As previously noted, this backend uses the legacy zone.tab file, which only gives one territory for each entry, where zone1970.tab gives several. Until we switch to using the latter, we thus don't know all the territories in which an IANA ID is used. However, we do have such data from CLDR and deploy it in the default backend to filter the list of all available IDs. As a result, the TZ backend lacks some results of filtering by territory that the base-class would get for it, because the CLDR data associates more territories with some IDs. It implements the overload because its lookups in a hash are potentially more efficient than generating its full list in order to iterate it while intersecting with a CLDR-derived list of candidates. Leverage that hash also to do efficient intersecting with the CLDR-derived list, which lets us avoid duplicates almost for free, to make the subsequent union with the directly-derived entries from zone.tab efficient. This incidentally resolves a TODO about handling of QLocale::AnyTerritory. In the process, add more notes to the zone{1970,}.tab details. Restructure the base-class implementations of filtered available IDs to separate their generation of a CLDR-derived list of candidates from the intersection of that with the available IDs. For the territory overload, this lets the TZ backend share the CLDR-derived list; for symmetry, do the same to the offset-filter, too. (This also makes each function responsible for fewer things.) Similar treatment may also be needed for the ICU and chrono::tzdb backends, since the former also overloads the territory filter and both overload the offset filter. However, leave those for now (with comments added on the possibility) until we have evidence they actually do need this. Pick-to: 6.8 Task-number: QTBUG-130877 Task-number: QTBUG-64941 Change-Id: Ibb347df88dd14b55f8f580bb4c9e37e5c56a533a Reviewed-by: Mate Barany <mate.barany@qt.io>