public void GroupBy_must_work_with_empty_input_stream()
{
this.AssertAllStagesStopped(() =>
{
var publisher =
Source.From(new List<int>())
.GroupBy(2, x => x%2)
.Lift(x => x%2)
.RunWith(Sink.AsPublisher<Tuple<int, Source<int, NotUsed>>>(false), Materializer);
var subscriber = TestSubscriber.CreateManualProbe<Tuple<int, Source<int, NotUsed>>>(this);
publisher.Subscribe(subscriber);
subscriber.ExpectSubscriptionAndComplete();
}, Materializer);
}