Combine's FlatMap makes request eagerly

I am having an issue with Publishers.FlatMap where a request is sent upstream immediately upon subscription, even though my Subscriber has not made a request at that time.

I make a simple chain of operators, like so: (forgive the error mangling -- it's a reduction.)

let pub = URLSession.shared.dataTaskPublisher(for: URL(string: "")!)
                           .handleEvents(receiveOutput: { print($0.0.count) })
                           .mapError({ _ in NetworkError() })
                           .flatMap({ ($0.response as? HTTPURLResponse)?.statusCode == 200
                                      ? Result.Publisher($
                                      : Result.Publisher(NetworkError()) })

let sub = TimedSubscriber<Data, NetworkError>()

My custom subscriber makes a request of 1 in receive(subscription:), but only after a 2-second delay. You'd expect the output of that handleEvents to appear after 2 seconds, but it appears immediately. (Well, after the 200-ish milliseconds of the network request.)

My custom subscriber is like so: (sorry for the code dump.)

class TimedSubscriber<Input, Failure: Error>: Subscriber {
  let combineIdentifier = CombineIdentifier()
  private var subscription: Subscription?
  private let q = DispatchQueue(label: "timed-requests", qos: .background)

  init() {}
  deinit { subscription?.cancel() }

  func receive(subscription: Subscription) {
    self.subscription = subscription
    request(after: .seconds(2))

  func receive(_ input: Input) -> Subscribers.Demand {
    request(after: .seconds(2))
    return .none

  private func request(after waitTime: DispatchTimeInterval) {
    q.schedule(after: .init(.now() + waitTime)) {

  func receive(completion: Subscribers.Completion<Failure>) {}

I have found that if I use tryMap to perform effectively the same operation, I get the output of handleEvents at moment I expect it.
If I stick print() in there, I see that an unlimited request has been made at the moment of subscription. What's the logic of this? Or is it a bug? It seems like the wrong behaviour to me.


No replies, but since it looks like a bug to me, I reported it as FB7721569.

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