Fix interface handling in bindings

If a type had a property with an interface type, we did not support the
case where the assigned binding value is convertible to the interface.

This is now fixed by adding a last new check to QQmlPrivate::write

Fixes: QTBUG-78721
Change-Id: I0b85fbfdf8561ba43610ac343001ae380287a674
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

3 files changed, 117 insertions, 1 deletions

diff --git a/src/qml/qml/qqmlproperty.cpp b/src/qml/qml/qqmlproperty.cpp
index 5f57e0eca1..b6baa86bd7 100644
--- a/src/qml/qml/qqmlproperty.cpp
+++ b/src/qml/qml/qqmlproperty.cpp
@@ -1373,8 +1373,9 @@ bool QQmlPropertyPrivate::write(QObject *object,
             }
         }
         if (!ok) {
-            // the only other option is that they are assigning a single value
+            // the only other options are that they are assigning a single value
             // to a sequence type property (eg, an int to a QList<int> property).
+            // or that we encountered an interface type
             // Note that we've already handled single-value assignment to QList<QUrl> properties.
             if (variantType == QVariant::Int && propertyType == qMetaTypeId<QList<int> >()) {
                 QList<int> list;
@@ -1405,6 +1406,15 @@ bool QQmlPropertyPrivate::write(QObject *object,
             }
         }
 
+        if (!ok && QQmlMetaType::isInterface(propertyType)) {
+            auto valueAsQObject = qvariant_cast<QObject *>(value);
+            if (valueAsQObject && valueAsQObject->qt_metacast(QQmlMetaType::interfaceIId(propertyType))) {
+                // this case can occur when object has an interface type
+                // and the variant contains a type implementing the interface
+                return property.writeProperty(object, const_cast<void *>(value.constData()), flags);
+            }
+        }
+
         if (ok) {
             return property.writeProperty(object, const_cast<void *>(v.constData()), flags);
         } else {
diff --git a/tests/auto/qml/qqmlproperty/data/interfaceBinding.qml b/tests/auto/qml/qqmlproperty/data/interfaceBinding.qml
new file mode 100644
index 0000000000..4e72a75f42
--- /dev/null
+++ b/tests/auto/qml/qqmlproperty/data/interfaceBinding.qml
@@ -0,0 +1,27 @@
+import QtQuick 2.12
+import io.qt.bugreports 1.0
+Item {
+    InterfaceConsumer {
+        objectName: "a1"
+        i: A {
+            property int i: 42
+        }
+    }
+
+    InterfaceConsumer {
+        objectName: "a2"
+        property A a: A {
+            property int i: 43
+        }
+        i: a
+    }
+
+    InterfaceConsumer {
+        objectName: "a3"
+        property A a: A {
+            id : aa
+            property int i: 44
+        }
+        i: aa
+    }
+}
diff --git a/tests/auto/qml/qqmlproperty/tst_qqmlproperty.cpp b/tests/auto/qml/qqmlproperty/tst_qqmlproperty.cpp
index bc407e97a2..7ebe0bac52 100644
--- a/tests/auto/qml/qqmlproperty/tst_qqmlproperty.cpp
+++ b/tests/auto/qml/qqmlproperty/tst_qqmlproperty.cpp
@@ -40,6 +40,7 @@
 #endif
 #include <QtCore/private/qobject_p.h>
 #include "../../shared/util.h"
+#include "qobject.h"
 
 #include <QDebug>
 class MyQmlObject : public QObject
@@ -148,6 +149,7 @@ private slots:
     void readOnlyDynamicProperties();
     void aliasToIdWithMatchingQmlFileNameOnCaseInsensitiveFileSystem();
     void nullPropertyBinding();
+    void interfaceBinding();
 
     void floatToStringPrecision_data();
     void floatToStringPrecision();
@@ -2083,6 +2085,83 @@ void tst_qqmlproperty::nullPropertyBinding()
     QMetaObject::invokeMethod(root.get(), "tog");
 }
 
+struct Interface {
+};
+
+QT_BEGIN_NAMESPACE
+#define MyInterface_iid "io.qt.bugreports.Interface"
+Q_DECLARE_INTERFACE(Interface, MyInterface_iid);
+QT_END_NAMESPACE
+
+class A : public QObject, Interface {
+    Q_OBJECT
+    Q_INTERFACES(Interface)
+};
+
+class B : public QObject, Interface {
+    Q_OBJECT
+    Q_INTERFACES(Interface)
+};
+
+class C : public QObject {
+    Q_OBJECT
+};
+
+class InterfaceConsumer : public QObject {
+    Q_OBJECT
+    Q_PROPERTY(Interface* i READ interface WRITE setInterface NOTIFY interfaceChanged)
+    Q_PROPERTY(int testValue READ testValue NOTIFY testValueChanged)
+
+
+public:
+
+    Interface* interface() const
+    {
+        return m_interface;
+    }
+    void setInterface(Interface* interface)
+    {
+        QObject* object = reinterpret_cast<QObject*>(interface);
+        m_testValue = object->property("i").toInt();
+        emit testValueChanged();
+        if (m_interface == interface)
+            return;
+
+        m_interface = interface;
+        emit interfaceChanged();
+    }
+
+    int testValue() {
+        return m_testValue;
+    }
+
+signals:
+    void interfaceChanged();
+    void testValueChanged();
+
+private:
+    Interface* m_interface = nullptr;
+    int m_testValue = 0;
+};
+void tst_qqmlproperty::interfaceBinding()
+{
+
+        qmlRegisterInterface<Interface>("Interface");
+        qmlRegisterType<A>("io.qt.bugreports", 1, 0, "A");
+        qmlRegisterType<B>("io.qt.bugreports", 1, 0, "B");
+        qmlRegisterType<C>("io.qt.bugreports", 1, 0, "C");
+        qmlRegisterType<InterfaceConsumer>("io.qt.bugreports", 1, 0, "InterfaceConsumer");
+
+        const QUrl url = testFileUrl("interfaceBinding.qml");
+        QQmlEngine engine;
+        QQmlComponent component(&engine, url);
+        QScopedPointer<QObject> root(component.create());
+        QVERIFY(root);
+        QCOMPARE(root->findChild<QObject*>("a1")->property("testValue").toInt(), 42);
+        QCOMPARE(root->findChild<QObject*>("a2")->property("testValue").toInt(), 43);
+        QCOMPARE(root->findChild<QObject*>("a3")->property("testValue").toInt(), 44);
+}
+
 void tst_qqmlproperty::floatToStringPrecision_data()
 {
     QTest::addColumn<QString>("propertyName");