[ruby-core:107265] [Ruby master Feature#18273] Class#subclasses

Issue #18273 has been updated by zverok (Victor Shepelev).


@fxn You know what? 

I think I actually got your point:
* `Class#ancestors` represent a "really existing thing", that is, objective ancestors chain. E.g. it is an API to an internal _structures_, helping to make sense of them;
* `Class#subclasses` look the same way, therefore implying it is also an API to internally existing structures (for example, to each class keeping track of its descendants because it is a part of an object model)—but this implication is completely untrue! The `#subclasses` is a dynamic thing—and in more formal language, maybe should've at least been called `#enumerate_subclasses` (implying by a verb form that it will _do_  something, not just expose internally existing reality)

Is that it?

----------------------------------------
Feature #18273: Class#subclasses
https://bugs.ruby-lang.org/issues/18273#change-96125

* Author: byroot (Jean Boussier)
* Status: Closed
* Priority: Normal
----------------------------------------
Ref: https://github.com/rails/rails/pull/43481

Something we forgot to mention in [Feature #14394], is either a parameter or another method to only get direct descendants.

Active Support has been offering `Class.subclasses` as:

```ruby
  def subclasses
    descendants.select { |descendant| descendant.superclass == self }
  end
```

It seems a bit silly to grab all descendants and then restrict the list when `Class#descendants` had to do some recursion to get them all in the first place.

### Proposal

We could either implement `Class#subclasses` directly, or accept a parameter in `Class#descendants`, e.g. `descendants(immediate = false)`.

cc @eregon



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