Include offset-zone's ID in the available IDs for an offset

While it would be perverse for availableTimeZoneIds() to list all
supported UTC-offset zone IDs, it makes reasonably good sense for its
offset-specific overload to include the ID QTimeZone would use for the
relevant UTC-offset, since passing this ID to the ID-based constructor
will indeed get a zone with this offset. In particular, it already
does include the offset-zone's ID if there is an IANA UTC-offset zone
with the given offset; and its list is apt to be empty otherwise.

Only applies to IDs we would in fact accept, checked with
offsetFromUtcString() to match the QTZ constructor's check.

Change-Id: I77bb60b166c3d3af5824d84952e1e10a5d32a5ad
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qtimezone.cpp

@@ -1504,9 +1504,13 @@ QList<QByteArray> QTimeZone::availableTimeZoneIds(QLocale::Territory territory)
     Returns a list of all available IANA time zone IDs with a given standard
     time offset of \a offsetSeconds.
 
+    Where the given offset is supported, \c{QTimeZone(offsetSeconds).id()} is
+    included in the list, even if it is not an IANA ID. This only arises when
+    there is no IANA UTC-offset ID with the given offset.
+
     This method is only available when feature \c timezone is enabled.
 
-    \sa isTimeZoneIdAvailable()
+    \sa isTimeZoneIdAvailable(), QTimeZone(int)
 */
 
 QList<QByteArray> QTimeZone::availableTimeZoneIds(int offsetSeconds)

src/corelib/time/qtimezoneprivate.cpp

@@ -989,6 +989,12 @@ QList<QByteArray> QUtcTimeZonePrivate::availableTimeZoneIds(qint32 offsetSeconds
         }
         result << id.toByteArray();
     }
+    // CLDR only has round multiples of a quarter hour, and only some of
+    // those. For anything else, throw in the ID we would use for this offset
+    // (if we'd accept that ID).
+    QByteArray isoName = isoOffsetFormat(offsetSeconds, QTimeZone::ShortName).toUtf8();
+    if (offsetFromUtcString(isoName) == qint64(offsetSeconds) && !result.contains(isoName))
+        result << isoName;
     // Not guaranteed to be sorted, so sort:
     std::sort(result.begin(), result.end());
     // ### assuming no duplicates