Restore previous QDateTime behavior in DST gap

Historic QDateTime behavior when being asked to create a
QDateTime in the DST gap was to interpret the given date
as if it was in the time before that gap, mapping it to a point
in time after the gap. This has changed with
a04411119ead3d4473e4f0ac4bceedc585977b2f .
Since then, the given date is interpreted as if it was in the
time after the gap, thus being mapped to a point in time
before the gap.
This patch restores the historic behavior.
This was not caught by Coin because machines ran in timezone
"Atlantic/Reykjavik" which does not have DST since 1967.
This patch changes tests to always run in "Europe/Oslo".
Driveby: Test function "findSpring" did some operations in
local time, even though being asked to work in a specific
time zone. Fixed that.

Fixes: QTBUG-86960
Fixes: QTBUG-89208
Change-Id: Iecce5898bf9711a10e7dfc0a25e4bbeaed1c8ade
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 3d785249ba43cf4bd895ed679bac2791e0130dc5)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/time/qdatetime.cpp

@@ -2856,21 +2856,21 @@ static void refreshZonedDateTime(QDateTimeData &d, Qt::TimeSpec spec)
         qint64 epochMSecs = 0;
         QDate testDate;
         QTime testTime;
-        if (spec == Qt::LocalTime) {
         auto dstStatus = extractDaylightStatus(status);
+        if (spec == Qt::LocalTime) {
             epochMSecs = localMSecsToEpochMSecs(msecs, &dstStatus, &testDate, &testTime);
-            status = mergeDaylightStatus(status, dstStatus);
 #if QT_CONFIG(timezone)
         // else spec == Qt::TimeZone, so check zone is valid:
         } else if (d->m_timeZone.isValid()) {
             epochMSecs = QDateTimePrivate::zoneMSecsToEpochMSecs(
-                msecs, d->m_timeZone, extractDaylightStatus(status), &testDate, &testTime);
+                msecs, d->m_timeZone, dstStatus, &testDate, &testTime);
 #endif // timezone
         } // else: testDate, testTime haven't been set, so are invalid.
         // Cache the offset to use in offsetFromUtc() &c.
         offsetFromUtc = (msecs - epochMSecs) / 1000;
         if (testDate.isValid() && testTime.isValid()
             && timeToMSecs(testDate, testTime) == msecs) {
+            status = mergeDaylightStatus(status, dstStatus);
             status |= QDateTimePrivate::ValidDateTime;
         } else {
             status &= ~QDateTimePrivate::ValidDateTime;

tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp

@@ -2821,6 +2821,9 @@ void tst_QDateTime::fromStringStringFormat()
 #endif
         // OffsetFromUTC needs an offset check - we may as well do it for all:
         QCOMPARE(dt.offsetFromUtc(), expected.offsetFromUtc());
+    } else {
+        QCOMPARE(dt.isValid(), expected.isValid());
+        QCOMPARE(dt.toMSecsSinceEpoch(), expected.toMSecsSinceEpoch());
     }
 }
 
@@ -3245,6 +3248,8 @@ void tst_QDateTime::daylightTransitions() const
         QVERIFY(!missing.isValid());
         QCOMPARE(missing.date(), QDate(2012, 3, 25));
         QCOMPARE(missing.time(), QTime(2, 0));
+        // datetimeparser relies on toMSecsSinceEpoch to still work:
+        QCOMPARE(missing.toMSecsSinceEpoch(), daylight2012);
 
         QDateTime after(QDate(2012, 3, 25), QTime(3, 0));
         QVERIFY(after.isValid());

tests/auto/widgets/widgets/qdatetimeedit/tst_qdatetimeedit.cpp

@@ -4622,7 +4622,7 @@ static QDateTime findSpring(int year, const QTimeZone &timeZone)
         return QDateTime();
 
     // Southern hemisphere spring is after midsummer
-    const QDateTime midSummer = QDate(year, 6, 21).startOfDay();
+    const QDateTime midSummer = QDate(year, 6, 21).startOfDay(timeZone);
     const QTimeZone::OffsetData transition =
         midSummer.isDaylightTime() ? timeZone.previousTransition(midSummer)
                                    : timeZone.nextTransition(midSummer);
@@ -4744,7 +4744,7 @@ void tst_QDateTimeEdit::stepIntoDSTGap_data()
     QTest::addColumn<int>("steps");
     QTest::addColumn<QDateTime>("end");
 
-    const QTimeZone timeZone = QTimeZone::systemTimeZone();
+    const QTimeZone timeZone = QTimeZone("Europe/Oslo");
     if (!timeZone.hasDaylightTime())
         QSKIP("This test needs to run in a timezone that observes DST!");