From: "knu (Akinori MUSHA) via ruby-core" <ruby-core@...>
Date: 2026-09-15T09:08:11+00:00
Subject: [ruby-core:126728] [Ruby Bug#22174] Set operations (&, ^, collect!, flatten, classify, divide) do not preserve compare_by_identity

Issue #22174 has been updated by knu (Akinori MUSHA).


I'd leave Ruby 3.4 and the pure Ruby implementation just as they are.

As for Ruby 4.x I'd expect:

- `collect!`/`map!` retain the identity flag of the receiver.
- `divide`/`classify` generate subsets inheriting the flag of the receiver.
- `&`/`^` return a set inheriting the flag of the LHS set.
- `flatten` returns a set inheriting the flag of the receiver.

And on these points, I would prefer consistency with what users who explicitly choose to use the flag would expect from this feature over backward compatibility.

----------------------------------------
Bug #22174: Set operations (&, ^, collect!, flatten, classify, divide) do not preserve compare_by_identity
https://bugs.ruby-lang.org/issues/22174#change-119025

* Author: gil.desmarais (Gil Desmarais)
* Status: Open
* ruby -v: ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [arm64-darwin25]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
Since `Set` was transitioned to a C-level core class in Ruby 4.0, several operations that allocate new sets or partition subsets (`&`, `^`, `collect!`, `flatten`, `classify`, and `divide`) lost the `compare_by_identity` behavior for the receiver set or its subsets.

* For `&`, `collect!`, `flatten`, `classify`, and `divide`, the resulting sets/subsets are allocated without propagating the `compare_by_identity` flag (returning `false` for `#compare_by_identity?`).
* For `^` (XOR) with a generic `Enumerable`, the returned set has the flag set (via `dup`), but it allocates a temporary set `tmp` to hold the RHS elements without propagating the `compare_by_identity` flag. This causes the RHS elements to be deduplicated incorrectly using value equality instead of identity equality.

A pull request has been opened with the fix and tests: https://github.com/ruby/ruby/pull/17633

### Reproduction

```ruby
require 'set'

# 1. Intersection &
s1 = Set.new.compare_by_identity
s2 = Set.new
s1 << "a"
s2 << "a"
puts "Intersection preserves compare_by_identity: #{(s1 & s2).compare_by_identity?}"
# Expected: true
# Actual:   false

# 2. XOR ^ (with duplicate objects by value on RHS)
s_xor = Set.new.compare_by_identity
x1 = +"x"
x2 = +"x"
result = s_xor ^ [x1, x2]
puts "XOR with Enumerable preserves identity comparison: #{result.size == 2}"
# Expected: true (size should be 2, because x1 and x2 are distinct objects)
# Actual:   false (size is 1)

# 3. collect! / map!
s_collect = Set.new(["a", "b"]).compare_by_identity
s_collect.collect! { |x| x }
puts "collect! preserves compare_by_identity: #{s_collect.compare_by_identity?}"
# Expected: true
# Actual:   false

# 4. flatten
s_flat = Set.new([Set.new([1])]).compare_by_identity
puts "flatten preserves compare_by_identity: #{s_flat.flatten.compare_by_identity?}"
# Expected: true
# Actual:   false

# 5. classify
s_classify = Set.new(["a", "b"]).compare_by_identity
classified = s_classify.classify { |x| x }
puts "classify subsets preserve compare_by_identity: #{classified.values.all?(&:compare_by_identity?)}"
# Expected: true
# Actual:   false

# 6. divide
s_divide = Set.new(["a", "b"]).compare_by_identity
divided = s_divide.divide { |x| x }
puts "divide subsets preserve compare_by_identity: #{divided.all?(&:compare_by_identity?)}"
# Expected: true
# Actual:   false



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