The documentation says any QFuture<T> can be assigned to QFuture<void> and while this works in an explicit assignment, it does not when an argument expect QFuture<void> because the constructor that takes a QFuture<T> and build a QFuture<void> is explicit:

   template<typename U, typename V = T, typename = QtPrivate::EnableForVoid<V>>
    explicit QFuture(const QFuture<U> &other) : d(other.d)
    {
    } 

Attached is a test case, uncomment line 34 to see the error:

qfuture/main.cpp:34:23: error: invalid initialization of reference of type ‘const QFuture<void>&’ from expression of type ‘QFuture<long long int>’
   34 |         debugProgress(future); // fails to compile