[ruby-core:107259] [Ruby master Feature#18273] Class#subclasses

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


TBH, I still fail to see the point of how the API clearly reflecting the reality should be considered a "broken window". 

Two subsequent calls returning different results without user's code affecting these results is not that uncommon: if you'll try to introspect the current state/list of threads, or fibers, or ractors; if you try to look at files and directories while something else changed them; well, if you just `Time.now` two times in a row...

From my perspective (being obsessed with docs and teaching people) it is rather fortunate that with `Class#subclasses` we'll have a clear and obvious place to write: yes, subclasses may vanish, please beware and remember; it is an important thing to say explicitly!

I don't think "that would be strange" is a strong enough argument here, if it clearly reflects *what's really going on*.

----------------------------------------
Feature #18273: Class#subclasses
https://bugs.ruby-lang.org/issues/18273#change-96120

* 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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