[ruby-core:122464] [Ruby Bug#21396] Set#initialize should call Set#add on items passed in

Issue #21396 has been updated by tenderlovemaking (Aaron Patterson).


ko1 (Koichi Sasada) wrote in #note-4:
> How about to redfine `initialize` on subclass of `Set` to call `#add`?

I think we could, but that means people have to change their code when upgrading.

Eregon (Benoit Daloze) wrote in #note-5:
> Is there any public code in some gem or so that relies on this? (the example is rather synthetic)

It's kind of hard to search for this on GitHub, but I was able to find two similar-ish examples:

[This one](https://github.com/alunny/confetti/blob/a10fab1180ab5a553bcda908834240ad277cd168/lib/typedset.rb#L13-L15) would not raise an exception in the same way, and [this one](https://github.com/bassnode/liner_notes/blob/26a899af271453c9ca7f95c11cee9e272189b456/lib/links.rb#L17-L21) could potentially be missing entries from its `@hash_lookup` instance variable.

Maybe we could publish a gem that just has a copy of the current `set.rb` file, but with `Set` renamed to `RbSet` or something. Then if people run in to issues they can use the gem and change the superclass to `RbSet`.  I think it would fix our case at work and probably the cases referenced above.  I agree with @jeremyevans0's points, but I'm sure this is going to cause friction for people upgrading and it would be nice if we can make it as easy as possible to upgrade.

----------------------------------------
Bug #21396: Set#initialize should call Set#add on items passed in
https://bugs.ruby-lang.org/issues/21396#change-113652

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
```ruby
class Foo < Set
  def add(item) = super(item.bytesize)
end

x = Foo.new(["foo"])
p x
p x.include?(3)
```

On Ruby 3.4 the output is this:

```
> ruby -v test.rb
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [arm64-darwin24]
#<Foo: {3}>
true
```

On Ruby master the output is this:

```
> make run
./miniruby -I./lib -I. -I.ext/common  -r./arm64-darwin24-fake  ./test.rb 
#<Set: {"foo"}>
false
```

The bug is that `initialize` is not calling `add` for the elements passed in, so the subclass doesn't get a chance to change them.

I've sent a PR here: https://github.com/ruby/ruby/pull/13518



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