QProperty: clean up unnecessary sentinel class

Instead of introducing the nested class InheritsQUntypedPropertyData
as a sentinel class for inheritance check, we can use BinaryTypeTrait
to handle the check. By doing this, we no longer need to maintain the
nested class.

Change-Id: Ie3aae976015d5fae6b6d072cad6ee52cd30b769d
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qproperty.h

@@ -263,7 +263,8 @@ public:
     QPropertyObserver &operator=(QPropertyObserver &&other) noexcept;
     ~QPropertyObserver();
 
-    template<typename Property, typename = typename Property::InheritsQUntypedPropertyData>
+    template <typename Property,
+              typename = std::enable_if_t<std::is_base_of_v<QUntypedPropertyData, Property>>>
     void setSource(const Property &property)
     { setSource(property.bindingData()); }
     void setSource(const QtPrivate::QPropertyBindingData &property);
@@ -302,7 +303,8 @@ public:
     {
     }
 
-    template<typename Property, typename = typename Property::InheritsQUntypedPropertyData>
+    template <typename Property,
+              typename = std::enable_if_t<std::is_base_of_v<QUntypedPropertyData, Property>>>
     Q_NODISCARD_CTOR
     QPropertyChangeHandler(const Property &property, Functor handler)
         : QPropertyObserver([](QPropertyObserver *self, QUntypedPropertyData *) {
@@ -332,7 +334,8 @@ public:
     {
     }
 
-    template<typename Functor, typename Property, typename = typename Property::InheritsQUntypedPropertyData>
+    template <typename Functor, typename Property,
+              typename = std::enable_if_t<std::is_base_of_v<QUntypedPropertyData, Property>>>
     Q_NODISCARD_CTOR
     QPropertyNotifier(const Property &property, Functor handler)
         : QPropertyObserver([](QPropertyObserver *self, QUntypedPropertyData *) {
@@ -906,7 +909,8 @@ public:
             iface->setObserver(aliasedProperty(), this);
     }
 
-    template<typename Property, typename = typename Property::InheritsQUntypedPropertyData>
+    template <typename Property,
+              typename = std::enable_if_t<std::is_base_of_v<QUntypedPropertyData, Property>>>
     QPropertyAlias(Property *property)
         : QPropertyObserver(property),
           iface(&QtPrivate::QBindableInterfaceForProperty<Property>::iface)

src/corelib/kernel/qpropertyprivate.h

@@ -125,8 +125,13 @@ struct QPropertyObserverPointer;
 class QUntypedPropertyData
 {
 public:
+#if QT_DEPRECATED_SINCE(6, 8)
     // sentinel to check whether a class inherits QUntypedPropertyData
-    struct InheritsQUntypedPropertyData {};
+    struct QT_DEPRECATED_VERSION_X_6_8("Use std::is_base_of instead.")
+            InheritsQUntypedPropertyData
+    {
+    };
+#endif
 };
 
 template <typename T>

tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp

@@ -29,6 +29,7 @@ class tst_QProperty : public QObject
 {
     Q_OBJECT
 private slots:
+    void inheritQUntypedPropertyData();
     void functorBinding();
     void basicDependencies();
     void multipleDependencies();
@@ -109,6 +110,59 @@ private slots:
     void propertyUpdateViaSignaledProperty();
 };
 
+namespace {
+template <class T>
+constexpr auto isDerivedFromQUntypedPropertyData = std::is_base_of_v<QUntypedPropertyData, T>;
+
+template <typename Property>
+constexpr auto isDerivedFromQUntypedPropertyDataFunc(const Property &property)
+{
+    Q_UNUSED(property);
+    return isDerivedFromQUntypedPropertyData<Property>;
+}
+
+template <typename Property>
+constexpr auto isDerivedFromQUntypedPropertyDataFunc(Property *property)
+{
+    Q_UNUSED(property);
+    return isDerivedFromQUntypedPropertyData<Property>;
+}
+} // namespace
+
+void tst_QProperty::inheritQUntypedPropertyData()
+{
+    class propertyPublic : public QUntypedPropertyData
+    {
+    };
+    class propertyPrivate : private QUntypedPropertyData
+    {
+    };
+
+    // Compile time test
+    static_assert(isDerivedFromQUntypedPropertyData<propertyPublic>);
+    static_assert(isDerivedFromQUntypedPropertyData<propertyPrivate>);
+    static_assert(isDerivedFromQUntypedPropertyData<QPropertyData<int>>);
+    static_assert(isDerivedFromQUntypedPropertyData<QProperty<int>>);
+
+    // Run time test
+    propertyPublic _propertyPublic;
+    propertyPrivate _propertyPrivate;
+    QPropertyData<int> qpropertyData;
+    QProperty<int> qproperty;
+    std::unique_ptr<propertyPublic> _propertyPublicPtr{ new propertyPublic };
+    std::unique_ptr<propertyPrivate> _propertyPrivatePtr{ new propertyPrivate };
+    std::unique_ptr<QPropertyData<int>> qpropertyDataPtr{ new QPropertyData<int> };
+    std::unique_ptr<QProperty<int>> qpropertyPtr{ new QProperty<int> };
+    QVERIFY(isDerivedFromQUntypedPropertyDataFunc(_propertyPublic));
+    QVERIFY(isDerivedFromQUntypedPropertyDataFunc(_propertyPrivate));
+    QVERIFY(isDerivedFromQUntypedPropertyDataFunc(qpropertyData));
+    QVERIFY(isDerivedFromQUntypedPropertyDataFunc(qproperty));
+    QVERIFY(isDerivedFromQUntypedPropertyDataFunc(_propertyPublicPtr.get()));
+    QVERIFY(isDerivedFromQUntypedPropertyDataFunc(_propertyPrivatePtr.get()));
+    QVERIFY(isDerivedFromQUntypedPropertyDataFunc(qpropertyDataPtr.get()));
+    QVERIFY(isDerivedFromQUntypedPropertyDataFunc(qpropertyPtr.get()));
+}
+
 void tst_QProperty::functorBinding()
 {
     QProperty<int> property([]() { return 42; });