getting instances for rrule yearly, 'every 2nd tuesday of January' fails

[GoldenSoju]

When requesting all instances for Rrule 'RRULE:FREQ=YEARLY;COUNT=4;INTERVAL=1;BYDAY=2TU;BYMONTH=1',
it fails with error message:

"The BYDAY rule part MUST NOT be specified with a numeric value when '
'the FREQ rule part is not set to MONTHLY or YEARLY." '
'— https://tools.ietf.org/html/rfc5545#section-3.3.10',

However, it seems to be a legit rrule. (Tested also with (https://jakubroztocil.github.io/rrule/) )

Error is in src/iteration/date_set_filtering.dart
Beginning from line 64 (_isFilteredByWeekDays);

if (rrule.frequency == Frequency.yearly && rrule.byMonths.isEmpty)
-> I think byMonths can be deleted here as this seems to be the reason. It works fine without it.
But I am not sure if there is a specific reason?

You can also test with below:

void main() {
  group('instancesAndRruleToRruleString', () {
    test('yearly every second Tuesday of January (for 4 years)', () {
      final rrule = RecurrenceRule(
          frequency: Frequency.yearly,
          count: 4,
          interval: 1,
          byMonths: {1},
          byWeekDays: {ByWeekDayEntry(DateTime.tuesday, 2)});

      final rruleString = rrule.toString();
      print('RRULE_STRING: $rruleString');

      final instances = rrule.getAllInstances(start: DateTime.now().toUtc());
      print('RRULE_INSTANCES: $instances');
      expect(rruleString,
          'RRULE:FREQ=YEARLY;COUNT=4;INTERVAL=1;BYDAY=2TU;BYMONTH=1');
    });
  });
}

[JonasWanke]

Thanks for all the details and the reproduction test! It turns out that the second part of the condition (rrule.byMonths.isEmpty) is relevant, but I forgot to handle the other case:

With the frequency set to yearly, something like 2TU means the second Tuesday of the year. However, when byMonths/BYMONTH is also specified (e.g., 1,4), 2TU means the second Tuesday of any of the specified months (e.g., the second Tuesday of January and the second Tuesday of April).

I fixed this in 8646af1 which is published as part of v0.2.4.