Class with a member variable with its own type causes a leak

[mortarroad]

Godot version:
3.3.rc8
(master seems to be unaffected)

Issue description:
If a class references itself in a variable type, it causes a leak.
Probably related:
#16959
#45986
#46467

Steps to reproduce:
Attach the following script to a node and run.

extends Spatial

class A:
	var a: A

The log is as follows:

ERROR: ~List: Condition "_first != __null" is true.
   At: ./core/self_list.h:112
ERROR: ~List: Condition "_first != __null" is true.
   At: ./core/self_list.h:112
WARNING: cleanup: ObjectDB instances leaked at exit (run with --verbose for details).
     At: core/object.cpp:2132
Leaked instance: GDScript:1192 - Resource path:
Leaked instance: GDScriptNativeClass:550
Hint: Leaked instances typically happen when nodes are removed from the scene tree (with `remove_child()`) but not freed (with `free()` or `queue_free()`).
Orphan StringName: ::0::A._init
Orphan StringName: Reference
Orphan StringName: GDScriptNativeClass
Orphan StringName: a
Orphan StringName: res://scene.gd
Orphan StringName: _init
Orphan StringName: GDScript
StringName: 7 unclaimed string names at exit.

Minimal reproduction project:
bug_unfreed.zip

Run, then close the running project.

[stengun]

Godot Version: 3.3 stable

I'm affected with a similar issue (I suppose is basically the same as this one so I avoid to spawn a new issue for this), in which the leak happens if you cast a variable to the same class instead.
We have a scene LeakTest.tscn with an attached script LeakTest.gd

class_name LeakTest

func something() -> void:
    var helo = load("res://LeakTest.tscn").instance() as LeakTest

This gives leaked instance of LeakTest.gd upon closing the program. With the same code, if I remove the cast it works correctly. I also noted that if you specify return values (or function parameters) typed with the same custom class it works fine and no leaks happens.