Regression when switchMap is used together with asynchronous generator

[ky1vstar]

Here is minimal reproducible example for regression which I found during migration from rxdart 0.27.7 to 0.28.0.

Dart SDK version: 3.5.1

Source code

import 'package:rxdart/rxdart.dart';

void main(List<String> arguments) async {
  getStream().listen(print);
}

Stream<String> getStream() {
  final stream1 = Future.delayed(Duration(seconds: 1), () => "1").asStream();
  final stream2 = stream1.startWith("2");

  final stream3 = stream2.switchMap((value) async* {
    await for (final innerValue in BehaviorSubject.seeded(value)) {
      yield innerValue;
    }
  });

  return stream3;
}

Expected result (rxdart 0.27.7):

2
1

Actual result (rxdart 0.28.0):

2

[hoc081098]

possibly related to dart-lang/sdk#42717 (comment), dart-lang/sdk#56619

[drstranges]

The issue is not only with the asynchronous generator, but also with someFuture.asStream too.
Here is the test that will fail on 0.28.0 but succeed on 0.27.7

  testWidgets('switchMap regression test', (tester) async {

    const delay = Duration(seconds: 1);

    Future<String> someFuture(String value) async {
      await Future<void>.delayed(delay);
      return value;
    }

    final subject = PublishSubject<String>();
    final beforeSwitchMap = <String>[];
    final afterSwitchMap = <String>[];

    subject
        .doOnData(beforeSwitchMap.add)
        .switchMap(
          (value) => someFuture(value).asStream(),
        )
        .doOnData(afterSwitchMap.add)
        .listen((event) {
      debugPrint('event: $event');
    });

    await tester.pump(const Duration(seconds: 2));
    subject.add('1');
    await tester.pump(const Duration(seconds: 2));
    subject.add('2');
    // Delay before next value < [delay] will fail the test in 0.28.0
    await tester.pump(delay - const Duration(microseconds: 1));
    subject.add('3');
    await tester.pump(const Duration(seconds: 2));
    subject.add('4');
    await tester.pump(const Duration(seconds: 2));

    expect(beforeSwitchMap, ['1', '2', '3', '4']);

    // Next line will succeed in 0.27.7 but fail in 0.28.0
    // with the message:
    // Expected: ['1', '3', '4']
    // Actual: ['1']
    expect(afterSwitchMap, ['1', '3', '4']);
  });

I can't reproduce the same behavior in runtime. Only widget tests seem to be affected (26 of our tests are red now ).