[ruby-core:122311] [Ruby Bug#21375] Set[] does not call #initialize

Issue #21375 has been updated by jeremyevans0 (Jeremy Evans).


nobu (Nobuyoshi Nakada) wrote in #note-1:
> I'm not sure if it is intentional or not.

It was intentional when I developed core Set.  `Set.[]` takes different arguments than `Set#initialize`.

`Array.[]` does not call `Array#initialize`, and `Hash.[]` does not call `Hash#initialize`, so there is precedent for the core collection classes to operate this way.

`Set.[]` was not documented as calling `Set#initialize` previously, and there were no tests or specs for it.  Relying on `Set.[]` calling `Set#initialize` was relying on undefined behavior.

> If unintentional, this patch may fix it.
> 
> ```diff
> diff --git a/set.c b/set.c
> index 8676c62cd35..c41781c446f 100644
> --- a/set.c
> +++ b/set.c
> @@ -409,13 +409,7 @@ static VALUE
>  set_s_create(int argc, VALUE *argv, VALUE klass)
>  {
>      VALUE set = set_alloc_with_size(klass, argc);
> -    set_table *table = RSET_TABLE(set);
> -    int i;
> -
> -    for (i=0; i < argc; i++) {
> -        set_table_insert_wb(table, set, argv[i], NULL);
> -    }
> -
> +    rb_obj_call_init(set, argc, argv);
>      return set;
>  }
>  
> ```

I'm not sure whether this is correct.  `#initialize` arguments are enumerables of elements, `.[]` arguments are elements.  I think if we wanted to call `#initialize`, we would have to allocate an array for the elements, and pass that, which would reduce performance.

----------------------------------------
Bug #21375: Set[] does not call #initialize
https://bugs.ruby-lang.org/issues/21375#change-113453

* Author: Ethan (Ethan -)
* Status: Open
* ruby -v: ruby 3.5.0dev (2025-05-26T17:42:35Z master 909a0daab6) +PRISM [x86_64-darwin22]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
I have a subclass of Set that overrides #initialize. Following #21216, .new does call #initialize but .[] does not.

```ruby
class MySet < Set
  def initialize(enum = nil)
    compare_by_identity
    super
  end
end

MySet.new.compare_by_identity?
# => true
MySet[].compare_by_identity?
# => false
```



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