QDateTime at beginning of DST is created wrongly with recent glibc

On Ubuntu20, QDateTimeEdit unit tests fail with errors like

FAIL! : tst_QDateTimeEdit::stepIntoDSTGap(hour down into 02:00 gap) Compared values are not the same
 Actual (edit.dateTime()): 2007/03/25 00:00:10.000[CET]
 Expected (end) : 2007/03/25 01:00:10.000[CET]
 Loc: [../tests/auto/widgets/widgets/qdatetimeedit/tst_qdatetimeedit.cpp(4747)]

A simpler reproducer is attached.

A patch which adds a unit test reproducing the problem in a simpler form in QDateTime (without QDateTimeEdit) is in Gerrit .

Observation:

When creating a QDateTime at e.g. 2007-03-25 02:00:00 (just at the beginning of the gap hour), we end up with a QDateTime at 2007-03-25 01:00:00 (one hour before the gap hour).

Analysis:

Creating the QDateTime at the beginning of the gap hour (I used 2007-03-25 02:00:00) creates a QDateTime with unknown DST status. It correctly calculates its local milliseconds since Epoch in winter time. Then in
QDateTimePrivate::create
is a call to
refreshZonedDateTime (qdatetime.cpp:2793).
This calls (qdatetime.cpp:2813)

epochMSecs = localMSecsToEpochMSecs(msecs, &dstStatus, &testDate, &testTime);

which correctly identifies the time to be (2007-03-25 03:00:00 DST). Here the problem arises. The DST information of the QDateTime object in question is updated from the result of localMSecsToEpochMSecs in (qdatetime.cpp:2814)

status = mergeDaylightStatus(status, dstStatus);

but the date/time information (given correctly in testDate/testTime) is not updated. So we end up with an object thinking it is 2007-03-25 02:00:00 DST, which is 2007-03-25 01:00:00. This is not what was asked for.