Object: Add usage hint to instantiate Object properties in editor

Fixes #36372 as Path2D/Path3D's `curve` property no longer uses a Curve
instance as default value, but instead it gets a (unique) default Curve
instance when created through the editor (CreateDialog).

ClassDB gets a sanity check to ensure that we don't do the same mistake
for other properties in the future, but instead use the dedicated
property usage hint.

Fixes #36372.
Fixes #36650.

Supersedes #36644 and #36656.

Co-authored-by: Thakee Nathees <thakeenathees@gmail.com>
Co-authored-by: simpuid <utkarsh.email@yahoo.com>

7 files changed, 54 insertions, 30 deletions

diff --git a/core/class_db.cpp b/core/class_db.cpp
index eed9ec17cb..05c9850c39 100644
--- a/core/class_db.cpp
+++ b/core/class_db.cpp
@@ -1386,7 +1386,23 @@ Variant ClassDB::class_get_default_property_value(const StringName &p_class, con
 	if (r_valid != nullptr) {
 		*r_valid = true;
 	}
-	return default_values[p_class][p_property];
+
+	Variant var = default_values[p_class][p_property];
+
+#ifdef DEBUG_ENABLED
+	// Some properties may have an instantiated Object as default value,
+	// (like Path2D's `curve` used to have), but that's not a good practice.
+	// Instead, those properties should use PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT
+	// to be auto-instantiated when created in the editor.
+	if (var.get_type() == Variant::OBJECT) {
+		Object *obj = var.get_validated_object();
+		if (obj) {
+			WARN_PRINT(vformat("Instantiated %s used as default value for %s's \"%s\" property.", obj->get_class(), p_class, p_property));
+		}
+	}
+#endif
+
+	return var;
 }
 
 RWLock *ClassDB::lock = nullptr;
diff --git a/core/object.h b/core/object.h
index 95662f6208..5b46a0f93a 100644
--- a/core/object.h
+++ b/core/object.h
@@ -124,6 +124,7 @@ enum PropertyUsageFlags {
 	PROPERTY_USAGE_RESOURCE_NOT_PERSISTENT = 1 << 24,
 	PROPERTY_USAGE_KEYING_INCREMENTS = 1 << 25, // Used in inspector to increment property when keyed in animation player
 	PROPERTY_USAGE_DEFERRED_SET_RESOURCE = 1 << 26, // when loading, the resource for this property can be set at the end of loading
+	PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT = 1 << 27, // For Object properties, instantiate them when creating in editor.
 
 	PROPERTY_USAGE_DEFAULT = PROPERTY_USAGE_STORAGE | PROPERTY_USAGE_EDITOR | PROPERTY_USAGE_NETWORK,
 	PROPERTY_USAGE_DEFAULT_INTL = PROPERTY_USAGE_STORAGE | PROPERTY_USAGE_EDITOR | PROPERTY_USAGE_NETWORK | PROPERTY_USAGE_INTERNATIONALIZED,
diff --git a/editor/create_dialog.cpp b/editor/create_dialog.cpp
index 4bd7371c21..73468f8ce0 100644
--- a/editor/create_dialog.cpp
+++ b/editor/create_dialog.cpp
@@ -513,30 +513,45 @@ String CreateDialog::get_selected_type() {
 Object *CreateDialog::instance_selected() {
 	TreeItem *selected = search_options->get_selected();
 
-	if (selected) {
-		Variant md = selected->get_metadata(0);
+	if (!selected) {
+		return nullptr;
+	}
 
-		String custom;
-		if (md.get_type() != Variant::NIL) {
-			custom = md;
-		}
+	Variant md = selected->get_metadata(0);
+	String custom;
+	if (md.get_type() != Variant::NIL) {
+		custom = md;
+	}
 
