QObject::connect(): fail to connect to a functor if UniqueConnection is passed

The connect() documentation makes it clear that UniqueConnection does
not work with free functions / function objects and the like; only with
actual PMFs. Rather than silently *ignoring* the flag, be vocal about its
presence by warning, and make the connection fail (as the user has
passed an illegal argument).

[ChangeLog][QtCore][QObject] QObject::connect() now will refuse to
connect a signal to a free function / function object if UniqueConnection
is passed. Note that UniqueConnection has never worked for such
connections -- the flag was simply ignored, and they were established
multiple times. Now, the flag is not ignored and results in a connection
failure (as well as a runtime warning by Qt).

Change-Id: I6509667018c74f9bd24910cde0a1b16c5f84f064
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qobject.cpp

@@ -4974,6 +4974,19 @@ QMetaObject::Connection QObject::connectImpl(const QObject *sender, void **signa
     return QObjectPrivate::connectImpl(sender, signal_index, receiver, slot, slotObj, type, types, senderMetaObject);
 }
 
+static void connectWarning(const QObject *sender,
+                           const QMetaObject *senderMetaObject,
+                           const QObject *receiver,
+                           const char *message)
+{
+    const char *senderString = sender ? sender->metaObject()->className()
+                                      : senderMetaObject ? senderMetaObject->className()
+                                      : "Unknown";
+    const char *receiverString = receiver ? receiver->metaObject()->className()
+                                          : "Unknown";
+    qCWarning(lcConnect, "QObject::connect(%s, %s): %s", senderString, receiverString, message);
+}
+
 /*!
     \internal
 
@@ -4986,18 +4999,24 @@ QMetaObject::Connection QObjectPrivate::connectImpl(const QObject *sender, int s
                                              QtPrivate::QSlotObjectBase *slotObj, int type,
                                              const int *types, const QMetaObject *senderMetaObject)
 {
-    if (!sender || !receiver || !slotObj || !senderMetaObject) {
+    auto connectFailureGuard = qScopeGuard([&]()
-        const char *senderString = sender ? sender->metaObject()->className()
+    {
-                                          : senderMetaObject ? senderMetaObject->className()
-                                          : "Unknown";
-        const char *receiverString = receiver ? receiver->metaObject()->className()
-                                              : "Unknown";
-        qCWarning(lcConnect, "QObject::connect(%s, %s): invalid nullptr parameter", senderString, receiverString);
         if (slotObj)
             slotObj->destroyIfLastRef();
+    });
+
+    if (!sender || !receiver || !slotObj || !senderMetaObject) {
+        connectWarning(sender, senderMetaObject, receiver, "invalid nullptr parameter");
         return QMetaObject::Connection();
     }
 
+    if (type & Qt::UniqueConnection && !slot) {
+        connectWarning(sender, senderMetaObject, receiver, "unique connections require a pointer to member function of a QObject subclass");
+        return QMetaObject::Connection();
+    }
+
+    connectFailureGuard.dismiss();
+
     QObject *s = const_cast<QObject *>(sender);
     QObject *r = const_cast<QObject *>(receiver);
 

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -6241,10 +6241,11 @@ void tst_QObject::connectFunctorWithContextUnique()
 
     SenderObject sender;
     ReceiverObject receiver;
-    QObject::connect(&sender, &SenderObject::signal1, &receiver, &ReceiverObject::slot1);
+    QVERIFY(QObject::connect(&sender, &SenderObject::signal1, &receiver, &ReceiverObject::slot1));
     receiver.count_slot1 = 0;
 
-    QObject::connect(&sender, &SenderObject::signal1, &receiver, SlotFunctor(), Qt::UniqueConnection);
+    QTest::ignoreMessage(QtWarningMsg, "QObject::connect(SenderObject, ReceiverObject): unique connections require a pointer to member function of a QObject subclass");
+    QVERIFY(!QObject::connect(&sender, &SenderObject::signal1, &receiver, [&](){ receiver.slot1(); }, Qt::UniqueConnection));
 
     sender.emitSignal1();
     QCOMPARE(receiver.count_slot1, 1);