QTimeZone(qint32 offsetSeconds): use IANA ID when one is available

Principle of least surprise: prefer IANA IDs over synthesized ones.
This also aligns what id() returns more nearly with what
availableTimeZoneIds() reports. Amend some tests to match the new
behavior, extend one test to verify id-round-tripping (also for the
IANA zones) and another to verify single-digit offset IDs get
zero-padded.

Document the complications in how id() relates to what is passed to
the constructor. (It was already complicated; the present change just
aligns it better with IANA IDs, where possible.) Mention, in
availableTimeZoneIds(), that (and why) it only includes IANA's offset
IDs. Drive-by: fix a typo in another availableTimeZoneIds() overload's
doc.

[ChangeLog][QtCore][QTimeZone] When created from (only) a UTC offset,
or from (only) a non-IANA UTC-offset ID, a QTimeZone instance now uses
an IANA UTC-offset ID, where one is available with a matching offset.
Previously it used a synthesized UTC±hh[:mm[:ss]] one which would omit
trailing :00 for minutes or seconds, which the IANA ID may well
include.

Task-number: QTBUG-118586
Change-Id: Ifc4976f36361c830c88a8bef0e8b963fe5a2ab43
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 29 insertions, 3 deletions

diff --git a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
index c03c7a89e9..7ae4a8b52e 100644
--- a/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
+++ b/tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp
@@ -4197,7 +4197,7 @@ void tst_QDateTime::timeZones() const
     QCOMPARE(nzStdOffset.date(), QDate(2012, 6, 1));
     QCOMPARE(nzStdOffset.time(), QTime(12, 0));
     QVERIFY(nzStdOffset.timeZone() == nzTzOffset);
-    QCOMPARE(nzStdOffset.timeZone().id(), QByteArray("UTC+12"));
+    QCOMPARE(nzStdOffset.timeZone().id(), QByteArray("UTC+12:00"));
     QCOMPARE(nzStdOffset.offsetFromUtc(), 43200);
     QVERIFY(!nzStdOffset.isDaylightTime());
     QCOMPARE(nzStdOffset.toMSecsSinceEpoch(), utcStd.toMSecsSinceEpoch());
diff --git a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
index 29643b2cac..6b0ce70456 100644
--- a/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
+++ b/tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp
@@ -537,12 +537,14 @@ void tst_QTimeZone::isTimeZoneIdAvailable()
     for (const QByteArray &id : available) {
         QVERIFY2(QTimeZone::isTimeZoneIdAvailable(id), id);
         QVERIFY2(QTimeZone(id).isValid(), id);
+        QCOMPARE(QTimeZone(id).id(), id);
     }
     for (qint32 offset = QTimeZone::MinUtcOffsetSecs;
          offset <= QTimeZone::MinUtcOffsetSecs; ++offset) {
         const QByteArray id = QTimeZone(offset).id();
         QVERIFY2(QTimeZone::isTimeZoneIdAvailable(id), id);
         QVERIFY2(QTimeZone(id).isValid(), id);
+        QCOMPARE(QTimeZone(id).id(), id);
     }
 }
 
@@ -607,7 +609,11 @@ void tst_QTimeZone::utcOffsetId_data()
     ROW("UTC-11", true, -39600);
     ROW("UTC-09", true, -32400);
     ROW("UTC-08", true, -28800);
+    ROW("UTC-8", true, -28800);
+    ROW("UTC-2:5", true, -7500);
     ROW("UTC-02", true, -7200);
+    ROW("UTC+2", true, 7200);
+    ROW("UTC+2:5", true, 7500);
     ROW("UTC+12", true, 43200);
     ROW("UTC+13", true, 46800);
     // Encountered in bug reports:
@@ -655,6 +661,19 @@ void tst_QTimeZone::utcOffsetId()
         QFETCH(int, offset);
         QCOMPARE(zone.offsetFromUtc(epoch), offset);
         QVERIFY(!zone.hasDaylightTime());
+
+        // zone.id() will be an IANA ID with zero minutes field if original was
+        // a UTC offset by a whole number of hours. It will also zero-pad a
+        // single-digit hour or minute to two digits.
+        if (const qsizetype cut = id.indexOf(':'); cut >= 0) {
+            if (id.size() == cut + 2) // "...:m" -> "...:0m"
+                id.insert(cut + 1, '0');
+        } else if (zone.id().contains(':')) {
+            id += ":00";
+        }
+        if (id.indexOf(':') == 5) // UTC±h:mm -> UTC±0h:mm
+            id.insert(4, '0');
+
         QCOMPARE(zone.id(), id);
     }
 }
@@ -1175,15 +1194,22 @@ void tst_QTimeZone::utcTest()
     QCOMPARE(tzp.hasDaylightTime(), false);
     QCOMPARE(tzp.hasTransitions(), false);
 
-    // Test create from UTC Offset (uses minimal id, skipping minutes if 0)
+    // Test create from UTC Offset:
     QDateTime now = QDateTime::currentDateTime();
     QTimeZone tz(36000);
     QVERIFY(tz.isValid());
-    QCOMPARE(tz.id(), QByteArray("UTC+10"));
+    QCOMPARE(tz.id(), QByteArray("UTC+10:00"));
     QCOMPARE(tz.offsetFromUtc(now), 36000);
     QCOMPARE(tz.standardTimeOffset(now), 36000);
     QCOMPARE(tz.daylightTimeOffset(now), 0);
 
+    tz = QTimeZone(15 * 3600); // no IANA ID, so uses minimal id, skipping :00 minutes
+    QVERIFY(tz.isValid());
+    QCOMPARE(tz.id(), QByteArray("UTC+15"));
+    QCOMPARE(tz.offsetFromUtc(now), 15 * 3600);
+    QCOMPARE(tz.standardTimeOffset(now), 15 * 3600);
+    QCOMPARE(tz.daylightTimeOffset(now), 0);
+
     // Test validity range of UTC offsets:
     int min = QTimeZone::MinUtcOffsetSecs;
     int max = QTimeZone::MaxUtcOffsetSecs;