Clip dataForLocalTime()'s bracketing window's start to minMSecs() + 1

The minMSecs() itself is one more than the type's min(), which is used
as invalidMSecs(). As (at least) the Windows back-end uses minMSecs()
as the time of a start-of-time transition (that we'd like to find as
our current transition), use one more than it as lower-bound for where
to search from for a previous transition, so that we do find that
first ever transition.

Pick-to: 6.3
Change-Id: Iae861e740e02bd38ffb2af77aff625d3b48182d2
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 9 insertions, 5 deletions

diff --git a/src/corelib/time/qtimezoneprivate.cpp b/src/corelib/time/qtimezoneprivate.cpp
index a85617718d..e3f392230a 100644
--- a/src/corelib/time/qtimezoneprivate.cpp
+++ b/src/corelib/time/qtimezoneprivate.cpp
@@ -225,14 +225,18 @@ QTimeZonePrivate::Data QTimeZonePrivate::dataForLocalTime(qint64 forLocalMSecs,
     static_assert(-sixteenHoursInMSecs / 1000 < QTimeZone::MinUtcOffsetSecs
                   && sixteenHoursInMSecs / 1000 > QTimeZone::MaxUtcOffsetSecs);
     qint64 millis;
+    // Clip the bracketing times to the bounds of the supported range. Exclude
+    // minMSecs(), because at least one backend (Windows) uses it for a
+    // start-of-time fake transition, that we want previousTransition() to find.
     const qint64 recent =
-        sub_overflow(forLocalMSecs, sixteenHoursInMSecs, &millis)
-        ? minMSecs() : millis;
+        sub_overflow(forLocalMSecs, sixteenHoursInMSecs, &millis) || millis <= minMSecs()
+        ? minMSecs() + 1 : millis; // Necessarily <= forLocalMSecs + 2.
+    // (Given that minMSecs() is std::numeric_limits<qint64>::min() + 1.)
     const qint64 imminent =
         add_overflow(forLocalMSecs, sixteenHoursInMSecs, &millis)
-        ? maxMSecs() : millis;
-    // At most one of those took the boundary value:
-    Q_ASSERT(recent < imminent && sixteenHoursInMSecs < imminent - recent + 1);
+        ? maxMSecs() : millis; // Necessarily >= forLocalMSecs
+    // At most one of those was clipped to its boundary value:
+    Q_ASSERT(recent < imminent && sixteenHoursInMSecs < imminent - recent + 2);
     /*
       Offsets are Local - UTC, positive to the east of Greenwich, negative to
       the west; DST offset always exceeds standard offset, when DST applies.