Proposal: allow to specify Union type for the module-self

It would be very helpful to use Union type for the module-self.

In my project, I have a Commentable concern module used across multiple ActiveRecord models. This Commentable module uses methods commonly defined in the modules where it is included. For example, it references methods like #comments and attributes such as #user_id.

Currently, in RBS, only classes or interfaces can be specified for module-self. As a workaround, I have defined an interface as follows:

interface _Commentable
  def comments: () -> Comment::ActiveRecord_Relation
  def user_id: () -> Integer
  ...
end

module Commentable : _Commentable
end

This interface allows for proper type checking, but as the number of methods grows, the interface definition becomes more cumbersome.

If it were possible to specify a Union type for module-self, it would allow for type definitions of modules without the need to define an interface.

module Commentable : (Article | Comment | ...)
end

However, generating an interface dynamically through Union types might negatively impact runtime performance. Do you think this idea is practical?

Start with the existing _Commentable interface workaround and the proposed module-self Union syntax in this issue. Review how RBS represents module-self and Union types, then assess the type-checking and runtime-performance implications raised here. Done means an accepted design with implementation and tests, if the proposal is approved.