A bug of QLayout

[cocalele]

Hi,
I just find a assert failure when I use QLayout. I want know is this a bug of QLayout.
The QLayout has a constructor like this, which need a parent of QWdiget

QLayout::QLayoutQWidget *parent)
    : QObject(*new QLayoutPrivate, parent)
{
    if (!parent)
        return;
    parent->setLayout(this);
}

QLayout::QLayoutQWidget *parent)
    : QObject(*new QLayoutPrivate, parent)
{
    if (!parent)
        return;
    parent->setLayout(this);
}

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But in the code of QLayout:arentWidget() (line 6), it trys to cast the parent to QLayout

QWidget *QLayout::parentWidget() const
{
    Q_D(const QLayout);
    if (!d->topLevel) {
        if (parent()) {
            QLayout *parentLayout = ::qobject_cast<QLayout*>(parent()); 
           Q_ASSERT(parentLayout);
            return parentLayout->parentWidget();
        } else {
            return 0;
        }
    } else {
        Q_ASSERT(parent() && parent()->isWidgetType());
        return static_cast<QWidget *>(parent());
    }
}

QWidget *QLayout::parentWidget() const
{
    Q_D(const QLayout);
    if (!d->topLevel) {
        if (parent()) {
            QLayout *parentLayout = ::qobject_cast<QLayout*>(parent()); 
           Q_ASSERT(parentLayout);
            return parentLayout->parentWidget();
        } else {
            return 0;
        }
    } else {
        Q_ASSERT(parent() && parent()->isWidgetType());
        return static_cast<QWidget *>(parent());
    }
}

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certainly, cast from QWidget* to QLayout* will failled, and the successed assertion will failed. Is this a bug or anything I don't know?
Thanks for your help!