Improve error message when calling method with "previously freed instance" argument

[akien-mga]

Godot version:
3.2 master (3af0400)

OS/device including version:
Any

Issue description:
When calling a GDScript method with a "previously freed instance" parameter, the error which is raised is not as clear as it could be. Maybe also for the "null instance" case, though I don't know how to reproduce it.

var node = Node.new()
node.free()
add_child(node)

raises:

Invalid type in function 'add_child' in base 'Node2D (Node2D.gd)'. The Object-derived class of argument 1 (previously freed instance) is not a subclass of the expected argument class.

Relevant code to adjust:

godot/modules/gdscript/gdscript_function.cpp

Lines 166 to 176 in 3af0400

	if (p_err.error == Variant::CallError::CALL_ERROR_INVALID_ARGUMENT) {
	int errorarg = p_err.argument;
	// Handle the Object to Object case separately as we don't have further class details.
	#ifdef DEBUG_ENABLED
	if (p_err.expected == Variant::OBJECT && argptrs[errorarg]->get_type() == p_err.expected) {
	err_text = "Invalid type in " + p_where + ". The Object-derived class of argument " + itos(errorarg + 1) + " (" + _get_var_type(argptrs[errorarg]) + ") is not a subclass of the expected argument class.";
	} else
	#endif // DEBUG_ENABLED
	{
	err_text = "Invalid type in " + p_where + ". Cannot convert argument " + itos(errorarg + 1) + " from " + Variant::get_type_name(argptrs[errorarg]->get_type()) + " to " + Variant::get_type_name(p_err.expected) + ".";
	}

Steps to reproduce:

• Use above snippet on any Node-derived script.

Minimal reproduction project:
See above.

[timothyqiu]

I'm a bit confused when reading the documentation for Object.free()

Deletes the object from memory. Any pre-existing reference to the freed object will now return null.

Isn't node a reference to the object? If yes, then it should become null after node.free() I think.

Or, maybe the documentation is referring to the Reference class?

[bojidar-bg]

Well, there is the thing.. pre-existing references become "previously freed instance" instead of "null".
Note that things can get much worse from there on as of now:

var node = Node.new()
node.free()
var node2 = Node.new()
add_child(node) # No error!
print(node, node2) # e.g. [Node:1370][Node:1370]
print(is_instance_valid(node)) # True ?!

But:

var node = Node.new()
node.free()
var node2 = Sprite.new()
add_child(node) # Error: Same as before (if you are lucky with malloc)

[remram44]

@bojidar-bg I think this is what was fixed by #38119

[akien-mga]

This is fixed indeed.