When using QDataStream with QDataStream::Qt_4_6 to write a QVariant containing a QUuid, the written data differs between Qt 5 and Qt 4. Please see the attached unit-test. The test succeeds with Qt 4.8.6 and fails with Qt 5 (and later). With the attached fix, Qt 5 succeeds, too.
The reason is that in Qt 4 QUuid contained in QVariant is handled as a custom-type in contrast to Qt 5 but that special case is not handled.