ImmediateClock not performing work immediately

[rhysm94]

Converting this question from the Slack into a discussion here, for visibility.
I'm having issues with the use of an ImmediateClock not actually performing work immediately, at least when used with my fairly basic debouncer type.

For context, the Debouncer looks like this:

actor Debouncer {
  private var task: Task<Void, any Error>?

  init() {}

  func debounce<C: Clock>(
    clock: C,
    duration: C.Duration,
    _ perform: @escaping @Sendable () async -> Void
  ) {
    task?.cancel()

    self.task = Task {
      try await clock.sleep(for: duration)
      await perform()
    }
  }
}

When using this debouncer with an ImmediateClock, I'd expect it to effectively bypass debouncing anything, bypass the clock.sleep call, and just immediately call perform(). Maybe my intuition on this is wrong?

I've got to pass an actual TestClock to get this to work in tests, which means actually calling clock.advance. In the real app, it also works fine with a ContinuousClock or SuspendingClock.

I've attempted using withMainSerialExecutor with some Task.yield() in case the debouncer's Task was the problem, but that didn't seem to fix anything.

This isn't a blocker, as I have a workaround in just using a TestClock and advancing manually, but it'd be really appreciated if someone could at least understand and explain why ImmediateClock isn't working, well, immediately in this situation! If there's something I can change in how I'm using clocks, or something that's technically incorrect anywhere, I'd love to know of a fix!

The full sample code is here: https://github.com/rhysm94/ImmediateDebounceIssue