ActiveRecord bug with unscope() and a grouped RH side

active_record_gem_failing.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" }

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "5.1.3"

  # Fixed here
  # gem "activerecord", "5.1.3", github: "jemminger/rails", branch: "unscope-with-group"
  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 :posts, force: true do |t|
    t.boolean :flagged
  end
end

class Post < ActiveRecord::Base

  default_scope -> { unflagged }
  scope :flagged, -> { unscope(where: :flagged).where(flagged: true) }
  scope :unflagged, -> { where(flagged: [nil, false]) }
end


class BugTest < Minitest::Test
  def test_unscope
    post1 = Post.create!
    post2 = Post.create!(flagged: true)


    assert_equal [post1], Post.all
    assert_equal [post2], Post.flagged
  end
end

active_record_gem_passing.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" }

  # Activate the gem you are reporting the issue against.
  # gem "activerecord", "5.1.3"

  # Fixed here
  gem "activerecord", "5.1.3", github: "jemminger/rails", branch: "unscope-with-group"
  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 :posts, force: true do |t|
    t.boolean :flagged
  end
end

class Post < ActiveRecord::Base

  default_scope -> { unflagged }
  scope :flagged, -> { unscope(where: :flagged).where(flagged: true) }
  scope :unflagged, -> { where(flagged: [nil, false]) }
end


class BugTest < Minitest::Test
  def test_unscope
    post1 = Post.create!
    post2 = Post.create!(flagged: true)


    assert_equal [post1], Post.all
    assert_equal [post2], Post.flagged
  end
end