[ruby-core:121082] [Ruby master Misc#21143] Speficy order of execution const_added vs inherited

Issue #21143 has been updated by fxn (Xavier Noria).


> For classes/modules not using the class/module keyword (e.g. C = Class.new(parent)), inherited must run first since the inheritance happens before the possible later assignment.

Yes, that is why the proposed docs specifically mention the `class` keyword, because it is there where the ambiguity lies.

If you do two things in series, it is to be expected that their side-effects happen in series. Although when that is done in one line, the programmer needs to understand that the expression is evaluated first, and then the result assigned to the constant.

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Misc #21143: Speficy order of execution const_added vs inherited
https://bugs.ruby-lang.org/issues/21143#change-112000

* Author: fxn (Xavier Noria)
* Status: Open
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The hooks `const_added` and `inherited` may need to be executed "together".

For example, consider:

```ruby
module M
  def self.const_added(cname) = ...

  class C
    def self.inherited(subclass) = ...
  end

  class D < C; end
end
```

When `D` is defined, two hooks are set to run, but in which order?

Both orders make sense in a way:

1. When `inherited` is called, you can observe that the subclass has a permanent name, ergo it was assigned to a constant, which must me stored in a class or module object. Therefore, the constant was added to said class/module before `inherited` was invoked.

1. When `const_added` is called, you can `const_get` the symbol and observe the object is a class, hence with a superclass, hence inheritance already happened.

The patch in [ruby#12759](https://github.com/ruby/ruby/pull/12759) documents and adds a test for (1). Rationale:

1. I believe it would be nice to specify this order.

1. Chose (1) because it is how it works today.

While the motivation for the patch was formal (to remove an ambiguity), after reflecting about this I realized users of Zeitwerk may depend on this. Nowadays, Zeitwerk uses `const_added` to set autoloads for child constants in namespaces. Thanks to the current order, code can be used in `inherited` hooks normally (it would not be ready if the order was different).




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