QtQml: Recognize JavaScript null as equivalent to nullptr QObject*

Otherwise we can go into infinite recursion there.

Fixes: QTBUG-108651
Change-Id: I73a7c524b28d0eacb151f2090e3dce6afbc029d1
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit c58d2e5229c39e8007ce5b93822c66af5e765a50)

3 files changed, 41 insertions, 1 deletions

diff --git a/src/qml/qml/qqmlvmemetaobject.cpp b/src/qml/qml/qqmlvmemetaobject.cpp
index 0cc26fe0fb..710aadda45 100644
--- a/src/qml/qml/qqmlvmemetaobject.cpp
+++ b/src/qml/qml/qqmlvmemetaobject.cpp
@@ -896,7 +896,7 @@ int QQmlVMEMetaObject::metaCall(QObject *o, QMetaObject::Call c, int _id, void *
                                     QObject *arg = *reinterpret_cast<QObject **>(a[0]);
                                     if (const auto *wrap = sv->as<QV4::QObjectWrapper>())
                                         needActivate = wrap->object() != arg;
-                                    else
+                                    else if (arg != nullptr || !sv->isNull())
                                         needActivate = true;
                                     if (needActivate)
                                         writeProperty(id, arg);
diff --git a/tests/auto/qml/qqmllanguage/data/nullIsNull.qml b/tests/auto/qml/qqmllanguage/data/nullIsNull.qml
new file mode 100644
index 0000000000..a6cbbee0bf
--- /dev/null
+++ b/tests/auto/qml/qqmllanguage/data/nullIsNull.qml
@@ -0,0 +1,28 @@
+import QtQml
+
+QtObject {
+    id: root
+    property alias someProperty: internal.someProperty
+
+    property Timer t: Timer {
+        interval: 1
+        running: true
+        onTriggered: root.someProperty = null
+    }
+
+    property QtObject a: QtObject {
+        id: someObjectInstance
+    }
+
+    property QtObject b: QtObject {
+        id: internal
+        property QtObject someProperty: someObjectInstance ? someObjectInstance : null
+    }
+
+    property Connections c: Connections {
+        target: internal
+        function onSomePropertyChanged() {
+              internal.someProperty = null
+        }
+    }
+}
diff --git a/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp b/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
index 0380456781..409c56d504 100644
--- a/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
+++ b/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
@@ -391,6 +391,7 @@ private slots:
     void badGroupedProperty();
     void bindableOnly();
     void signalInlineComponentArg();
+    void nullIsNull();
 
 private:
     QQmlEngine engine;
@@ -7421,6 +7422,17 @@ void tst_qqmllanguage::signalInlineComponentArg()
     }
 }
 
+void tst_qqmllanguage::nullIsNull()
+{
+    QQmlEngine engine;
+    QQmlComponent c(&engine, testFileUrl("nullIsNull.qml"));
+    QVERIFY2(c.isReady(), qPrintable(c.errorString()));
+    QScopedPointer<QObject> o(c.create());
+    QVERIFY(!o.isNull());
+    QVERIFY(o->property("someProperty").value<QObject*>() != nullptr);
+    QTRY_COMPARE(o->property("someProperty").value<QObject*>(), nullptr);
+}
+
 QTEST_MAIN(tst_qqmllanguage)
 
 #include "tst_qqmllanguage.moc"