Deprecate QPropertyAlias

That use case for the class never materialized, and it was actually
meant for internal use. However, we put it into the public header, so
we cannot remove it (and while undocumented, someone actually used it at
some point, compare e4d62651c278c468f71ce097979bc1183ffd0713).

Mark it as deprecated instead so that it can be finally be removed in Qt
7.

Change-Id: I058c5831a44610121fbec6eaddebd8b33d4a16c9
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/kernel/qproperty.cpp

@@ -641,11 +641,13 @@ QPropertyObserver::QPropertyObserver(ChangeHandler changeHandler)
     d.setChangeHandler(changeHandler);
 }
 
+#if QT_DEPRECATED_SINCE(6, 6)
 QPropertyObserver::QPropertyObserver(QUntypedPropertyData *data)
 {
     aliasData = data;
     next.setTag(ObserverIsAlias);
 }
+#endif
 
 /*! \internal
 */

src/corelib/kernel/qproperty.h

@@ -217,7 +217,9 @@ public:
         ObserverNotifiesBinding, // observer was installed to notify bindings that obsverved property changed
         ObserverNotifiesChangeHandler, // observer is a change handler, which runs on every change
         ObserverIsPlaceholder,  // the observer before this one is currently evaluated in QPropertyObserver::notifyObservers.
+#if QT_DEPRECATED_SINCE(6, 6)
         ObserverIsAlias
+#endif
     };
 protected:
     using ChangeHandler = void (*)(QPropertyObserver*, QUntypedPropertyData *);
@@ -257,7 +259,9 @@ public:
 
 protected:
     QPropertyObserver(ChangeHandler changeHandler);
-    QPropertyObserver(QUntypedPropertyData *aliasedPropertyPtr);
+#if QT_DEPRECATED_SINCE(6, 6)
+    QT_DEPRECATED QPropertyObserver(QUntypedPropertyData *aliasedPropertyPtr);
+#endif
 
     QUntypedPropertyData *aliasedProperty() const
     {
@@ -867,13 +871,16 @@ public:
     }
 };
 
+#if QT_DEPRECATED_SINCE(6, 6)
 template<typename T>
-class QPropertyAlias : public QPropertyObserver
+class QT_DEPRECATED_X("Class was only meant for internal use, use a QProperty and add a binding to the target")
+QPropertyAlias : public QPropertyObserver
 {
     Q_DISABLE_COPY_MOVE(QPropertyAlias)
     const QtPrivate::QBindableInterface *iface = nullptr;
 
 public:
+    QT_WARNING_PUSH QT_WARNING_DISABLE_DEPRECATED
     QPropertyAlias(QProperty<T> *property)
         : QPropertyObserver(property),
           iface(&QtPrivate::QBindableInterfaceForProperty<QProperty<T>>::iface)
@@ -989,7 +996,9 @@ public:
     {
         return aliasedProperty() != nullptr;
     }
+    QT_WARNING_POP
 };
+#endif
 
 template<typename Class, typename T, auto Offset, auto Signal = nullptr>
 class QObjectBindableProperty : public QPropertyData<T>

src/corelib/kernel/qproperty_p.h

@@ -124,13 +124,17 @@ struct QPropertyObserverPointer
     void unlink()
     {
         unlink_common();
+#if QT_DEPRECATED_SINCE(6, 6)
         if (ptr->next.tag() == QPropertyObserver::ObserverIsAlias)
             ptr->aliasData = nullptr;
+#endif
     }
 
     void unlink_fast()
     {
+#if QT_DEPRECATED_SINCE(6, 6)
         Q_ASSERT(ptr->next.tag() != QPropertyObserver::ObserverIsAlias);
+#endif
         unlink_common();
     }
 
@@ -881,8 +885,10 @@ inline void QPropertyObserverPointer::notify(QUntypedPropertyData *propertyDataP
         case QPropertyObserver::ObserverIsPlaceholder:
             // recursion is already properly handled somewhere else
             break;
+#if QT_DEPRECATED_SINCE(6, 6)
         case QPropertyObserver::ObserverIsAlias:
             break;
+#endif
         default: Q_UNREACHABLE();
         }
         observer = next;

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

@@ -2240,6 +2240,8 @@ void tst_QProperty::selfBindingShouldNotCrash()
 
 void tst_QProperty::qpropertyAlias()
 {
+#if QT_DEPRECATED_SINCE(6, 6)
+    QT_WARNING_PUSH QT_WARNING_DISABLE_DEPRECATED
     std::unique_ptr<QProperty<int>> i {new QProperty<int>};
     QPropertyAlias<int> alias(i.get());
     QVERIFY(alias.isValid());
@@ -2251,6 +2253,8 @@ void tst_QProperty::qpropertyAlias()
     QCOMPARE(alias.value(), 42);
     i.reset();
     QVERIFY(!alias.isValid());
+    QT_WARNING_POP
+#endif
 }
 
 void tst_QProperty::scheduleNotify()