Singleton does not have method `instance`

Typechecking for singletons (the pattern) doesn't seem to work as expected.

E.g. given an rb file

# frozen_string_literal: true
require "singleton"

module A
  module Example
    class TestClass
      include Singleton
    end
  end
end

A::Example::TestClass.instance

and and RBS file:

module A
  module Example
    VERSION: String

    class TestClass
      include Singleton
    end
  end
end

steep complains about: Type singleton(::A::Example::TestClass) does not have method instance.

The simples possible Steepfile i could come up with for the repro case:

 target :lib do
    signature "sig"
    check "lib"
    library "singleton"
 end

Given that there are some moving parts involved but also there's still active development going on, I wonder whether this supposed to even work right now?

In the original project The class including Singleton also extends Forwardable and so i was somehow hoping that it's just some weirdness. But even in this simple repro case it just doesn't work as expected.

Start by running the supplied Ruby and RBS reproduction with the minimal Steepfile, focusing on the singleton library and include Singleton behavior. Done means the type checker recognizes A::Example::TestClass.instance without reporting that the singleton type lacks the method.