Revert "QObject: simplify part of connection logic"

This reverts commit 1918c689d7.
The template gets always instantiated in QObjectPrivate::connect, even
if the connection types is not Qt::(Blocking)QueuedConnection. For
non-queued connections we however support using incomplete types in
connect. The only way to fix this would be to make the connection type a
template parameter of QObjectPrivate::connect (or at lesat pass some
compile time constant indicating "blocking"-ness) along, so that we can
use if constexpr instead of if. As all involved classes are private, we
can postpone investigating this solution to 6.1

Change-Id: Ieffaf015f8e60ca6ac6f85eb9e2756e480060b4f
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qobject_impl.h

@@ -56,10 +56,23 @@ namespace QtPrivate {
         Logic to statically generate the array of qMetaTypeId
         ConnectionTypes<FunctionPointer<Signal>::Arguments>::types() returns an array
         of int that is suitable for the types arguments of the connection functions.
+
+        The array only exist of all the types are declared as a metatype
+        (detected using the TypesAreDeclaredMetaType helper struct)
+        If one of the type is not declared, the function return 0 and the signal
+        cannot be used in queued connection.
     */
-    template <typename T> struct ConnectionTypes;
-    template <typename... Args> struct ConnectionTypes<List<Args...>>
-    { static const int *types() { static const int t[sizeof...(Args) + 1] = { QMetaType::fromType<Args>().id()..., 0 }; return t; } };
+    template <typename ArgList> struct TypesAreDeclaredMetaType { enum { Value = false }; };
+    template <> struct TypesAreDeclaredMetaType<List<>> { enum { Value = true }; };
+    template <typename Arg, typename... Tail> struct TypesAreDeclaredMetaType<List<Arg, Tail...> >
+    { enum { Value = QMetaTypeId2<Arg>::Defined && TypesAreDeclaredMetaType<List<Tail...>>::Value }; };
+
+    template <typename ArgList, bool Declared = TypesAreDeclaredMetaType<ArgList>::Value > struct ConnectionTypes
+    { static const int *types() { return nullptr; } };
+    template <> struct ConnectionTypes<List<>, true>
+    { static const int *types() { return nullptr; } };
+    template <typename... Args> struct ConnectionTypes<List<Args...>, true>
+    { static const int *types() { static const int t[sizeof...(Args) + 1] = { (QtPrivate::QMetaTypeIdHelper<Args>::qt_metatype_id())..., 0 }; return t; } };
 
     // implementation of QSlotObjectBase for which the slot is a static function
     // Args and R are the List of arguments and the return type of the signal to which the slot is connected.

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

@@ -71,6 +71,7 @@ private slots:
     void disconnectNotify_metaObjConnection();
     void connectNotify_connectSlotsByName();
     void connectDisconnectNotify_shadowing();
+    void connectReferenceToIncompleteTypes();
     void emitInDefinedOrder();
     void customTypes();
     void streamCustomTypes();
@@ -881,6 +882,25 @@ void tst_QObject::connectDisconnectNotify()
     QCOMPARE(s.connectedSignals.size(), 1);
 }
 
+struct Incomplete;
+class QObjectWithIncomplete : public QObject {
+    Q_OBJECT
+
+public:
+    QObjectWithIncomplete(QObject *parent=nullptr) : QObject(parent) {}
+signals:
+    void signalWithIncomplete(const Incomplete &);
+public slots:
+    void slotWithIncomplete(const Incomplete &) {}
+};
+
+void tst_QObject::connectReferenceToIncompleteTypes() {
+    QObjectWithIncomplete withIncomplete;
+    auto connection = QObject::connect(&withIncomplete, &QObjectWithIncomplete::signalWithIncomplete,
+                                       &withIncomplete, &QObjectWithIncomplete::slotWithIncomplete);
+    QVERIFY(connection);
+}
+
 static void connectDisconnectNotifyTestSlot() {}
 
 void tst_QObject::connectDisconnectNotifyPMF()