Can you create a reproduction test script using https://github.com/rails/rails/blob/main/guides/bug_report_templates/active_record_gem.rb template?

Here it is:

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "~> 7.1.0"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

class BugTest < Minitest::Test
  def test_attribute_set
    my_attribute_set = ActiveModel::AttributeSet.new({})

    assert_equal false, my_attribute_set == nil
  end
end

Would you like to open a PR? An additional check for self.class == other.class and a test case would be enough.

Yep, I will do it.

Update to the suggestion: instead of self.class == other.class it is correct to use other.is_a?(self.class), because there is also a LazyAttributeSet class.

AttributeSet is not part of Rails' public API. We shouldn't fix this problem unless there's a way to trigger it using only methods documented on https://api.rubyonrails.org/.

Good point! @DmitryPogrebnoy How did you got that error? Can you share a real-world example?

I have a debugger gem that does object introspection. And there is a nil check in there like this:
my_attribute_set == nil. So it's quite unnatural to me that it will throw an exception in any case. This is an issue for the context: https://youtrack.jetbrains.com/issue/RUBY-31972.

Also, the current implementation of the ==(other) method violates the symmetry contract, because nil == my_attribute_set returns false without any exception.

From my point of view this is definitely a bug. I don't know why we need to keep it just because this is not a part of public api.