Ruby 3.1 adds Class#subclasses

Ruby 3.1 introduces the Class#subclasses method, which returns all classes directly inheriting from the receiver without including singleton classes.

We can see many implementations for calculating all subclasses of a particular class from the Ruby community with different gems. The ActiveSupport::DescendantsTracker is one of such implementations used in Rails framework. Finally, Ruby has added the Class#subclasses native implementation for it's 3.1 version release.

=> class User; end
=> class Employee < User; end
=> class Client < User; end

=> class Manager < Employee; end
=> class Developer < Employee; end

=> User.subclasses
=> [Employee, Client]

=> Employee.subclasses
=> [Manager, Developer]

=> Developer.subclasses
=> []

Here's the relevant pull request and feature discussion for this change.