[ruby-core:109394] [Ruby master Feature#18930] Officially deprecate class variables

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


Regarding @austin's example, there is Class#subclasses now which might be an easier way to do that.
Using class variables for that only works based on a fairly subtle constraint that Base needs to be inherited before any Base subclass can be inherited. Otherwise it would be broken (two subclasses could have separate state).
IMHO using a constant there is what makes most sense, it is clear then where the state lives, and it is convenient to access.

@Dan0042 Right, but most developers do not expect that. And those "class variables" are just prefixed global variables.
With this additional catch (can be a lot less obvious if the cvar is set in a method):
```ruby
class A
end

class B < A
  @@foo = 2
end

class A
  @@foo = 1
  
  def self.foo
    @@foo
  end
end

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

> deprecating them is just removing a tool from the programmer's toolbox

Yes, in this case I think this is good because it's a tool which is very difficult to use correctly and can easily be replaced by clearer ways like instance variables, constants, etc.

----------------------------------------
Feature #18930: Officially deprecate class variables
https://bugs.ruby-lang.org/issues/18930#change-98545

* 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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