//...
auto future = someFutureWork().then([&](QPromise<void> &myPromise, const QFuture<int> &previousResults) {  // either (optionally) pass QPromise
    Q_UNUSED(previousResults);
    for (int i = 0; i < (1 << 31) && !myPromise.isCanceled(); ++i) {
      QThread::msleep(100); // simulate some heavy work    }
    myPromise.finish();
}).then([&](std::stop_token stopToken) { // or use std::stop_token, though it's a C++20 feature ...
    for (int i = 0; i < (1 << 31) && !stopToken.stop_requested(); ++i) {
       QThread::msleep(100); // simulate some heavy work
     }
});

future.cancelChain();

//...