QString::toUpper() fails at temporary objects with c++11 if resulting string grows

template <typename Traits, typename T>
Q_NEVER_INLINE
static QString detachAndConvertCase(T &str, QStringIterator it)
{
    QString s = qMove(str);             // will copy if T is const QString
    QChar *pp = s.begin() + it.index(); // will detach if necessary
    uint uc = it.nextUnchecked();
    forever {
        const QUnicodeTables::Properties *prop = qGetProp(uc);
        signed short caseDiff = Traits::caseDiff(prop);

        if (Q_UNLIKELY(Traits::caseSpecial(prop))) {
            // slow path
            const ushort *specialCase = specialCaseMap + caseDiff;
            ushort length = *specialCase++;
            int pos = pp - s.constBegin();
            s.replace(pos, 1, reinterpret_cast<const QChar *>(specialCase), length);
            pp = const_cast<QChar *>(s.constBegin()) + pos + length;
        } else if (QChar::requiresSurrogates(uc)) {
            *pp++ = QChar::highSurrogate(uc + caseDiff);
            *pp++ = QChar::lowSurrogate(uc + caseDiff);
        } else {
            *pp++ = QChar(uc + caseDiff);
        }

        if (!it.hasNext())
            return s;

        uc = it.nextUnchecked();
    }
}