Cannot use `Option<Gd<T>>` as signal argument when `T` is a user declared class

[jsternberg]

If you create a user declared class such as:

#[derive(GodotClass)]
#[class(init, base=Node2D)]
pub struct MyCustomNode {
    base: Base<Node2D>,
}

If you attempt to create a signal that takes this node as a parameter, it will allow it.

#[godot_api]
impl MyCustomNode {
    #[signal]
    pub fn my_custom_event(node: Gd<MyCustomNode>);
}

You can then emit this with a reference to the node. On the other hand, if you try to do the same thing with Option<Gd<MyCustomNode>>, I can't figure out what type it accepts.

#[godot_api]
impl MyCustomNode {
    #[signal]
    pub fn my_custom_event(node: Option<Gd<MyCustomNode>>);
}

There's no compile error when you do this. But, if you attempt to use it.

let my_node = Some(MyCustomNode::new_alloc());
// type mismatch resolving <Option<Gd<MyCustomNode>> as ToGodot>::Pass == ByRef
// expected enum ByRef
// found enum ByOption<godot::prelude::Gd<MyCustomNode>>
// required for &Option<godot::prelude::Gd<MyCustomNode>> to implement AsArg<Option<godot::prelude::Gd<MyCustomNode>>>
self.signals().my_custom_event().emit(&my_node);
// type mismatch resolving <MyCustomNode as Bounds>::Declarer == DeclEngine
// required for Option<&godot::prelude::Gd<MyCustomNode>> to implement AsArg<Option<godot::prelude::Gd<MyCustomNode>>>
self.signals().my_custom_event().emit(my_node.as_ref());

I've also tried other combinations but I can't figure out which type it expects. I could define the signal to take a type parameter that is the base class that's in the engine, but it would be nice to have it more strongly typed than that.

I can find other ways to work around this so it's not a pressing issue, but I didn't know if this was intentional or not.

[Yarwin]

Will be fixed by: #1371