QLayoutItem: make QLayoutItem::widget() const in Qt6

QLayoutItem::widget() should be const since it does not modify the
class. Since this can not be done within Qt5 in a binary compatible way,
change it for Qt6.

Fixes: QTBUG-41997
Change-Id: I9211eb1c36a5bc4f06ab417a9df790ebedb7fcda
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/kernel/qlayoutengine.cpp

@@ -376,7 +376,11 @@ Q_WIDGETS_EXPORT QSize qSmartMinSize(const QSize &sizeHint, const QSize &minSize
 
 Q_WIDGETS_EXPORT QSize qSmartMinSize(const QWidgetItem *i)
 {
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     QWidget *w = const_cast<QWidgetItem *>(i)->widget();
+#else
+    QWidget *w = i->widget();
+#endif
     return qSmartMinSize(w->sizeHint(), w->minimumSizeHint(),
                             w->minimumSize(), w->maximumSize(),
                             w->sizePolicy());
@@ -414,8 +418,11 @@ Q_WIDGETS_EXPORT QSize qSmartMaxSize(const QSize &sizeHint,
 
 Q_WIDGETS_EXPORT QSize qSmartMaxSize(const QWidgetItem *i, Qt::Alignment align)
 {
-    QWidget *w = const_cast<QWidgetItem*>(i)->widget();
-
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
+    QWidget *w = const_cast<QWidgetItem *>(i)->widget();
+#else
+    QWidget *w = i->widget();
+#endif
     return qSmartMaxSize(w->sizeHint().expandedTo(w->minimumSizeHint()), w->minimumSize(), w->maximumSize(),
                             w->sizePolicy(), align);
 }

src/widgets/kernel/qlayoutitem.cpp

@@ -291,14 +291,12 @@ void QSpacerItem::changeSize(int w, int h, QSizePolicy::Policy hPolicy,
 /*!
     Destructor.
 */
-QWidgetItem::~QWidgetItem() {}
+QWidgetItem::~QWidgetItem() = default;
 
 /*!
     Destroys the QLayoutItem.
 */
-QLayoutItem::~QLayoutItem()
-{
-}
+QLayoutItem::~QLayoutItem() = default;
 
 /*!
     Invalidates any cached information in this layout item.
@@ -362,7 +360,11 @@ QSpacerItem * QSpacerItem::spacerItem()
 
     \sa layout(), spacerItem()
 */
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 QWidget *QLayoutItem::widget()
+#else
+QWidget *QLayoutItem::widget() const
+#endif
 {
     return nullptr;
 }
@@ -370,7 +372,11 @@ QWidget *QLayoutItem::widget()
 /*!
     Returns the widget managed by this item.
 */
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 QWidget *QWidgetItem::widget()
+#else
+QWidget *QWidgetItem::widget() const
+#endif
 {
     return wid;
 }

src/widgets/kernel/qlayoutitem.h

@@ -74,7 +74,11 @@ public:
     virtual int minimumHeightForWidth(int) const;
     virtual void invalidate();
 
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     virtual QWidget *widget();
+#else
+    virtual QWidget *widget() const;
+#endif
     virtual QLayout *layout();
     virtual QSpacerItem *spacerItem();
 
@@ -133,7 +137,11 @@ public:
     bool isEmpty() const override;
     void setGeometry(const QRect&) override;
     QRect geometry() const override;
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     QWidget *widget() override;
+#else
+    QWidget *widget() const override;
+#endif
 
     bool hasHeightForWidth() const override;
     int heightForWidth(int) const override;

src/widgets/widgets/qdockwidget_p.h

@@ -201,7 +201,11 @@ inline QLayoutItem *QDockWidgetItem::dockWidgetChildItem() const
 
 inline QDockWidgetLayout *QDockWidgetItem::dockWidgetLayout() const
 {
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     QWidget *w = const_cast<QDockWidgetItem*>(this)->widget();
+#else
+    QWidget *w = widget();
+#endif
     if (w != nullptr)
         return qobject_cast<QDockWidgetLayout*>(w->layout());
     return nullptr;