Add check to ensure registered classes are declared

Checks that all classes registered to `ClassDB` have been properly
declared with `GDCLASS`

(cherry picked from commit c638238fae6c7a6d9c544fcec083ccb6ccc263b7)

core/object/class_db.h

@@ -187,6 +187,7 @@ public:
 	template <class T>
 	static void register_class(bool p_virtual = false) {
 		GLOBAL_LOCK_FUNCTION;
+		static_assert(TypesAreSame<typename T::self_type, T>::value, "Class not declared properly, please use GDCLASS.");
 		T::initialize_class();
 		ClassInfo *t = classes.getptr(T::get_class_static());
 		ERR_FAIL_COND(!t);
@@ -201,6 +202,7 @@ public:
 	template <class T>
 	static void register_abstract_class() {
 		GLOBAL_LOCK_FUNCTION;
+		static_assert(TypesAreSame<typename T::self_type, T>::value, "Class not declared properly, please use GDCLASS.");
 		T::initialize_class();
 		ClassInfo *t = classes.getptr(T::get_class_static());
 		ERR_FAIL_COND(!t);
@@ -221,6 +223,7 @@ public:
 	template <class T>
 	static void register_custom_instance_class() {
 		GLOBAL_LOCK_FUNCTION;
+		static_assert(TypesAreSame<typename T::self_type, T>::value, "Class not declared properly, please use GDCLASS.");
 		T::initialize_class();
 		ClassInfo *t = classes.getptr(T::get_class_static());
 		ERR_FAIL_COND(!t);

core/object/object.h

@@ -382,6 +382,7 @@ private:
 	friend class ::ClassDB;                                                                                                                      \
                                                                                                                                                  \
 public:                                                                                                                                          \
+	typedef m_class self_type;                                                                                                                   \
 	static constexpr bool _class_is_enabled = !bool(GD_IS_DEFINED(ClassDB_Disable_##m_class)) && m_inherits::_class_is_enabled;                  \
 	virtual String get_class() const override {                                                                                                  \
 		if (_get_extension()) {                                                                                                                  \
@@ -551,6 +552,8 @@ class ScriptInstance;
 
 class Object {
 public:
+	typedef Object self_type;
+
 	enum ConnectFlags {
 		CONNECT_DEFERRED = 1,
 		CONNECT_PERSIST = 2, // hint for scene to save this connection