Revert "QObject: simplify part of connection logic"

This reverts commit 1918c689d78b0f6a718343e7ebceb387acc32a97.
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>

1 files changed, 20 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
index f4c028c83e..5cf1f0e50f 100644
--- a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
+++ b/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()