ActiveRecord fails to correctly merge scopes containing IN constraints on the same column

active_record_master.rb

# frozen_string_literal: true

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"

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

  gem "rails", github: "rails/rails"
  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)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
    t.integer :value
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def setup
    Post.create!([{ id: 1 }, { id: 2 }, { id: 3 }])
    assert_equal 3, Post.count

    Comment.create!([{ post_id: 1, value: 1 },
                     { post_id: 2, value: 1 }, { post_id: 2, value: 2 },
                     { post_id: 3, value: 2 }])
    assert_equal 4, Comment.count
  end

  def teardown
    Comment.delete_all
    Post.delete_all
  end

  # Merging relational constraints works:
  def test_merging_relational_constraints
    posts = Post.where(Post.arel_table[:id].gt(1))
            .merge(Post.where(Post.arel_table[:id].lt(3)))
    assert_equal 1, posts.count # passes
  end

  # But merging IN constraints doesn't work:
  def test_merging_in_constraints
    posts = Post.where(id: [2, 3]).where(id: [1, 2])
    assert_equal 1, posts.count # passes

    posts = Post.where(id: [2, 3])
              .merge(Post.where(id: [1, 2]))
    assert_equal 1, posts.count # fails
  end

  # It also fails when using arel explicitly:
  def test_merging_arel_in_constraints
    posts = Post.where(Post.arel_table[:id].in([2, 3]))
              .where(Post.arel_table[:id].in([1, 2]))
    assert_equal 1, posts.count # passes

    posts = Post.where(Post.arel_table[:id].in([2, 3]))
              .merge(Post.where(Post.arel_table[:id].in([1, 2])))
    assert_equal 1, posts.count # fails
  end

  # and when using subquery IN constraints
  def test_merging_subquery_in_constraints
    posts = Post.where(id: Comment.where(value: 1).select(:post_id))
              .where(id: Comment.where(value: 2).select(:post_id))
    assert_equal 1, posts.count # passes

    posts = Post.where(id: Comment.where(value: 1).select(:post_id))
              .merge(Post.where(id: Comment.where(value: 2).select(:post_id)))
    assert_equal 1, posts.count # fails
  end
end