Threads dont work with optional arguments

[2shady4u]

Godot version:
v3.2.1

OS/device including version:
Windows 10

Issue description:
Starting a new thread on a function with optional arguments doesn't work as expected.
Both calling the function normally and calling the function by emitting a signal works perfectly as expected.

Minimal reproduction project:

extends Node2D

var thread : Thread

signal test_signal

func _ready():
	thread = Thread.new()
	thread.start(self, "do_stuff")

	connect("test_signal", self, "do_stuff")
	emit_signal("test_signal")
	
	do_stuff()

func do_stuff(optional_var : int = 0):
	print("HURRAY!")

func _exit_tree():
	thread.wait_to_finish()

Gives following error when running:

E 0:00:00.812   _start_func: Could not call function 'do_stuff' to start thread 13480: Invalid Argument #0.
  <C++ Error>   Method failed.
  <C++ Source>  core/bind/core_bind.cpp:2694 @ _start_func()

Evidently running the start thread command with an actual integer as argument does work.

[Calinou]

I think this is because the userdata passed to Thread.start() must not be null. This issue is probably a duplicate of another one.

[Faymoon]

I think it may be because on this line we can see that the arg number is hard coded :
https://github.com/godotengine/godot/blob/master/core/bind/core_bind.cpp#L2104

[Faymoon]

I tried to replace the 1 with a zero on the line i linked before and it worked with this code :

extends Node2D

var thread : Thread

func _ready():
	thread = Thread.new()
	thread.start(self, "do_stuff")
	
	do_stuff()

func do_stuff(optional_var : int = 0):
	print("HURRAY!")

func _exit_tree():
	thread.wait_to_finish()

[Faymoon]

Threads don't really works as signals, signals can accept multiples parameters but thread just let you give one userdata, i think it's the reason why it doesn't work here. I'm working on a real fix. (The fix just before prevent all threads to receive userdata.