[ruby-core:109382] [Ruby master Feature#18930] Officially deprecate class variables

Issue #18930 has been updated by Eregon (Benoit Daloze).


@Dan0042 It seems likely you might not know well their semantics then and how much a trap they are.
Using @byroot's example from https://bugs.ruby-lang.org/issues/14541#note-25:
```ruby
class A
  @@foo = 1
  
  def self.foo
    @@foo
  end
end

class B < A
  @@foo = 2 # actually writes in A
end

p A.foo # => 2, but should be 1
```

And if one does not write to them then constants are a much better choice.

Could someone point to a gem using class variables where it is not fairly easy to replace them with instance variables?
Or even better, a gem where the semantics shown above are actually desirable?

I believe nobody wants these semantics (it's buggy behavior, if one wants that they might as well use an instance variable on a given class+accessors or a global variable), hence class variables should be discouraged or deprecated as they are just likely to cause bugs and are not necessary (plus, they look like a Perl mess of sigils).

Because fixing their semantics is deemed too incompatible (#14541), then deprecation seems the only way to avoid people running into these buggy class variables.

----------------------------------------
Feature #18930: Officially deprecate class variables
https://bugs.ruby-lang.org/issues/18930#change-98532

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
----------------------------------------
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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