QStackedLayout: Fix UB (invalid cast) in qt_wasDeleted()

Found by UBSan:

  qstackedlayout.cpp:261:98: runtime error: downcast of address 0x60400003cd10 which does not point to an object of type 'QtFriendlyLayoutWidget'
  0x60400003cd10: note: object is of type 'QWidget'
   0e 00 80 76  70 0b 06 3f d6 2a 00 00  00 99 00 00 50 61 00 00  20 0d 06 3f d6 2a 00 00  00 00 be be
                ^~~~~~~~~~~~~~~~~~~~~~~
                vptr for 'QWidget'
    #0 0x2ad637b8f58f in qt_wasDeleted qstackedlayout.cpp:261
    #1 0x2ad637b8f58f in QStackedLayout::takeAt(int) qstackedlayout.cpp:285
    #2 0x2ad637b4afcb in QLayout::removeWidget(QWidget*) qlayout.cpp:1369
    #3 0x413534 in tst_QStackedLayout::testCase() tst_qstackedlayout.cpp:155

Caused by using a struct to get access to QWidgetPrivate::wasDeleted
using a cast.  That cast is invalid, of course, so simply use
QWidgetPrivate::get().

Caveat: we need a const version of that function, but that didn't
exist, yet, so added one.

Change-Id: I27d449b90be7e2072646d950c676b500ef698349
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/widgets/kernel/qwidget_p.h b/src/widgets/kernel/qwidget_p.h
index 5f07a8802a..37f2c0e5c7 100644
--- a/src/widgets/kernel/qwidget_p.h
+++ b/src/widgets/kernel/qwidget_p.h
@@ -323,6 +323,7 @@ public:
     ~QWidgetPrivate();
 
     static QWidgetPrivate *get(QWidget *w) { return w->d_func(); }
+    static const QWidgetPrivate *get(const QWidget *w) { return w->d_func(); }
 
     QWExtra *extraData() const;
     QTLWExtra *topData() const;