[ruby-core:122455] [Ruby Bug#21376] Inconsistent equality between Sets with different compare_by_identity, different classes

Issue #21376 has been updated by knu (Akinori MUSHA).


This bug in optimization in Set#== deserves to be fixed, I think.  In hindsight, adding Set#compare_by_identity might not have been the best idea.

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Bug #21376: Inconsistent equality between Sets with different compare_by_identity, different classes
https://bugs.ruby-lang.org/issues/21376#change-113640

* 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
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This is an inconsistency between the new core Set and the ruby-implemented Set. I think probably the new implementation's behavior seems correct and the ruby implementation is an incorrect edge case.

```ruby
class MySet < Set
end

o = Object.new
Set.new([o]).compare_by_identity == MySet.new([o])
# => false on 3.5.0; true with the ruby implementation
```

My understanding is that, for Hash, if two hashes have unequal `compare_by_identity?` they are never equal (even if containing identical contents), unless both are empty. It would make sense for Set to be the same.

Hmm, analyzing the ruby Set#== a bit more, I see a further edge case where a non-identity set is considered equal to an identity set with as few as one element in common.

```ruby
set_by_eq = Set.new(['a', 'b', 'c'])
# => #<Set: {"a", "b", "c"}>
set_by_id = MySet.new.compare_by_identity.merge(['a', 'a'.dup, 'a'.dup])
# => #<MySet: {"a", "a", "a"}>
set_by_eq == set_by_id
# => true in the ruby set, false in core set
set_by_id == set_by_eq
# => false in both
```

The latter seems unlikely it would ever be triggered unintentionally. The former I did happen across, but since I think the new behavior is correct, I will not be affected when I fix my code to consistently compare_by_identity. Given that, it may be questionable whether it's worth fixing.



-- 
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