QWidgetWindow: fix UB (invalid downcast) in Private::handleDragEnterEvent()

The handleDragEnterEvent() function is not only called for
QEvent::DragEnter, but also, in handleDragMoveEvent(), for
QEvent::DragMove, in which case the fully-derived event passed as an
argument is a QDragMoveEvent, and not its subclass QDragEnterEvent.

Code in Qt seems to assume that it's ok to cast an object down to a
more-derived class than the most-derived dynamic type, as long as no
extra state is being added between the two, but C++ does not chare
that view.

Says UBSan:

  qwidgetwindow.cpp:1000:38: runtime error: downcast of address 0x7ffe7b34c6e0 which does not point to an object of type 'QDragEnterEvent'
  0x7ffe7b34c6e0: note: object is of type 'QDragMoveEvent'
   00 00 00 00  e0 62 70 42 aa 7f 00 00  3d 00 00 00 00 00 00 00  00 00 00 00 00 c0 58 40  00 00 00 00
                ^~~~~~~~~~~~~~~~~~~~~~~
                vptr for 'QDragMoveEvent'

Furtunately, handleDragEnterEvent() doesn't actually need its argument
to be a QDragEnterEvent; QDragMoveEvent suffices, so we can just
change the argument type back to QDragMoveEvent and remove the cast in
handleDragMoveEvent().

Add a bit of \internal docs to describe the discrepancy between the
function's name and argument type.

Amends f8944a7f07112c85dc4f66848cabb490514cd28e.

Picking all the way, because this is risk-less: Either the code compiles
and then it works as before (minus the UB), or it doesn't compile.

Pick-to: 6.8 6.5 5.15
Change-Id: I5be53b023d389902b43e9a896d074edea1c4ff2d
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit e89c0a7fb54a15b8df1d2938e2f01ef14f3bf7e5)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/kernel/qwidgetwindow.cpp

@@ -948,7 +948,17 @@ static QWidget *findDnDTarget(QWidget *parent, const QPoint &pos)
     return widget;
 }
 
-void QWidgetWindow::handleDragEnterEvent(QDragEnterEvent *event, QWidget *widget)
+/*!
+    \internal
+
+    Sends \a event to \a widget.
+
+    Also called from dragMoveEvent(), in which case \a event is-a
+    QDragMoveEvent only, not a full QDragEnterEvent, which is why this function
+    takes \a event as a QDragMoveEvent and not, as one would expect,
+    QDragEnterEvent (downcast would be UB).
+*/
+void QWidgetWindow::handleDragEnterEvent(QDragMoveEvent *event, QWidget *widget)
 {
     Q_ASSERT(m_dragTarget == nullptr);
     if (!widget)
@@ -997,7 +1007,7 @@ void QWidgetWindow::handleDragMoveEvent(QDragMoveEvent *event)
             // widget might have been deleted when handling the leaveEvent
             if (widget) {
                 // Send DragEnter to new widget.
-                handleDragEnterEvent(static_cast<QDragEnterEvent*>(event), widget);
+                handleDragEnterEvent(event, widget);
                 // Handling 'DragEnter' should suffice for the application.
                 translated.setDropAction(event->dropAction());
                 translated.setAccepted(event->isAccepted());

src/widgets/kernel/qwidgetwindow_p.h

@@ -65,7 +65,7 @@ protected:
     void handleWheelEvent(QWheelEvent *);
 #endif
 #if QT_CONFIG(draganddrop)
-    void handleDragEnterEvent(QDragEnterEvent *, QWidget *widget = nullptr);
+    void handleDragEnterEvent(QDragMoveEvent *, QWidget *widget = nullptr);
     void handleDragMoveEvent(QDragMoveEvent *);
     void handleDragLeaveEvent(QDragLeaveEvent *);
     void handleDropEvent(QDropEvent *);