#include "mainwindow.h"
#include <QApplication>
#include <QtNetwork/QTcpServer>
#include <QNetworkProxy>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qDebug()<< QNetworkProxy::applicationProxy();
    //QNetworkProxy::setApplicationProxy(QNetworkProxy());
    QTcpServer *server = new QTcpServer();
    qDebug() << "RESULT " <<server->listen()<< " " << server->serverPort();
    qDebug()<< server->errorString();

    return 0;
}

Outputs :
HttpProxy ""proxy.test.com":8080" ["Tunnel Caching NameLookup"]
RESULT false 0
"The proxy type is invalid for this operation"

If i uncomment the line:

QNetworkProxy::setApplicationProxy(QNetworkProxy());

It prints:
HttpProxy ""proxy.test.com":8080" ["Tunnel Caching NameLookup"]
RESULT true 0
""

Even having a configured proxy, i may want to listen at localhost or by other network that don't use the proxy.

If it is now required... it should be well documented in the QTcpServer... but i think it shouldn't be a problem to "listen" under a http proxy as you may ignore the proxy.