Make it possible to use QObjectPrivate::connect in private code

In most QObjectPrivate-subclasses, the Q_DECLARE_PUBLIC macro is used in
the private segment of the class declaration. In that case, the q_ptr
becomes a private member of the private class, and then the
QObjectPrivate::connect function can no longer be used, as it needs to
access the d_ptr.

Fix this by declaring QObjectPrivate, and the static-assert-helper, as
friends of the class using the Q_DECLARE_PUBLIC macro.

Adapt the QObject test by moving the Q_DECLARE_PUBLIC macro into
the private section of the test-private, and add a compile test.

Pick-to: 6.5 6.5.0
Change-Id: Ifc04be3b305221e138b1e08bb3a3838d871f4fcb
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/global/qtclasshelpermacros.h

@@ -78,6 +78,13 @@ template <typename T> inline T *qGetPtrHelper(T *ptr) noexcept { return ptr; }
 template <typename Ptr> inline auto qGetPtrHelper(Ptr &ptr) noexcept -> decltype(ptr.get())
 { static_assert(noexcept(ptr.get()), "Smart d pointers for Q_DECLARE_PRIVATE must have noexcept get()"); return ptr.get(); }
 
+class QObject;
+class QObjectPrivate;
+namespace QtPrivate {
+    template <typename ObjPrivate> void assertObjectType(QObjectPrivate *d);
+    inline const QObject *getQObject(const QObjectPrivate *d);
+}
+
 #define Q_DECLARE_PRIVATE(Class) \
     inline Class##Private* d_func() noexcept \
     { Q_CAST_IGNORE_ALIGN(return reinterpret_cast<Class##Private *>(qGetPtrHelper(d_ptr));) } \
@@ -95,7 +102,9 @@ template <typename Ptr> inline auto qGetPtrHelper(Ptr &ptr) noexcept -> decltype
 #define Q_DECLARE_PUBLIC(Class)                                    \
     inline Class* q_func() noexcept { return static_cast<Class *>(q_ptr); } \
     inline const Class* q_func() const noexcept { return static_cast<const Class *>(q_ptr); } \
-    friend class Class;
+    friend class Class; \
+    friend const QObject *QtPrivate::getQObject(const QObjectPrivate *d); \
+    template <typename ObjPrivate> friend void QtPrivate::assertObjectType(QObjectPrivate *d);
 
 #define Q_D(Class) Class##Private * const d = d_func()
 #define Q_Q(Class) Class * const q = q_func()

src/corelib/kernel/qobject_p.h

@@ -243,6 +243,7 @@ inline void QObjectPrivate::disconnectNotify(const QMetaMethod &signal)
 }
 
 namespace QtPrivate {
+inline const QObject *getQObject(const QObjectPrivate *d) { return d->q_func(); }
 
 template <typename Func>
 struct FunctionStorageByValue
@@ -322,7 +323,7 @@ inline QMetaObject::Connection QObjectPrivate::connect(const typename QtPrivate:
         types = QtPrivate::ConnectionTypes<typename SignalType::Arguments>::types();
 
     return QObject::connectImpl(sender, reinterpret_cast<void **>(&signal),
-        receiverPrivate->q_ptr, reinterpret_cast<void **>(&slot),
+        QtPrivate::getQObject(receiverPrivate), reinterpret_cast<void **>(&slot),
         new QtPrivate::QPrivateSlotObject<Func2, typename QtPrivate::List_Left<typename SignalType::Arguments, SlotType::ArgumentCount>::Value,
                                         typename SignalType::ReturnType>(slot),
         type, types, &SignalType::Object::staticMetaObject);

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

@@ -5982,15 +5982,15 @@ class ConnectToPrivateSlotPrivate;
 
 class ConnectToPrivateSlot :public QObject {
     Q_OBJECT
+    Q_DECLARE_PRIVATE(ConnectToPrivateSlot)
 public:
     ConnectToPrivateSlot();
     void test(SenderObject *obj1) ;
-    Q_DECLARE_PRIVATE(ConnectToPrivateSlot)
 };
 
 class ConnectToPrivateSlotPrivate : public QObjectPrivate {
-public:
     Q_DECLARE_PUBLIC(ConnectToPrivateSlot)
+public:
     int receivedCount;
     QVariant receivedValue;
 
@@ -6002,6 +6002,8 @@ public:
         receivedCount++;
         receivedValue = v;
     };
+
+    void testFromPrivate(SenderObject *obj);
 };
 
 ConnectToPrivateSlot::ConnectToPrivateSlot(): QObject(*new ConnectToPrivateSlotPrivate) {}
@@ -6028,6 +6030,14 @@ void ConnectToPrivateSlot::test(SenderObject* obj1) {
     QVERIFY(!QObjectPrivate::disconnect(obj1, &SenderObject::signal2, d, &ConnectToPrivateSlotPrivate::thisIsAPrivateSlot));
 }
 
+// Compile test to verify that we can use QObjectPrivate::connect in
+// the code of the private class, even if Q_DECLARE_PUBLIC is used in the
+// private section of the private class.
+void ConnectToPrivateSlotPrivate::testFromPrivate(SenderObject *obj)
+{
+    QVERIFY(QObjectPrivate::connect(obj, &SenderObject::signal1, this, &ConnectToPrivateSlotPrivate::thisIsAPrivateSlot));
+}
+
 void tst_QObject::connectPrivateSlots()
 {
     SenderObject sender;