From: "jeremyevans0 (Jeremy Evans) via ruby-core" <ruby-core@...>
Date: 2025-07-09T06:35:07+00:00
Subject: [ruby-core:122694] [Ruby Bug#21396] Set#initialize should call Set#add on items passed in

Issue #21396 has been updated by jeremyevans0 (Jeremy Evans).


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.

> The way Set was written, the accompanying tests, and the existence of OrderedSet all make it clear that this behaviour was a deliberate design choice, so after those long-standing signals, I feel like it's hard to tell users that those were implementation details they shouldn't have relied on.

Thank you for providing that context.  Since there were not explicit tests that methods should call other methods, my assumption was that the behavior was an implementation detail and not by design.  If we consider these to be deliberate design decisions and not implementation details, I recommend we add tests for them. I can handle that if you would like.

----------------------------------------
Bug #21396: Set#initialize should call Set#add on items passed in
https://bugs.ruby-lang.org/issues/21396#change-113972

* 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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