Ensure GDExtension class is the correct type for the Godot engine class

src/core/class_db.cpp

@@ -40,6 +40,7 @@
 namespace godot {
 
 std::unordered_map<StringName, ClassDB::ClassInfo> ClassDB::classes;
+std::unordered_map<StringName, const GDExtensionInstanceBindingCallbacks *> ClassDB::instance_binding_callbacks;
 GDExtensionInitializationLevel ClassDB::current_level = GDEXTENSION_INITIALIZATION_CORE;
 
 MethodDefinition D_METHOD(StringName p_name) {
@@ -314,6 +315,12 @@ GDExtensionClassCallVirtual ClassDB::get_virtual_func(void *p_userdata, GDExtens
 	return nullptr;
 }
 
+const GDExtensionInstanceBindingCallbacks *ClassDB::get_instance_binding_callbacks(const StringName &p_class) {
+	std::unordered_map<StringName, const GDExtensionInstanceBindingCallbacks *>::iterator callbacks_it = instance_binding_callbacks.find(p_class);
+	ERR_FAIL_COND_V_MSG(callbacks_it == instance_binding_callbacks.end(), nullptr, String("Cannot find instance binding callbacks for class '{0}'.").format(Array::make(p_class)));
+	return callbacks_it->second;
+}
+
 void ClassDB::bind_virtual_method(const StringName &p_class, const StringName &p_method, GDExtensionClassCallVirtual p_call) {
 	std::unordered_map<StringName, ClassInfo>::iterator type_it = classes.find(p_class);
 	ERR_FAIL_COND_MSG(type_it == classes.end(), String("Class '{0}' doesn't exist.").format(Array::make(p_class)));

src/core/object.cpp

@@ -30,8 +30,38 @@
 
 #include <godot_cpp/core/object.hpp>
 
+#include <godot_cpp/core/class_db.hpp>
+
 namespace godot {
 
+namespace internal {
+
+Object *get_object_instance_binding(GodotObject *p_engine_object) {
+	if (p_engine_object == nullptr) {
+		return nullptr;
+	}
+
+	// Get existing instance binding, if one already exists.
+	GDExtensionObjectPtr instance = gdextension_interface_object_get_instance_binding(p_engine_object, token, nullptr);
+	if (instance != nullptr) {
+		return reinterpret_cast<Object *>(instance);
+	}
+
+	// Otherwise, try to look up the correct binding callbacks.
+	const GDExtensionInstanceBindingCallbacks *binding_callbacks = nullptr;
+	StringName class_name;
+	if (gdextension_interface_object_get_class_name(p_engine_object, library, reinterpret_cast<GDExtensionStringNamePtr>(class_name._native_ptr()))) {
+		binding_callbacks = ClassDB::get_instance_binding_callbacks(class_name);
+	}
+	if (binding_callbacks == nullptr) {
+		binding_callbacks = &Object::___binding_callbacks;
+	}
+
+	return reinterpret_cast<Object *>(gdextension_interface_object_get_instance_binding(p_engine_object, token, binding_callbacks));
+}
+
+} // namespace internal
+
 MethodInfo::MethodInfo() :
 		flags(GDEXTENSION_METHOD_FLAG_NORMAL) {}
 

src/godot.cpp

@@ -163,6 +163,7 @@ GDExtensionInterfaceGlobalGetSingleton gdextension_interface_global_get_singleto
 GDExtensionInterfaceObjectGetInstanceBinding gdextension_interface_object_get_instance_binding = nullptr;
 GDExtensionInterfaceObjectSetInstanceBinding gdextension_interface_object_set_instance_binding = nullptr;
 GDExtensionInterfaceObjectSetInstance gdextension_interface_object_set_instance = nullptr;
+GDExtensionInterfaceObjectGetClassName gdextension_interface_object_get_class_name = nullptr;
 GDExtensionInterfaceObjectCastTo gdextension_interface_object_cast_to = nullptr;
 GDExtensionInterfaceObjectGetInstanceFromId gdextension_interface_object_get_instance_from_id = nullptr;
 GDExtensionInterfaceObjectGetInstanceId gdextension_interface_object_get_instance_id = nullptr;
@@ -342,6 +343,7 @@ GDExtensionBool GDExtensionBinding::init(GDExtensionInterfaceGetProcAddress p_ge
 	LOAD_PROC_ADDRESS(object_get_instance_binding, GDExtensionInterfaceObjectGetInstanceBinding);
 	LOAD_PROC_ADDRESS(object_set_instance_binding, GDExtensionInterfaceObjectSetInstanceBinding);
 	LOAD_PROC_ADDRESS(object_set_instance, GDExtensionInterfaceObjectSetInstance);
+	LOAD_PROC_ADDRESS(object_get_class_name, GDExtensionInterfaceObjectGetClassName);
 	LOAD_PROC_ADDRESS(object_cast_to, GDExtensionInterfaceObjectCastTo);
 	LOAD_PROC_ADDRESS(object_get_instance_from_id, GDExtensionInterfaceObjectGetInstanceFromId);
 	LOAD_PROC_ADDRESS(object_get_instance_id, GDExtensionInterfaceObjectGetInstanceId);
@@ -371,6 +373,7 @@ GDExtensionBool GDExtensionBinding::init(GDExtensionInterfaceGetProcAddress p_ge
 	ERR_FAIL_COND_V_MSG(init_callback == nullptr, false, "Initialization callback must be defined.");
 
 	Variant::init_bindings();
+	register_engine_classes();
 
 	return true;
 }

src/variant/variant.cpp

@@ -33,6 +33,7 @@
 #include <godot_cpp/godot.hpp>
 
 #include <godot_cpp/core/binder_common.hpp>
+#include <godot_cpp/core/class_db.hpp>
 #include <godot_cpp/core/defs.hpp>
 
 #include <utility>
@@ -411,7 +412,7 @@ Variant::operator Object *() const {
 	if (obj == nullptr) {
 		return nullptr;
 	}
-	return reinterpret_cast<Object *>(internal::gdextension_interface_object_get_instance_binding(obj, internal::token, &Object::___binding_callbacks));
+	return internal::get_object_instance_binding(obj);
 }
 
 Variant::operator ObjectID() const {