Find the bug in this DispatchGroup code

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Task

The function must run work for all items in parallel, collect the results in the original order and call completion once. Where's the bug?

func performAll<T>(_ items: [T],
                   work: @escaping (T, @escaping (T) -> Void) -> Void,
                   completion: @escaping ([T]) -> Void) {
    let group = DispatchGroup()
    let queue = DispatchQueue(label: "perform", attributes: .concurrent)
    var result = items

    group.notify(queue: .main) {
        completion(result)
    }

    for (index, item) in items.enumerated() {
        queue.async {
            group.enter()
            work(item) { response in
                queue.async(flags: .barrier) {
                    result[index] = response
                    group.leave()
                }
            }
        }
    }
}