Possible signed integer overflow in QProgressBar::reset is undefined behaviour as per C++11:

    d->value = d->minimum - 1;
    if (d->minimum == INT_MIN)
        d->value = INT_MIN;

If d->minimum is INT_MIN already, subtracting one from it results in UB. A minor transformation helps already:

    if (d->minimum == INT_MIN)
        d->value = INT_MIN;
    else
        d->value = d->minimum - 1;