QTimeZone: add construction from std::chrono::time_zone*

A time_zone represents a timezone identified by its IANA ID. This
allows for a straightforward conversion to QTimeZone.

[ChangeLog][QtCore][QTimeZone] QTimeZone can now be constructed
from a std::chrono::time_zone pointer.

Change-Id: I093d9fc2e989475d30730a9dcdef491903a2aeb2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qtimezone.cpp

@@ -974,6 +974,15 @@ QList<QByteArray> QTimeZone::windowsIdToIanaIds(const QByteArray &windowsId, QLo
     return QTimeZonePrivate::windowsIdToIanaIds(windowsId, territory);
 }
 
+/*!
+    \fn QTimeZone QTimeZone::fromStdTimeZonePtr(const std::chrono::time_zone *timeZone)
+    \since 6.4
+
+    Returns a QTimeZone object representing the same time zone as \a timeZone.
+    The IANA ID of \a timeZone must be one of the available system IDs,
+    otherwise an invalid time zone will be returned.
+*/
+
 #ifndef QT_NO_DATASTREAM
 // Invalid, as an IANA ID: too long, starts with - and has other invalid characters in it
 static inline QString invalidId() { return QStringLiteral("-No Time Zone Specified!"); }

src/corelib/time/qtimezone.h

@@ -45,6 +45,8 @@
 #include <QtCore/qlocale.h>
 #include <QtCore/qdatetime.h>
 
+#include <chrono>
+
 QT_REQUIRE_CONFIG(timezone);
 
 #if (defined(Q_OS_DARWIN) || defined(Q_QDOC)) && !defined(QT_NO_SYSTEMLOCALE)
@@ -164,6 +166,17 @@ public:
     NSTimeZone *toNSTimeZone() const Q_DECL_NS_RETURNS_AUTORELEASED;
 #endif
 
+#if __cpp_lib_chrono >= 201907L || defined(Q_QDOC)
+    QT_POST_CXX17_API_IN_EXPORTED_CLASS
+    static QTimeZone fromStdTimeZonePtr(const std::chrono::time_zone *timeZone)
+    {
+        if (!timeZone)
+            return QTimeZone();
+        const std::string_view timeZoneName = timeZone->name();
+        return QTimeZone(QByteArrayView(timeZoneName).toByteArray());
+    }
+#endif
+
 private:
     QTimeZone(QTimeZonePrivate &dd);
 #ifndef QT_NO_DATASTREAM

tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp

@@ -77,6 +77,8 @@ private Q_SLOTS:
     void winTest();
     void localeSpecificDisplayName_data();
     void localeSpecificDisplayName();
+    void stdCompatibility_data();
+    void stdCompatibility();
 
 private:
     void printTimeZone(const QTimeZone &tz);
@@ -341,6 +343,11 @@ void tst_QTimeZone::systemZone()
     };
     for (const auto &date : dates)
         QCOMPARE(date.startOfDay(Qt::LocalTime), date.startOfDay(zone));
+
+#if __cpp_lib_chrono >= 201907L
+    const std::chrono::time_zone *currentTimeZone = std::chrono::current_zone();
+    QCOMPARE(QByteArrayView(currentTimeZone->name()), QByteArrayView(zone.id()));
+#endif
 }
 
 void tst_QTimeZone::dataStreamTest()
@@ -1609,5 +1616,46 @@ void tst_QTimeZone::testEpochTranPrivate(const QTimeZonePrivate &tzp)
 }
 #endif // QT_BUILD_INTERNAL
 
+#if __cpp_lib_chrono >= 201907L
+Q_DECLARE_METATYPE(const std::chrono::time_zone *);
+#endif
+
+void tst_QTimeZone::stdCompatibility_data()
+{
+#if __cpp_lib_chrono >= 201907L
+    QTest::addColumn<const std::chrono::time_zone *>("timeZone");
+    const std::chrono::tzdb &tzdb = std::chrono::get_tzdb();
+    qDebug() << "Using tzdb version:" << QByteArrayView(tzdb.version);
+
+    for (const std::chrono::time_zone &zone : tzdb.zones)
+        QTest::addRow(zone.name().data()) << &zone;
+#else
+    QSKIP("This test requires C++20's <chrono>.");
+#endif
+}
+
+void tst_QTimeZone::stdCompatibility()
+{
+#if __cpp_lib_chrono >= 201907L
+    QFETCH(const std::chrono::time_zone *, timeZone);
+    QByteArrayView zoneName = QByteArrayView(timeZone->name());
+    QTimeZone tz = QTimeZone::fromStdTimeZonePtr(timeZone);
+    if (tz.isValid()) {
+        QCOMPARE(tz.id(), zoneName);
+    } else {
+        // QTBUG-102187: a few timezones reported by tzdb might not be
+        // recognized by QTimeZone. This happens for instance on Windows, where
+        // tzdb is using ICU, whose database does not match QTimeZone's.
+        const bool isKnownUnknown =
+                !zoneName.contains('/')
+                || zoneName == "Antarctica/Troll"
+                || zoneName.startsWith("SystemV/");
+        QVERIFY(isKnownUnknown);
+    }
+#else
+    QSKIP("This test requires C++20's <chrono>.");
+#endif
+}
+
 QTEST_APPLESS_MAIN(tst_QTimeZone)
 #include "tst_qtimezone.moc"