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Collection downcasts in cast patterns are now supported

Available from Swift 5.8

Paul Hudson      @twostraws

This resolves another small but potentially annoying inconsistency in Swift where downcasting a collection – e.g. casting an array of ClassA to an array of another type that inherits from ClassA – would not be allowed in some circumstances.

For example, this code is now valid in Swift 5.8, whereas it would not have worked previously:

class Pet { }
class Dog: Pet {
    func bark() { print("Woof!") }
}

func bark(using pets: [Pet]) {
    switch pets {
    case let pets as [Dog]:
        for pet in pets {
            pet.bark()
        }
    default:
        print("No barking today.")
    }
}

Before Swift 5.8 that would have led to the error message, “Collection downcast in cast pattern is not implemented; use an explicit downcast to '[Dog]' instead.” In practice, syntax such as if let dogs = pets as? [Dog] { worked just fine, so I would imagine that error was rarely seen. However, this change does mean another language inconsistency is resolved, which is always welcome.

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Other changes in Swift 5.8…

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