QWeakPointer: use an alternative work-round for internalData() users

The previous work-around fails, probably because of
cross-dependencies. E.g. we have this in QtScXml:

In file included from /home/qt/work/install/include/QtCore/qsharedpointer.h:48:0,
                  from ../../src/scxml/qscxmltabledata_p.h:55,
                  from ../../src/scxml/qscxmltabledata.cpp:40:
 /home/qt/work/install/include/QtCore/qsharedpointer_impl.h:687:12: error: ‘QPointer’ does not name a type; did you mean ‘pointer’?
      friend QPointer<X>
             ^~~~~~~~
             pointer
 /home/qt/work/install/include/QtCore/qsharedpointer_impl.h:689:23: error: ‘QSmartPointerConvertFunctor’ in namespace ‘QtPrivate’ does not name a template type
      friend QtPrivate::QSmartPointerConvertFunctor<QWeakPointer>;
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~

To fix, grand friendship only to a non-template class with a templated
static method that returns internalData(). This fixes most users,
except in qmetatype.h, which does not include qsharedpointer.h. In
order to use the non-template class in there, we need to delay its
name lookup to instantiation time. We do this by artificially making
it a dependent name, by using a class template that inherits from
our befrieded class.

Change-Id: I12b427f1fe9503df819ea5436d780972d6402e68
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>

src/corelib/kernel/qmetatype.h

@@ -1715,12 +1715,17 @@ namespace QtPrivate {
         }
     };
 
+    // hack to delay name lookup to instantiation time by making
+    // EnableInternalData a dependent name:
+    template <typename T>
+    struct EnableInternalDataWrap;
+
     template<typename T>
     struct QSmartPointerConvertFunctor<QWeakPointer<T> >
     {
         QObject* operator()(const QWeakPointer<T> &p) const
         {
-            return p.internalData();
+            return QtPrivate::EnableInternalDataWrap<T>::internalData(p);
         }
     };
 }

src/corelib/kernel/qpointer.h

@@ -143,7 +143,8 @@ template<typename T>
 QPointer<T>
 qPointerFromVariant(const QVariant &variant)
 {
-    return QPointer<T>(qobject_cast<T*>(QtSharedPointer::weakPointerFromVariant_internal(variant).internalData()));
+    const auto wp = QtSharedPointer::weakPointerFromVariant_internal(variant);
+    return QPointer<T>{qobject_cast<T*>(QtPrivate::EnableInternalData::internalData(wp))};
 }
 
 template <class T>

src/corelib/tools/qsharedpointer_impl.h

@@ -107,6 +107,10 @@ template <class X, class T>
 QSharedPointer<X> qSharedPointerObjectCast(const QSharedPointer<T> &ptr);
 #endif
 
+namespace QtPrivate {
+struct EnableInternalData;
+}
+
 namespace QtSharedPointer {
     template <class T> class ExternalRefCount;
 
@@ -672,21 +676,12 @@ public:
 #endif
 
 private:
-
+    friend struct QtPrivate::EnableInternalData;
 #if defined(Q_NO_TEMPLATE_FRIENDS)
 public:
 #else
     template <class X> friend class QSharedPointer;
     template <class X> friend class QPointer;
-#  ifndef QT_NO_QOBJECT
-    template<typename X>
-    friend QWeakPointer<typename std::enable_if<QtPrivate::IsPointerToTypeDerivedFromQObject<X*>::Value, X>::type>
-    qWeakPointerFromVariant(const QVariant &variant);
-#  endif
-    template<typename X>
-    friend QPointer<X>
-    qPointerFromVariant(const QVariant &variant);
-    friend QtPrivate::QSmartPointerConvertFunctor<QWeakPointer>;
 #endif
 
     template <class X>
@@ -721,6 +716,17 @@ public:
     T *value;
 };
 
+namespace QtPrivate {
+struct EnableInternalData {
+    template <typename T>
+    static T *internalData(const QWeakPointer<T> &p) noexcept { return p.internalData(); }
+};
+// hack to delay name lookup to instantiation time by making
+// EnableInternalData a dependent name:
+template <typename T>
+struct EnableInternalDataWrap : EnableInternalData {};
+}
+
 template <class T>
 class QEnableSharedFromThis
 {
@@ -996,7 +1002,7 @@ template<typename T>
 QWeakPointer<typename std::enable_if<QtPrivate::IsPointerToTypeDerivedFromQObject<T*>::Value, T>::type>
 qWeakPointerFromVariant(const QVariant &variant)
 {
-    return QWeakPointer<T>(qobject_cast<T*>(QtSharedPointer::weakPointerFromVariant_internal(variant).internalData()));
+    return QWeakPointer<T>(qobject_cast<T*>(QtPrivate::EnableInternalData::internalData(QtSharedPointer::weakPointerFromVariant_internal(variant))));
 }
 template<typename T>
 QSharedPointer<typename std::enable_if<QtPrivate::IsPointerToTypeDerivedFromQObject<T*>::Value, T>::type>