Persistent child indexes refer to incorrect model when source model changed

#include <QApplication>
#include <QSortFilterProxyModel>
#include <QStandardItemModel>

#include <QMainWindow>
#include <QTreeView>


class ItemModel : public QStandardItemModel
{
public:
    QModelIndex index(int row, int column, const QModelIndex &parent = QModelIndex()) const Q_DECL_OVERRIDE
    {
        // Would we always expect the parent to belong to the model?
        Q_ASSERT(!parent.isValid() || parent.model() == this);
        return QStandardItemModel::index(row, column, parent);
    }
};

void fillModel(QAbstractItemModel &model)
{
    model.insertRow(0);
    model.insertColumn(0);
    QModelIndex index = model.index(0, 0);
    model.setData(index, "Level0");
    {
        model.insertRow(0, index);
        model.insertColumn(0, index);
        index = model.index(0, 0, index);
        model.setData(index, "Level1");
        {
            model.insertRow(0, index);
            model.insertColumn(0, index);
            index = model.index(0, 0, index);
            model.setData(index, "Level2");
        }
    }
}


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMainWindow mainWindow;
    mainWindow.resize(200, 200);
    mainWindow.show();

    QTreeView treeView(&mainWindow);
    treeView.resize(mainWindow.width(), mainWindow.height());

    ItemModel model;
    fillModel(model);

    QSortFilterProxyModel proxy;
    proxy.setSourceModel(&model);

    treeView.setModel(&proxy);
    treeView.expandAll();           // Required to create 'persistent' indexes.

    // Now set a new model: see assert.
    ItemModel model2;
    fillModel(model2);
    proxy.setSourceModel(&model2);

    return a.exec();
}