Native style: improve type safety

A QuickStyleItem instantiated inside a QML file set property "control" to
point to the control it styles. It's required that this item matches the
control it styles, so that e.g a NativeStyle.Button is only used for styling
a Button.qml (and not e.g a Slider.qml), since NativeStyle.Button
will crash otherwise. We do a quick runtime check in debug mode rather than
a compile time check to avoid to much churn in the c++ code to enforce this.

Change-Id: I31dca31ab4f1fdf08780ffb01c533ac5837784f2
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

2 files changed, 11 insertions, 2 deletions

diff --git a/src/imports/nativestyle/items/qquickstyleitem.cpp b/src/imports/nativestyle/items/qquickstyleitem.cpp
index bf6ac25e..a7bce506 100644
--- a/src/imports/nativestyle/items/qquickstyleitem.cpp
+++ b/src/imports/nativestyle/items/qquickstyleitem.cpp
@@ -36,7 +36,6 @@
 
 #include "qquickstyleitem.h"
 
-#include <QtQml/qqmlinfo.h>
 #include <QtQuick/qsgninepatchnode.h>
 #include <QtQuick/private/qquickwindow_p.h>
 
diff --git a/src/imports/nativestyle/items/qquickstyleitem.h b/src/imports/nativestyle/items/qquickstyleitem.h
index 2535abc2..5d0b6671 100644
--- a/src/imports/nativestyle/items/qquickstyleitem.h
+++ b/src/imports/nativestyle/items/qquickstyleitem.h
@@ -39,6 +39,7 @@
 
 #include <QtCore/qdebug.h>
 #include <QtQml/qqml.h>
+#include <QtQml/qqmlinfo.h>
 #include <QtQuick/private/qquickitem_p.h>
 #include <QtQuickTemplates2/private/qquickcontrol_p.h>
 
@@ -209,7 +210,16 @@ protected:
 
     inline QSize contentSize() { return m_contentSize.toSize(); }
     inline static QStyle *style() { return QQuickNativeStyle::style(); }
-    template <class T> inline const T* control() const { return static_cast<T *>(m_control.data()); }
+
+    template <class T> inline const T* control() const {
+#ifdef QT_DEBUG
+        if (!dynamic_cast<T *>(m_control.data())) {
+            qmlWarning(this) << "control property is not of correct type";
+            Q_UNREACHABLE();
+        }
+#endif
+        return static_cast<T *>(m_control.data());
+    }
 
 #ifdef QT_DEBUG
     bool m_debug = false;