[Reg 5.15->6.x] QVariant::isNull() can return true for non-null values

#include <QtTest>

class VariantNullTest : public QObject
{
    Q_OBJECT
    const QDate validDate = QDate(2024, 05, 06);

private slots:
    void initialiazeDataByConstructor()
    {
        QVariant v(validDate);
        QCOMPARE(v.toString(), "2024-05-06");
        QCOMPARE(v.isNull(), false);
        QCOMPARE(v.isValid(), true);
    }

    void initialiazeDataFromValue()
    {
        QVariant v = QVariant::fromValue(validDate);
        QCOMPARE(v.toString(), "2024-05-06");
        QCOMPARE(v.isNull(), false);
        QCOMPARE(v.isValid(), true);
    }

#if QT_VERSION_MAJOR >= 6
    void initializeDataFromType()
    {
        QVariant v(QMetaType::fromType<QDate>(), &validDate);
        QCOMPARE(v.toString(), "2024-05-06");
        QCOMPARE(v.isNull(), false);
        QCOMPARE(v.isValid(), true);
    }
#endif

    void storeDataInInvalidVariant()
    {
        QVariant v;
        QCOMPARE(v.toString(), "");
        QCOMPARE(v.isNull(), true);
        QCOMPARE(v.isValid(), false);

        v.setValue(validDate);
        QCOMPARE(v.toString(), "2024-05-06");
        QCOMPARE(v.isNull(), false);
        QCOMPARE(v.isValid(), true);
    }

    void storeDataInValidVariant()
    {
#if QT_VERSION_MAJOR >= 6
        QVariant v(QMetaType::fromType<QDate>()); // Same result if we use the  deprecated QVariant(QVariant::Date) constructor in Qt 6
#else
        QVariant v(QVariant::Date);
#endif
        QCOMPARE(v.toString(), "");
        QCOMPARE(v.isNull(), true);
        QCOMPARE(v.isValid(), true);

        v.setValue(validDate);
        QCOMPARE(v.toString(), "2024-05-06");
        QCOMPARE(v.isNull(), false); // ERROR: This should definitely be non-null!
        QCOMPARE(v.isValid(), true);
    }
};

QTEST_MAIN(VariantNullTest)
#include "variantnulltest.moc"