`BYSETPOS` not fully supported

[minichma]

BYSETPOS seems to be supported only for certain frequencies, i.e. for YEARLY and for MONTHLY (see #21) but seemingly not for others.

E.g. the following test case fails:

RRULE:FREQ=HOURLY;BYMINUTE=0,10,20,30,40,50;BYSETPOS=-2,3;INTERVAL=2;COUNT=5
DTSTART:20241023T154000
INSTANCES:20241023T154000,20241023T172000,20241023T174000,20241023T192000,20241023T194000

[Edit] Could also easily get something wrong. The RFC says

Valid values are 1 to 366 or -366 to -1.

This seems to imply that no time-related BY rules may be applied, therefore the above example would be wrong. However, the following example doesn't seem to work either:

RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR;BYSETPOS=1,3;COUNT=3
DTSTART:20241023
Expected INSTANCES:20241023,20241028,20241030
Actual   INSTANCES:20241023,20241024,20241025

[minichma]

I assume this is a pretty tough one. If we find that we don't have the resources for implementing this any time near, we should probably list this limitation somewhere. Generally I think we should maintain a list of limitations in our docs, which I think we don't have at this time. Correct @winterz? Things that come to my mind:

• BYSETPOS is only supported for MONTHLY and YEARLY frequencies (BYSETPOS not fully supported #795)
• When used as a limit (rather than for expansion) negative values are not supported for BYMONTHDAY, BYYEARDAY. Any others? (FREQ=HOURLY;BYYEARDAY=-366 doesn't return any recurrences #791)
• FREQ=YEARLY, BYYEARDAY can't be combined with BYWEEKNO, BYMONTH or BYMONTHDAY (

  libical/src/libical/icalrecur.c

  Lines 2882 to 2888 in cfd401b

  	if (has_by_data(impl, ICAL_BY_YEAR_DAY)) {
  	/* We only support BYYEARDAY + BYDAY */
  	if (has_by_data(impl, ICAL_BY_WEEK_NO) ||
  	has_by_data(impl, ICAL_BY_MONTH) || has_by_data(impl, ICAL_BY_MONTH_DAY)) {
  	icalerror_set_errno(ICAL_UNIMPLEMENTED_ERROR);
  	return 0;
  	}

  )
• FREQ=YEARLY, BYWEEKNO can't be combined with BYYEARDAY, BYMONTH or BYMONTHDAY (

  libical/src/libical/icalrecur.c

  Lines 2928 to 2937 in cfd401b

  	} else if (has_by_data(impl, ICAL_BY_WEEK_NO)) {
  	int weekno, start_doy;
  	/* We only support BYWEEKNO + BYDAY */
  	if (has_by_data(impl, ICAL_BY_YEAR_DAY) ||
  	has_by_data(impl, ICAL_BY_MONTH_DAY) ||
  	(has_by_data(impl, ICAL_BY_MONTH) && !has_by_data(impl, ICAL_BY_DAY))) {
  	icalerror_set_errno(ICAL_UNIMPLEMENTED_ERROR);
  	return 0;
  	}

  )
• what else?