QStringBuilder: one more problem with 'auto'

#include <QCoreApplication>
#include <QTimer>
#include <QStringBuilder>

void foo()
{
    QString s1 = "somesting1";
    QString s2 = "somesting2";
    const auto str = s1 % s2;
    QTimer::singleShot(100, [str] { qDebug() << str; });
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    foo();
    return a.exec();
}

                                                                                                                                                                                                                                                                                                                                                                                                  
1  _platform_memmove                                                                                                                           (arm64e) /usr/lib/system/libsystem_platform.dylib      0x188c61870 
2  QConcatenable<QString>::appendTo(QString const&, QChar *&)                                                                                  qstringbuilder.h                                  255  0x1000037c8 
3  void QConcatenable<QStringBuilder<QString&, QString&>>::appendTo<QChar>(QStringBuilder<QString&, QString&> const&, QChar *&)                qstringbuilder.h                                  398  0x100003624 
4  QString QStringBuilder<QString&, QString&>::convertTo<QString>() const                                                                      qstringbuilder.h                                  112  0x1000034d8 
5  QStringBuilder<QString&, QString&>::operator QString() const                                                                                qstringbuilder.h                                  129  0x100003260 
6  foo()::$_0::operator()() const                                                                                                              main.cpp                                          10   0x100003108 
7  QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, foo()::$_0>::call(foo()::$_0&, void * *)                          qobjectdefs_impl.h                                129  0x10000309c 
8  void QtPrivate::Functor<foo()::$_0, 0>::call<QtPrivate::List<>, void>(foo()::$_0&, void *, void * *)                                        qobjectdefs_impl.h                                243  0x100003034 
9  QtPrivate::QFunctorSlotObject<foo()::$_0, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void * *, bool *) qobjectdefs_impl.h                                457  0x100002f94 
10 QtPrivate::QSlotObjectBase::call(QObject *, void * *)                                                                                       qobjectdefs_impl.h                                374  0x100c589e0 
11 void doActivate<false>(QObject *, int, void * *)                                                                                            qobject.cpp                                       4036 0x100c589c4 
12 QSingleShotTimer::timeout()                                                                                                                 qtimer.moc                                        159  0x100c6e13c 
13 QSingleShotTimer::timerEvent(QTimerEvent *)                                                                                                 qtimer.cpp                                        304  0x100c6e124 
14 QObject::event(QEvent *)                                                                                                                    qobject.cpp                                            0x100c510dc 
15 QCoreApplicationPrivate::notify_helper(QObject *, QEvent *)                                                                                 qcoreapplication.cpp                              1285 0x100c0e21c 
16 QCoreApplication::notifyInternal2(QObject *, QEvent *)                                                                                      qcoreapplication.cpp                              1118 0x100c0dd9c 
17 QTimerInfoList::activateTimers()                                                                                                            qtimerinfo_unix.cpp                               613  0x100d7bd48 
18 QEventDispatcherUNIXPrivate::activateTimers()                                                                                               qeventdispatcher_unix.cpp                         213  0x100d78b3c 
19 QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>)                                                                  qeventdispatcher_unix.cpp                         482  0x100d78b34 
20 QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>)                                                                            qeventloop.cpp                                    100  0x100c17714 
21 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>)                                                                                     qeventloop.cpp                                    182  0x100c176f0 
22 QCoreApplication::exec()                                                                                                                    qcoreapplication.cpp                              1439 0x100c0e428 
23 main                                                                                                                                        main.cpp                                          17   0x100002560