[ruby-core:109389] [Ruby master Feature#18930] Officially deprecate class variables

Issue #18930 has been updated by zverok (Victor Shepelev).


@austin

About your example

I'd say that referencing a class method in another class method in inheritance hierarchy always makes one ask whether you meant the method/data of the base class or of the current one. 

Because, like

> but `Rand.dependents` producing `{1=>V1}` and `Timestamp.dependents` producing `{2=>V2,3=>V3,4=>V4}`.

I can easily imagine it as a _desired_ outcome (e.g., "every non-Vx class is root for its own list of versions, and stores them independently"). 

So, without using class variables, I'd rewrote that with `Base.dependents` to make the intention unambiguous: "yes, please be aware, whenever in class hierarchy this will be reused, we are putting it into the **Base's** list of dependents."

You mention it as a "hacky workaround," but I honestly see it as a most explicit (and not that verbose) way of stating the intention; I use it instead of class vars even today.

(It wasn't that way always for me: I adored class vars for this kind of trick... But at some point, I came to an understanding that I needed to challenge "well, I learned the class vars quirk long ago, it is natural to me," and now see them as a way of too cryptic coding of the same, not that frequently needed meaning, "yes, that's data shared throughout the hierarchy")

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Feature #18930: Officially deprecate class variables
https://bugs.ruby-lang.org/issues/18930#change-98538

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