-		if (custom != String()) {
-			if (ScriptServer::is_global_class(custom)) {
-				Object *obj = EditorNode::get_editor_data().script_class_instance(custom);
-				Node *n = Object::cast_to<Node>(obj);
-				if (n) {
-					n->set_name(custom);
-				}
-				return obj;
+	Object *obj = nullptr;
+
+	if (!custom.empty()) {
+		if (ScriptServer::is_global_class(custom)) {
+			obj = EditorNode::get_editor_data().script_class_instance(custom);
+			Node *n = Object::cast_to<Node>(obj);
+			if (n) {
+				n->set_name(custom);
 			}
-			return EditorNode::get_editor_data().instance_custom_type(selected->get_text(0), custom);
+			obj = n;
 		} else {
-			return ClassDB::instance(selected->get_text(0));
+			obj = EditorNode::get_editor_data().instance_custom_type(selected->get_text(0), custom);
+		}
+	} else {
+		obj = ClassDB::instance(selected->get_text(0));
+	}
+
+	// Check if any Object-type property should be instantiated.
+	List<PropertyInfo> pinfo;
+	obj->get_property_list(&pinfo);
+
+	for (List<PropertyInfo>::Element *E = pinfo.front(); E; E = E->next()) {
+		if (E->get().type == Variant::OBJECT && E->get().usage & PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT) {
+			Object *prop = ClassDB::instance(E->get().class_name);
+			obj->set(E->get().name, prop);
 		}
 	}
 
-	return nullptr;
+	return obj;
 }
 
 void CreateDialog::_item_selected() {
diff --git a/scene/2d/path_2d.cpp b/scene/2d/path_2d.cpp
index 00e9af3bb7..f2f549e851 100644
--- a/scene/2d/path_2d.cpp
+++ b/scene/2d/path_2d.cpp
@@ -149,11 +149,7 @@ void Path2D::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_curve", "curve"), &Path2D::set_curve);
 	ClassDB::bind_method(D_METHOD("get_curve"), &Path2D::get_curve);
 
-	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "curve", PROPERTY_HINT_RESOURCE_TYPE, "Curve2D"), "set_curve", "get_curve");
-}
-
-Path2D::Path2D() {
-	set_curve(Ref<Curve2D>(memnew(Curve2D))); //create one by default
+	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "curve", PROPERTY_HINT_RESOURCE_TYPE, "Curve2D", PROPERTY_USAGE_DEFAULT | PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT), "set_curve", "get_curve");
 }
 
 /////////////////////////////////////////////////////////////////////////////////
diff --git a/scene/2d/path_2d.h b/scene/2d/path_2d.h
index 288ef698e7..38fcca0323 100644
--- a/scene/2d/path_2d.h
+++ b/scene/2d/path_2d.h
@@ -55,7 +55,7 @@ public:
 	void set_curve(const Ref<Curve2D> &p_curve);
 	Ref<Curve2D> get_curve() const;
 
-	Path2D();
+	Path2D() {}
 };
 
 class PathFollow2D : public Node2D {
diff --git a/scene/3d/path_3d.cpp b/scene/3d/path_3d.cpp
index dcfdf8efcf..40d988ff9f 100644
--- a/scene/3d/path_3d.cpp
+++ b/scene/3d/path_3d.cpp
@@ -77,15 +77,11 @@ void Path3D::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_curve", "curve"), &Path3D::set_curve);
 	ClassDB::bind_method(D_METHOD("get_curve"), &Path3D::get_curve);
 
-	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "curve", PROPERTY_HINT_RESOURCE_TYPE, "Curve3D"), "set_curve", "get_curve");
+	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "curve", PROPERTY_HINT_RESOURCE_TYPE, "Curve3D", PROPERTY_USAGE_DEFAULT | PROPERTY_USAGE_EDITOR_INSTANTIATE_OBJECT), "set_curve", "get_curve");
 
 	ADD_SIGNAL(MethodInfo("curve_changed"));
 }
 
-Path3D::Path3D() {
-	set_curve(Ref<Curve3D>(memnew(Curve3D))); //create one by default
-}
-
 //////////////
 
 void PathFollow3D::_update_transform() {
diff --git a/scene/3d/path_3d.h b/scene/3d/path_3d.h
index 5a33016bc6..7f227a8a6f 100644
--- a/scene/3d/path_3d.h
+++ b/scene/3d/path_3d.h
@@ -49,7 +49,7 @@ public:
 	void set_curve(const Ref<Curve3D> &p_curve);
 	Ref<Curve3D> get_curve() const;
 
-	Path3D();
+	Path3D() {}
 };
 
 class PathFollow3D : public Node3D {