It is not possible to send null as the last argument of an UndoRedo method

[Zylann]

Godot 3.2

To register a method to be executed when doing or undoing a change, UndoRedo.add_do_method() or UndoRedo.add_undo_method() have to be used.
Those methods were implemented by declaring 5 Variant arguments, all null by default.
Unfortunately, the argument count is then determined by checking how many remaining arguments are null, including cases where you want to call the method with null.
That means it is impossible to write this:

undo_redo.add_do_method(object, "foo", 42, true, null)

Because the null at the end will be confused as absence of argument. Then when committing the action, undoing or redoing it, it won't work and this kind of error will be printed:

ERROR: UndoRedo::_process_operation_list: Error calling method from signal 'foo': Method expected 3 arguments, but called with 2.

See

godot/core/undo_redo.cpp

Line 284 in 78adce8

for (int i = 0; i < VARIANT_ARG_MAX; i++) {

I had to workaround this limitation by binding extra methods in my objects, and check if variants are null before using add_do_method and add_undo_method.

To fix this, UndoRedo should not determine argument count in this way. Instead, it should use reflection to query how many arguments the called method actually has. Other solutions include sending an array explicitely, adding a count argument, or having a Variant::UNDEFINED type, but that would break compatibility.

[KoBeWi]

Related to #29204

[kopcion]

I've been working on this bug. For now, I only managed to solve this issue by having map in UndoRedo class with argc for current functions, but I would like to do some more, so feedback would be great. I don't see how to apply reflection to this problem, nor what exactly do you mean by reflection in c++ (until recently it wasn't included afaik).

Here is basic project reproducing the bug.
Issue#36895.zip

[Jummit]

Minimal reproduction project I made for #43141:

undo_test.zip

Note that Object.call handles this correctly, so maybe one could look how it does it.

[me2beats]

just faced this too, so I do it in my own functions like

undo.add_do_method(self, "_do")

[akien-mga]

Fixed by #66070.

Edit: Not sure actually, based on #43872 (comment).

[KoBeWi]

This was actually resolved by #58929