Issue #21375 has been updated by jeremyevans0 (Jeremy Evans). I've submitted a PR that implements backwards compatibility for Set subclasses, without affecting Set itself: https://github.com/ruby/ruby/pull/15228 @Eregon can you try it with concurrent-ruby and see if it resolves your issue? ---------------------------------------- Bug #21375: Set[] does not call #initialize https://bugs.ruby-lang.org/issues/21375#change-115240 * 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 ``` -- https://bugs.ruby-lang.org/