In Swift 5.7 that would have issued the rather baffling error message “Type 'any Numeric' cannot conform to 'Numeric’”, so it’s good to see this inconsistency resolved.

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