Add DUPLICATE_INTERNAL_STATE flag

Author: KoBeWi

doc/classes/Node.xml

@@ -280,6 +280,7 @@
 			<description>
 				Duplicates the node, returning a new node with all of its properties, signals, groups, and children copied from the original. The behavior can be tweaked through the [param flags] (see [enum DuplicateFlags]). Internal nodes are not duplicated.
 				[b]Note:[/b] For nodes with a [Script] attached, if [method Object._init] has been defined with required parameters, the duplicated node will not have a [Script].
+				[b]Note:[/b] By default, this method will duplicate only properties marked for serialization (i.e. using [constant @GlobalScope.PROPERTY_USAGE_STORAGE], or in GDScript, [annotation @GDScript.@export]). If you want to duplicate all properties, use [constant DUPLICATE_INTERNAL_STATE].
 			</description>
 		</method>
 		<method name="find_child" qualifiers="const">
@@ -1378,6 +1379,16 @@
 		<constant name="DUPLICATE_USE_INSTANTIATION" value="8" enum="DuplicateFlags">
 			Duplicate using [method PackedScene.instantiate]. If the node comes from a scene saved on disk, reuses [method PackedScene.instantiate] as the base for the duplicated node and its children.
 		</constant>
+		<constant name="DUPLICATE_INTERNAL_STATE" value="16" enum="DuplicateFlags">
+			Duplicate also non-serializable variables (i.e. without [constant @GlobalScope.PROPERTY_USAGE_STORAGE]).
+		</constant>
+		<constant name="DUPLICATE_DEFAULT" value="15" enum="DuplicateFlags">
+			Duplicate using default flags. This constant is useful to add or remove a single flag.
+			[codeblock]
+			# Duplicate non-exported variables.
+			var dupe = duplicate(DUPLICATE_DEFAULT | DUPLICATE_INTERNAL_STATE)
+			[/codeblock]
+		</constant>
 		<constant name="INTERNAL_MODE_DISABLED" value="0" enum="InternalMode">
 			The node will not be internal.
 		</constant>

scene/main/node.cpp

@@ -3012,7 +3012,7 @@ void Node::_duplicate_properties(const Node *p_root, const Node *p_original, Nod
 		}
 	}
 	for (const PropertyInfo &E : props) {
-		if (!(E.usage & PROPERTY_USAGE_STORAGE)) {
+		if (!(p_flags & DUPLICATE_INTERNAL_STATE) && !(E.usage & PROPERTY_USAGE_STORAGE)) {
 			continue;
 		}
 		const StringName name = E.name;
@@ -3800,7 +3800,7 @@ void Node::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_tree"), &Node::get_tree);
 	ClassDB::bind_method(D_METHOD("create_tween"), &Node::create_tween);
 
-	ClassDB::bind_method(D_METHOD("duplicate", "flags"), &Node::duplicate, DEFVAL(DUPLICATE_USE_INSTANTIATION | DUPLICATE_SIGNALS | DUPLICATE_GROUPS | DUPLICATE_SCRIPTS));
+	ClassDB::bind_method(D_METHOD("duplicate", "flags"), &Node::duplicate, DEFVAL(DUPLICATE_DEFAULT));
 	ClassDB::bind_method(D_METHOD("replace_by", "node", "keep_groups"), &Node::replace_by, DEFVAL(false));
 
 	ClassDB::bind_method(D_METHOD("set_scene_instance_load_placeholder", "load_placeholder"), &Node::set_scene_instance_load_placeholder);
@@ -3955,6 +3955,8 @@ void Node::_bind_methods() {
 	BIND_ENUM_CONSTANT(DUPLICATE_GROUPS);
 	BIND_ENUM_CONSTANT(DUPLICATE_SCRIPTS);
 	BIND_ENUM_CONSTANT(DUPLICATE_USE_INSTANTIATION);
+	BIND_ENUM_CONSTANT(DUPLICATE_INTERNAL_STATE);
+	BIND_ENUM_CONSTANT(DUPLICATE_DEFAULT);
 
 	BIND_ENUM_CONSTANT(INTERNAL_MODE_DISABLED);
 	BIND_ENUM_CONSTANT(INTERNAL_MODE_FRONT);

scene/main/node.h

@@ -100,8 +100,10 @@ class Node : public Object {
 		DUPLICATE_GROUPS = 2,
 		DUPLICATE_SCRIPTS = 4,
 		DUPLICATE_USE_INSTANTIATION = 8,
+		DUPLICATE_INTERNAL_STATE = 16,
+		DUPLICATE_DEFAULT = DUPLICATE_SIGNALS | DUPLICATE_GROUPS | DUPLICATE_SCRIPTS | DUPLICATE_USE_INSTANTIATION,
 #ifdef TOOLS_ENABLED
-		DUPLICATE_FROM_EDITOR = 16,
+		DUPLICATE_FROM_EDITOR = 32,
 #endif
 	};