The build failed when the macro was set:

QT_DISABLE_DEPRECATED_UP_TO=0x060000 

Because on line 49 of the file 'qt-solutions/qtsingleapplication/src/qtlocalpeer.cpp', a deprecated API in Qt6 was called:

 quint16 idNum = qChecksum(idc.constData(), idc.size());

The API has changed in Qt6 to:

quint16 qChecksum(QByteArrayView data, Qt::ChecksumType standard = Qt::ChecksumIso3309)

Therefore, I think that a check for the Qt version number should be added here so that different APIs can be called depending on the version.