[ruby-core:120126] [Ruby master Feature#18930] Officially deprecate class variables

Issue #18930 has been updated by Dan0042 (Daniel DeLorme).


I'm still against deprecating, either soft or hard, but I believe the errors could be improved.

```ruby
class A
  def self.foo
    p @@foo ||= rand
  end
end

class B < A
  p @@foo = 2
end

B.foo 
```

In this code, the @@foo variable is overtaken but the "overtaken" error isn't triggered because even though the `foo` method is invoked on B, it was defined in A. If this could be improved to raise an error, I believe it might relieve some of the confusion over class variables.

----------------------------------------
Feature #18930: Officially deprecate class variables
https://bugs.ruby-lang.org/issues/18930#change-110877

* Author: Eregon (Benoit Daloze)
* Status: Rejected
----------------------------------------
Ruby's class variables are very confusing, and it seem many people agree they should be avoided (#18927).

How about we deprecate them officially?

Concretely:
* Mention in the documentation that class variables are deprecated and should be avoided/should not be used.
* Add a parse-time deprecation warning, now that we only see those with `Warning[:deprecation] = true` it seems reasonable to add.



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