var a = Array(repeating: 0, count: 10) let q = DispatchQueue(label: "array") let group = DispatchGroup() group.enter() for _i in a.indices { let i = _i DispatchQueue.global().async { q.sync { a[i] = i } group.leave() // EXC_BAD_INSTRUCTION } } group.wait() print(a) // [0, 1, ..., 9] と出て欲しい