qint8 (here: typedef for signed char) is obviously not declared as metatype, unlike, for example, qint16 or qint32

example:

QVariant v;
// v gets some value
v.canConvert<qint32>(); // works
v.canConvert<qint16>(); // works
v.canConvert<qint8>(); // does not work: "'qt_metatype_id' is not a member of 'QMetaTypeId<signed char>'"

workaround:
therefore, i have to put a
Q_DECLARE_METATYPE(qint8)
into my sources myself to make it all work