[ruby-core:106086] [Ruby master Feature#18273] Class#subclasses

Issue #18273 has been updated by Eregon (Benoit Daloze).


AFAIK superclass is always understand as the direct superclass.
And given we already have Class#descendants and Module#ancestors, I feel there is very little room for confusion for Class#subclasses (i.e., almost no confusion possible).
Of course we can document it clearly in that method's documentation it's only "classes which have C has the superclass", and link to `Class#descendants`.

IMHO `Class#direct_subclasses` is verbose and not helpful.
Rails already established that `subclasses` is a clear meaning for this.

Regarding a boolean argument for `Class#descendants` I'm neutral, I think that's less good and one definitely need to read the docs just to find the meaning of the boolean, plus they might not guess it even takes an argument.
`Class#subclasses` is clearer and is already an established name for this.

----------------------------------------
Feature #18273: Class#subclasses
https://bugs.ruby-lang.org/issues/18273#change-94677

* Author: byroot (Jean Boussier)
* Status: Open
* 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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