1

Sorry the title is a mouthful.

I have a protocol that refines Collection with a new subscript that overloads subscript(position: Index) -> Element:

public protocol Foo: Collection {
    subscript(array: [Int]) -> Element { get set }
}

extension Foo where Index == Int {
    public subscript(array: [Int]) -> Element {
        get { self[array.count] }
        set { self[array.count] = newValue }
        //               ^
        // error: cannot convert value of type 'Int' to expected argument type '[Int]'
    }
}

It seems that it tried to call subscript(array: [Int]) instead of subscript(position: Index). It also seems to only happen with collection. For example, this following code works:

protocol A {
    subscript(index: Int) -> Bool { get set }
}

protocol B: A {
    subscript(array: [Int]) -> Bool { get set }
}

extension B {
    subscript(array: [Int]) -> Bool {
        get { self[array.count] }
        set { self[array.count] = newValue }
    }
}

Did I do some thing wrong in the first code snippet, or is there any way to work around it?

2

Yes. Notice that you're getting an error on the setter and not the getter, and notice the difference between the requirement on Collection and the requirement in your protocol A:

`Collection`
subscript(position: Self.Index) -> Self.Element { get }
                                                  ^^^
`A`
subscript(index: Int) -> Bool { get set }
                                ^^^^^^^

In other words, it's not calling what you think it should be calling because there's no such setter.

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