Object-less callables rejected by `Object::connect` et al

[maiself]

Godot version

master, e3e2528

System information

linux

Issue description

Creating this issue to keep track of something discovered while working working on #79005.

Basically, in some places Godot has checks for object as a way to check if a callable is valid. This seems wrong, especially for custom callables, which may not actually have an associated object. While this doesn't seem to be a problem within the engine itself, it can be a problem for extensions. In my extension I had to use a workaround that creates an object that exists for no reason other than to pass those checks.

Here are some examples of object-less callables being rejected:

• Object::emit_signalp
• Object::connect
• Object::_disconnect

I haven't looked to see if there are more cases (I merely searched object.cpp for get_object()).

Original comments from #79005:

• Allow CallableCustom objects to be created from GDExtensions #79005 (comment)
• Allow CallableCustom objects to be created from GDExtensions #79005 (comment)

Steps to reproduce

N/A

Minimal reproduction project

N/A

[Mickeon]

Does simply removing the check for a valid object completely fix this or is there something more going on?

[dsnopek]

Does simply removing the check for a valid object completely fix this or is there something more going on?

I think we probably want to replace those checks with callable.is_valid().

The examples in the description seem to be doing their own manual version of a validity check by checking callable.get_object(). However, there's several types of callables, some of which are perfectly valid without objects (and those should have overridden is_valid()).

If I had to guess what happened here, maybe there didn't used to be a is_valid() function (so checking get_object() made sense), but after adding is_valid() some bits of the code didn't get updated?