// *** before: ***
final class CounterOld {
    private let queue = DispatchQueue(label: "counter")
    private var value = 0

    func increment() { queue.sync { value += 1 } }
    var current: Int { queue.sync { value } }
    // nothing checked that every path went through the queue: one direct
    // read added later, and the race was back
}

// *** in version 5.5: ***
actor Counter {
    private var value = 0

    func increment() { value += 1 }      // inside the actor there is no await
    var current: Int { value }
}

let counter = Counter()
await counter.increment()
print(await counter.current)

// the compiler itself forbids touching the state of an actor from the
// outside without await, so the protection cannot be forgotten.
// A method that touches nothing of the actor is marked nonisolated