The documentation for QButtonGroup::addButton() states:

If id is -1, an id will be assigned to the button. Automatically assigned ids are guaranteed to be negative, starting with -2. If you are assigning your own ids, use positive values to avoid conflicts.

However, this is evidently not true from reading the implementation (qtbase/src/widgets/widgets/qbuttongroup.cpp, starting at line 206):

if (id == -1) {
    const QHash<QAbstractButton*, int>::const_iterator it
            = std::min_element(d->mapping.cbegin(), d->mapping.cend());
    if (it == d->mapping.cend())
        d->mapping[button] = -2;
    else
        d->mapping[button] = *it - 1;
} 

If there are already items in the map, this will choose an id one less than the minimum, which does not guarantee a negative number. For instance, if a user first adds a single item with id == 2, then adds another with id == -1, the second will be automatically assigned an id of 1, not -2, as the documentation states.