Fix usage of std::enable_if_t to SFINAE out QProperty APIs

Declare an IfUntypedPropertyData alias and use that consistently.

Amends 311f889.

Pick-to: 6.7
Change-Id: If36ef8e2f9ce25e0ffe7b4b448c31ea5866acfc3
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qproperty.h

@@ -263,8 +263,7 @@ class Q_CORE_EXPORT QPropertyObserver : public QPropertyObserverBase
     QPropertyObserver &operator=(QPropertyObserver &&other) noexcept;
     ~QPropertyObserver();
 
-    template <typename Property,
-              typename = std::enable_if_t<std::is_base_of_v<QUntypedPropertyData, Property>>>
+    template <typename Property, QtPrivate::IsUntypedPropertyData<Property> = true>
     void setSource(const Property &property)
     { setSource(property.bindingData()); }
     void setSource(const QtPrivate::QPropertyBindingData &property);
@@ -303,8 +302,7 @@ class QPropertyChangeHandler : public QPropertyObserver
     {
     }
 
-    template <typename Property,
-              typename = std::enable_if_t<std::is_base_of_v<QUntypedPropertyData, Property>>>
+    template <typename Property, QtPrivate::IsUntypedPropertyData<Property> = true>
     Q_NODISCARD_CTOR
     QPropertyChangeHandler(const Property &property, Functor handler)
         : QPropertyObserver([](QPropertyObserver *self, QUntypedPropertyData *) {
@@ -335,7 +333,7 @@ class QPropertyNotifier : public QPropertyObserver
     }
 
     template <typename Functor, typename Property,
-              typename = std::enable_if_t<std::is_base_of_v<QUntypedPropertyData, Property>>>
+              QtPrivate::IsUntypedPropertyData<Property> = true>
     Q_NODISCARD_CTOR
     QPropertyNotifier(const Property &property, Functor handler)
         : QPropertyObserver([](QPropertyObserver *self, QUntypedPropertyData *) {
@@ -909,8 +907,7 @@ QPropertyAlias : public QPropertyObserver
             iface->setObserver(aliasedProperty(), this);
     }
 
-    template <typename Property,
-              typename = std::enable_if_t<std::is_base_of_v<QUntypedPropertyData, Property>>>
+    template <typename Property, QtPrivate::IsUntypedPropertyData<Property> = true>
     QPropertyAlias(Property *property)
         : QPropertyObserver(property),
           iface(&QtPrivate::QBindableInterfaceForProperty<Property>::iface)

src/corelib/kernel/qpropertyprivate.h

@@ -124,16 +124,13 @@ struct QPropertyObserverPointer;
 
 class QUntypedPropertyData
 {
-public:
-#if QT_DEPRECATED_SINCE(6, 8)
-    // sentinel to check whether a class inherits QUntypedPropertyData
-    struct QT_DEPRECATED_VERSION_X_6_8("Use std::is_base_of instead.")
-            InheritsQUntypedPropertyData
-    {
-    };
-#endif
 };
 
+namespace QtPrivate {
+template <typename T>
+using IsUntypedPropertyData = std::enable_if_t<std::is_base_of_v<QUntypedPropertyData, T>, bool>;
+}
+
 template <typename T>
 class QPropertyData;