relation merge turns an inner join into outer

[bughit]

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  # Activate the gem you are reporting the issue against.
  gem 'activerecord', '4.2.4'
  gem 'sqlite3'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# 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 :blogs, force: true do |t|

  end

  create_table :posts, force: true do |t|
    t.belongs_to :blog
  end

  create_table :comments, force: true do |t|
    t.belongs_to :post

  end

end

class Blog < ActiveRecord::Base
  has_one :post

  scope :with_post_with_comment_good, ->{joins(post: :comment)}
  scope :with_post_with_comment_bad, ->{joins(:post).merge(Post.with_comment)}
end

class Post < ActiveRecord::Base
  belongs_to :blog
  has_one :comment

  scope :with_comment, ->{joins(:comment)}
end

class Comment < ActiveRecord::Base
  belongs_to :post
end


class BugTest < Minitest::Test
  def test_association_stuff

    Blog.create! post: Post.create!

    puts Blog.with_post_with_comment_good.to_sql
    puts Blog.with_post_with_comment_bad.to_sql

    assert_equal 0, Blog.with_post_with_comment_good.count
    assert_equal 0, Blog.with_post_with_comment_bad.count

  end
end

SELECT "blogs".* FROM "blogs" INNER JOIN "posts" ON "posts"."blog_id" = "blogs"."id" INNER JOIN "comments" ON "comments"."post_id" = "posts"."id"
SELECT "blogs".* FROM "blogs" INNER JOIN "posts" ON "posts"."blog_id" = "blogs"."id" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id"

[maclover7]

Reproduction script failing on master.

[sevaorlov]

related to #24281

[stale]

This issue has been automatically marked as stale because it has not been commented on for at least three months.
The resources of the Rails team are limited, and so we are asking for your help.
If you can still reproduce this error on the 5-1-stable branch or on master, please reply with all of the information you have about it in order to keep the issue open.
Thank you for all your contributions.