Failed to initialize Array with Range

[rayx (Huan Xiong)]

Hi, I guess I must be missing something, but couldn't figure out what exactly it is. Can anyone please explain why this fails to compile?

protocol Foo: BidirectionalCollection {
    var index: Index { get }

    func getIndices() -> [Index]
}

extension Foo {
    func getIndices() -> [Index] {
        Array(index..<endIndex)
        // Error: No exact matches in call to initializer
    }
}

If I do the similar on a concreate type (e.g. an array), it works fine.

let x: [Int] = [1, 2, 3, 4]
let y = Array(x.startIndex..<x.endIndex)

I think the initializer in question is this. I can't see there are any reason why it doesn't work in the first case.

[mayoff (Rob Mayoff)]

That Array initializer requires its argument to conform to Sequence.

Range conforms to Sequence conditionally:

Conforms when Bound conforms to Strideable and Bound.Stride conforms to SignedInteger .

You need to add those constraints to your getIndices method:

extension Foo {
    func getIndices() -> [Index] where Index: Strideable, Index.Stride: SignedInteger {
        Array(index..<endIndex)
    }
}

[rayx (Huan Xiong)]

Thanks!

[cukr]

By default Xcode is hiding the full error message, to get the notes you have to click on the issue navigator in the left pane (FB11369761)

[rayx (Huan Xiong)]

Thanks all for your reply. They are valuable. I probably would never notice them (the conditional conformance of Range and the full error message in navigation pane) if you didn't say it.