We must ensure that a Fetcher may be immediately destroyed after emplacing a value in the Fetcher::_completionPromise. This prevents deadlock since the Fetcher destructor calls shutdown which also acquires the Fetcher::_mutex.

Consider the following snippet:

{
  auto fetcher = makeFetcherSharedPointer(...);
  fetcher.onCompletion().thenRunOn(executorWhichRejectsWork).then([fetcher] {
    // this will never run
  });
}

An executor which rejects work (likely due to being shutdown) will never run the completion, and so the fetcher destructor will be run inline with the fulfillment of the future.