Nested QtConcurrent::map cause deadlock

// code placeholder
#include <cassert>
#include <QtConcurrentMap>
#include <QtConcurrentRun>
#include <QFuture>
#include <QThreadPool>
#include <QtTest/QTest>
#include <QFutureSynchronizer>

struct Task2 { // only calculation
    typedef void result_type;
    void operator()(int count) {
        int k = 0;
        k += count;
        assert(k >= 0);
    }
};

struct Task1 { // will launch some other concurrent map
    typedef void result_type;
    void operator()(int count) {

        QVector<int> vec;
        vec.push_back(count); // only one element
        Task2 task;

        QFuture<void> f = QtConcurrent::map(vec.begin(), vec.end(), task);
        {
            // with out releaseThread before wait, it will hang directly
            QThreadPool::globalInstance()->releaseThread();
            f.waitForFinished(); // BUG: may hang there
            QThreadPool::globalInstance()->reserveThread();
        }
    }
};


int main() {
    int count = 0;
    for (;;) {
        QVector<int> vec;
        vec.push_back(0); // only one element
        // launch a task with nested map
        Task1 task; // Task1 will have nested concurrent map
        QFuture<void> f = QtConcurrent::map(vec.begin(), vec.end(),task);

        f.waitForFinished(); // BUG: may hang there

        count++;

        qDebug() << count;
    }
    return 0;
}