Fix memory leaks when instantiating Mixin manually for testing

core/script/mixin_script/mixin_script.cpp

@@ -518,6 +518,12 @@ Variant Mixin::call(const StringName &p_method, const Variant **p_args, int p_ar
 	if (real_owner) {
 		return real_owner->call(p_method, p_args, p_argcount, r_error);
 	}
+#ifdef DEBUG_ENABLED
+	// The following allows to call `Mixin.free()`.
+	// This is not strictly needed because Mixin objects should not be instantiated directly,
+	// and doing so is discouraged, but this can make debug builds more robust for fuzz testing. 
+	Object::call(p_method, p_args, p_argcount, r_error);
+#endif
 	return Variant();
 }
 

doc/Mixin.xml

@@ -5,7 +5,7 @@
 	</brief_description>
 	<description>
 		A class which extends the run-time functionality of objects that use [MixinScript]. All scripts added to [MixinScript] must inherit from [Mixin].
-		Mixin objects are instantiated internally and cannot be used as a [Node] in the scene tree.
+		[b]Warning:[/b] [Mixin] objects are instantiated internally and cannot be used as a [Node] in the scene tree. Do not instantiate [Mixin] objects manually.
 	</description>
 	<tutorials>
 	</tutorials>

tests/project/goost/core/script/test_mixin_script.gd

@@ -125,3 +125,11 @@ func test_move_script():
 	assert_eq(n.tell(), "First", "Did not move")
 	ms.move_mixin(1, MixinFirst)
 	assert_eq(n.tell(), "Second")
+
+
+func test_mixin_get_owner():
+	var m = Mixin.new()
+	var o = m.get_owner()
+	assert_null(o)
+	m.free()
+	assert_freed(m, "Freed")