QML: Add markers corresponding to various QML attributes

QML attributes are transitive: a base type's attributes leak
into derived type

This causes issues in type registration since we cannot tell
whether a type has a specific attribute is really present
for it. For instance, things like QML_EXTENDED, QML_ATTACHED,
QML_SINGLETON and a few others are the offenders

The way to overcome this, in the lack of the own type, is to
store a static meta object of the enclosing type as a marker
when we encounter a particular attribute declaration. We can
then attest whether the marker stored matches the static meta
object of the type we assess: if so, it is the same type;
if not, we are in a derived type. Consider:

class X : public QObject {
  Q_OBJECT

  QML_EXTENDED
  //-> unpacks roughly in:
  //   (old) using QmlExtendedType = ...;
  //   (new) inline constexpr void marker() {}
};

class Y : public X {
  Q_OBJECT
};

Before:
X::QmlExtendedType - exists
Y::QmlExtendedType - exists (due to inheritance)
After:
X::QmlExtendedType - exists
Y::QmlExtendedType - exists
classType<decltype(X::marker)> == X // truly QmlExtendedType
classType<decltype(Y::marker)> != Y // falsely QmlExtendedType

Note that this commit does not use the markers, but only
introduces them

Change-Id: I22fdee056c2fc63bc64386dd5de62e6021f15a02
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

2 files changed, 14 insertions, 7 deletions

diff --git a/src/qml/qml/qqmlregistration.h b/src/qml/qml/qqmlregistration.h
index edbae1c654..db387e09f3 100644
--- a/src/qml/qml/qqmlregistration.h
+++ b/src/qml/qml/qqmlregistration.h
@@ -51,7 +51,8 @@ QT_BEGIN_NAMESPACE
     using QmlForeignType = FOREIGN_TYPE; \
     template<class, class> friend struct QML_PRIVATE_NAMESPACE::QmlResolved; \
     template<typename... Args> \
-    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *);
+    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *); \
+    inline constexpr void qt_qmlMarker_foreign() {}
 
 #define QML_FOREIGN_NAMESPACE(FOREIGN_NAMESPACE) \
     Q_CLASSINFO("QML.Foreign", #FOREIGN_NAMESPACE)
diff --git a/src/qmlintegration/qqmlintegration.h b/src/qmlintegration/qqmlintegration.h
index 5333ed9ce3..a70c49d8d8 100644
--- a/src/qmlintegration/qqmlintegration.h
+++ b/src/qmlintegration/qqmlintegration.h
@@ -92,7 +92,8 @@ QT_END_NAMESPACE
     enum class QmlIsSingleton {yes = true}; \
     template<typename, typename> friend struct QML_PRIVATE_NAMESPACE::QmlSingleton; \
     template<typename... Args> \
-    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *);
+    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *); \
+    inline constexpr void qt_qmlMarker_singleton() {}
 
 #define QML_ADDED_IN_MINOR_VERSION(VERSION) \
     Q_CLASSINFO("QML.AddedInVersion", Q_REVISION(VERSION))
@@ -113,14 +114,16 @@ QT_END_NAMESPACE
     Q_CLASSINFO("QML.Attached", #ATTACHED_TYPE) \
     using QmlAttachedType = ATTACHED_TYPE; \
     template<class, class, bool> friend struct QML_PRIVATE_NAMESPACE::QmlAttached; \
-    template<class> friend struct QML_PRIVATE_NAMESPACE::QmlAttachedAccessor;
+    template<class> friend struct QML_PRIVATE_NAMESPACE::QmlAttachedAccessor; \
+    inline constexpr void qt_qmlMarker_attached() {}
 
 #define QML_EXTENDED(EXTENDED_TYPE) \
     Q_CLASSINFO("QML.Extended", #EXTENDED_TYPE) \
     using QmlExtendedType = EXTENDED_TYPE; \
     template<class, class> friend struct QML_PRIVATE_NAMESPACE::QmlExtended; \
     template<typename... Args> \
-    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *);
+    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *); \
+    inline constexpr void qt_qmlMarker_extended() {}
 
 #define QML_EXTENDED_NAMESPACE(EXTENDED_NAMESPACE) \
     Q_CLASSINFO("QML.Extended", #EXTENDED_NAMESPACE) \
@@ -128,7 +131,8 @@ QT_END_NAMESPACE
     static constexpr const QMetaObject *qmlExtendedNamespace() { return &EXTENDED_NAMESPACE::staticMetaObject; } \
     template<class, class> friend struct QML_PRIVATE_NAMESPACE::QmlExtendedNamespace; \
     template<typename... Args> \
-    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *);
+    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *); \
+    inline constexpr void qt_qmlMarker_extendedNamespace() {}
 
 #define QML_NAMESPACE_EXTENDED(EXTENDED_NAMESPACE) \
     Q_CLASSINFO("QML.Extended", #EXTENDED_NAMESPACE)
@@ -138,7 +142,8 @@ QT_END_NAMESPACE
     enum class QmlIsInterface {yes = true}; \
     template<typename, typename> friend struct QML_PRIVATE_NAMESPACE::QmlInterface; \
     template<typename... Args> \
-    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *);
+    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *); \
+    inline constexpr void qt_qmlMarker_interface() {}
 
 #define QML_IMPLEMENTS_INTERFACES(INTERFACES) \
     Q_INTERFACES(INTERFACES) \
@@ -151,7 +156,8 @@ QT_END_NAMESPACE
     enum class QmlIsSequence {yes = true}; \
     template<typename, typename> friend struct QML_PRIVATE_NAMESPACE::QmlSequence; \
     template<typename... Args> \
-    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *);
+    friend void QML_REGISTER_TYPES_AND_REVISIONS(const char *uri, int versionMajor, QList<int> *); \
+    inline constexpr void qt_qmlMarker_sequence() {}
 
 #define QML_UNAVAILABLE \
     QML_FOREIGN(QQmlTypeNotAvailable)