[ruby-core:122319] [Ruby Bug#21375] Set[] does not call #initialize

Issue #21375 has been updated by Ethan (Ethan -).


It seems like a regression to me. I mean, it broke my code - maybe I'm alone, I can't say whether other people override #initialize and expect Set[] to call it, but it seems like a reasonable assumption to make, particularly since it always has done that. I wasn't aware that Array[]/Hash[] don't call initialize and I find that counterintuitive, and would advocate against changing other classes/constructors to do that.

> `Set.[]` was not documented as calling `Set#initialize` previously

My feeling is that the base expectation is that #initialize is called on new objects. I know constructors can allocate and do things without #initialize, but I think that is quite rare. Maybe I'm wrong, I just learned Array[] and Hash[] do that, but I've almost never seen it.

I'll also note that the effects of not calling #initialize are easy to miss, since the set will still be a perfectly functional set, but will have lost whatever checks or changes are intended on initialization. At least in my case this was not straightforward to trace back to this change, it took me a while in the debugger to figure out that some of my sets were unexpectedly no longer compare_by_identity as my #initialize sets.

Having figured that out it is of course trivial for me to override MySet.[], but I'd rather Set behave like it used to, especially if that would save other people having to debug subtle bugs and fix.


----------------------------------------
Bug #21375: Set[] does not call #initialize
https://bugs.ruby-lang.org/issues/21375#change-113460

* 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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