4 files changed, 57 insertions, 12 deletions

diff --git a/src/corelib/kernel/qproperty.cpp b/src/corelib/kernel/qproperty.cpp
index 3123858d83..942abfe6ba 100644
--- a/src/corelib/kernel/qproperty.cpp
+++ b/src/corelib/kernel/qproperty.cpp
@@ -558,9 +558,10 @@ QPropertyObserver::QPropertyObserver(ChangeHandler changeHandler)
     d.setChangeHandler(changeHandler);
 }
 
-QPropertyObserver::QPropertyObserver(QUntypedPropertyData *)
+QPropertyObserver::QPropertyObserver(QUntypedPropertyData *data)
 {
-    // ### Qt 7: Remove, currently left for binary compatibility
+    aliasData = data;
+    next.setTag(ObserverIsAlias);
 }
 
 /*! \internal
@@ -609,16 +610,38 @@ QPropertyObserver &QPropertyObserver::operator=(QPropertyObserver &&other) noexc
     return *this;
 }
 
+#define UNLINK_COMMON \
+    if (ptr->next) \
+        ptr->next->prev = ptr->prev; \
+    if (ptr->prev) \
+        ptr->prev.setPointer(ptr->next.data()); \
+    ptr->next = nullptr; \
+    ptr->prev.clear();
+
+/*!
+    \internal
+    Unlinks
+ */
 void QPropertyObserverPointer::unlink()
 {
-    if (ptr->next)
-        ptr->next->prev = ptr->prev;
-    if (ptr->prev)
-        ptr->prev.setPointer(ptr->next.data());
-    ptr->next = nullptr;
-    ptr->prev.clear();
+    UNLINK_COMMON
+    if (ptr->next.tag() == QPropertyObserver::ObserverIsAlias)
+        ptr->aliasData = nullptr;
 }
 
+/*!
+    \internal
+    Like unlink, but does not handle ObserverIsAlias.
+    Must only be called in places where we know that we are not dealing
+    with such an observer.
+ */
+void QPropertyObserverPointer::unlink_fast()
+{
+    Q_ASSERT(ptr->next.tag() != QPropertyObserver::ObserverIsAlias);
+    UNLINK_COMMON
+}
+#undef UNLINK_COMMON
+
 void QPropertyObserverPointer::setChangeHandler(QPropertyObserver::ChangeHandler changeHandler)
 {
     Q_ASSERT(ptr->next.tag() != QPropertyObserver::ObserverIsPlaceholder);
@@ -666,7 +689,7 @@ struct [[nodiscard]] QPropertyObserverNodeProtector {
 
     ~QPropertyObserverNodeProtector() {
         QPropertyObserverPointer d{static_cast<QPropertyObserver *>(&m_placeHolder)};
-        d.unlink();
+        d.unlink_fast();
     }
 };
 
@@ -721,6 +744,8 @@ void QPropertyObserverPointer::notify(QUntypedPropertyData *propertyDataPtr)
         case QPropertyObserver::ObserverIsPlaceholder:
             // recursion is already properly handled somewhere else
             break;
+        case QPropertyObserver::ObserverIsAlias:
+            break;
         default: Q_UNREACHABLE();
         }
         observer = next;
diff --git a/src/corelib/kernel/qproperty.h b/src/corelib/kernel/qproperty.h
index 133c7a7e14..356eb43cd3 100644
--- a/src/corelib/kernel/qproperty.h
+++ b/src/corelib/kernel/qproperty.h
@@ -223,7 +223,8 @@ public:
     enum ObserverTag {
         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.
+        ObserverIsPlaceholder,  // the observer before this one is currently evaluated in QPropertyObserver::notifyObservers.
+        ObserverIsAlias
     };
 protected:
     using ChangeHandler = void (*)(QPropertyObserver*, QUntypedPropertyData *);
@@ -244,6 +245,7 @@ private:
     union {
         QPropertyBindingPrivate *binding = nullptr;
         ChangeHandler changeHandler;
+        QUntypedPropertyData *aliasData;
     };
 };
 
@@ -266,7 +268,7 @@ protected:
 
     QUntypedPropertyData *aliasedProperty() const
     {
-        return nullptr;
+        return aliasData;
     }
 
 private:
diff --git a/src/corelib/kernel/qproperty_p.h b/src/corelib/kernel/qproperty_p.h
index e77bc4d834..55f0dc032c 100644
--- a/src/corelib/kernel/qproperty_p.h
+++ b/src/corelib/kernel/qproperty_p.h
@@ -103,6 +103,7 @@ struct QPropertyObserverPointer
     QPropertyObserver *ptr = nullptr;
 
     void unlink();
+    void unlink_fast();
 
     void setBindingToNotify(QPropertyBindingPrivate *binding);
     void setBindingToNotify_unsafe(QPropertyBindingPrivate *binding);
@@ -289,7 +290,7 @@ public:
     void clearDependencyObservers() {
         for (size_t i = 0; i < qMin(dependencyObserverCount, inlineDependencyObservers.size()); ++i) {
             QPropertyObserverPointer p{&inlineDependencyObservers[i]};
-            p.unlink();
+            p.unlink_fast();
         }
         if (heapObservers)
             heapObservers->clear();
diff --git a/tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp b/tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp
index 214512f20d..3cfdcfc7b0 100644
--- a/tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp
+++ b/tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp
@@ -115,6 +115,8 @@ private slots:
     void bindableInterfaceOfCompatPropertyUsesSetter();
 
     void selfBindingShouldNotCrash();
+
+    void qpropertyAlias();
 };
 
 void tst_QProperty::functorBinding()
@@ -1856,6 +1858,21 @@ void tst_QProperty::selfBindingShouldNotCrash()
     QVERIFY(i.binding().error().hasError());
 }
 
+void tst_QProperty::qpropertyAlias()
+{
+    std::unique_ptr<QProperty<int>> i {new QProperty<int>};
+    QPropertyAlias<int> alias(i.get());
+    QVERIFY(alias.isValid());
+    alias.setValue(42);
+    QCOMPARE(i->value(), 42);
+    QProperty<int> j;
+    i->setBinding([&]() -> int { return j; });
+    j.setValue(42);
+    QCOMPARE(alias.value(), 42);
+    i.reset();
+    QVERIFY(!alias.isValid());
+}
+
 QTEST_MAIN(tst_QProperty);
 
 #undef QT_SOURCE_LOCATION_NAMESPACE