[ruby-core:107261] [Ruby master Feature#18273] Class#subclasses

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


We are going in circles here :( It is "non-deterministic" all right, but also reflects the reality.

It is more or less the same as with
```ruby
Thread.new {}.run
Thread.list
```
...isn't it? We can't predict what the 2nd line is going to return. 

Should we argue that `Time.now` value can't be predicted and therefore non-deterministic, and therefore a bad API?

(I am not trolling, I am trying to see what lies in the gap between just repeating that it is non-deterministic and therefore not acceptable from one side, and it is how it really is and therefore totally sane API from another.)

----------------------------------------
Feature #18273: Class#subclasses
https://bugs.ruby-lang.org/issues/18273#change-96122

* 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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