Future<T> doWhile(executor, ...) {
  // When this return statement completes, the thread that called
  // doWhile will call propagateResultsTo.
  // But if the executor completes the continuation before propagate
  // completes, the futures chain builds instead of collapsing due
  // to isJustForContinuation.
  // When the iterations are fast enough and the thread pool large
  // enough this temporary chaining breaches the depth limit on
  // future_impl.h:438 
  return executor.schedule([&] {
    return doWhile(executor, ...);
  });
}