QString::number with negative qint64 produces positive, out-of-range numbers

void testToLongLong(qint64 value) {
    QString hexStrValue = QString::number(value, 16);

    bool ok;
    qint64 intValue;
    intValue = (qint64) hexStrValue.toULongLong(&ok, 16);

    Q_ASSERT_X(ok, "testToLongLong using cast", "returned not ok");
    Q_ASSERT_X(intValue == value, "testToLongLong using cast", "values differ");

    intValue = hexStrValue.toLongLong(&ok, 16);

    Q_ASSERT_X(ok, "testToLongLong using toLongLong", "returned not ok");
    Q_ASSERT_X(intValue == value, "testToLongLong using toLongLong", "values differ");
}

void testToLongLong() {
    testToLongLong(0x7FFFFFFFFFFFFFFF); // is working
    testToLongLong(0x8000000000000000); // doesn't work
    testToLongLong(0xAAAAAAAAAAAAAAAA); // doesn't work too
}