infinite loop in changed_for_autosave https://github.com/rails/rails/pull/7824 and https://github.com/rails/rails/issues/7809

gistfile1.txt

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
    gem 'byebug'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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)

ActiveRecord::Schema.define do
  create_table :users do |t| end
  create_table :employees do |t| t.integer "user_id" end
end

class User < ActiveRecord::Base
  has_one :employee
  accepts_nested_attributes_for :employee
end

class Employee < ActiveRecord::Base
  belongs_to :user
  accepts_nested_attributes_for :user
end

class BugTest < MiniTest::Unit::TestCase
  def test_changed_for_autosave_inf_loop
    u = User.create!
    e = Employee.create!(user:u)
    u.employee=e
    e.save!
  end
end

I have this problem on 4.2. Unfortunately, the issue is locked and I can't comment. I applied the patch locally and it fixes the problem.