From: "Eregon (Benoit Daloze) via ruby-core" <ruby-core@...>
Date: 2025-11-17T09:03:43+00:00
Subject: [ruby-core:123824] [Ruby Bug#21396] Set#initialize should call Set#add on items passed in

Issue #21396 has been updated by Eregon (Benoit Daloze).


jeremyevans0 (Jeremy Evans) wrote in #note-11:
> knu (Akinori MUSHA) wrote in #note-10:
> > Jeremy, thanks for the reply.
> > 
> > Your point about thread-safety is well taken.  It is an important advantage.  As a possible compromise, we could keep the C backing and switch the behavior of methods when the class is subclassed in exchange for losing thread-safety.  It's just an idea and not the cleanest solution, though.
> 
> I think that is a reasonable compromise.  It keeps the advantages of core Set when not subclassed, and keeps backwards compatibility for subclasses (at least, subclasses that don't access `@hash`).  Please let me know if you would like me to work on that.

Yes, this sounds like a good way, it avoids breaking code and keeps performance optimal for Set.
The only downside is some extra complexity but only some conditions in a couple methods, no big deal.
And the thread-safety aspect is pretty minor as explained in my other comment (and obviously it wasn't guaranteed before so that's not breaking anything).

----------------------------------------
Bug #21396: Set#initialize should call Set#add on items passed in
https://bugs.ruby-lang.org/issues/21396#change-115227

* 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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