C# Generics Deriving Godot.Object Cannot Be Marshalled Over Signals

[kayomn]

Godot version:
Godot 3.3 RC 7

OS/device including version:
OS: Manjaro Linux x86_64
Kernel: 5.9.16-1-MANJARO
CPU: AMD Ryzen 7 1700X (16) @ 3.400GHz
GPU: AMD ATI Radeon RX 5600 XT

Issue description:
Godot's marshalling logic does not account for signals with generic argument types extend Godot.Object.

This means that patterns to avoid the issue of not being able to pass structs over Godot signals, such as using a generic Box<T> type, do not work.

godot/modules/mono/mono_gd/gd_mono_marshal.cpp

Line 237 in 2a66714

case MONO_TYPE_GENERICINST: {

The issue appears to be in how Godot handles generic instances when doing coersion from Mono type info to Godot variant type info, as it will discard any generic that is not a hardcoded exception, such as the generic collection interfaces.

It should be noted that passing generic types that extend Godot.Object to regular Godot functions works fine.

Steps to reproduce:

public class Box<T> : Godot.Object where T : struct
{
    public T Value
    {
        get;
        private set;
    }

    public Box(in T value)
    {
        Value = value;
    }
}

Attempt to declare a Godot signal that uses the above generic type

Minimal reproduction project:
MinimalReproduction.zip