From f98e75a3e2b1665bd1da28b9456a296cc3c35bbc Mon Sep 17 00:00:00 2001
From: Kristijan Novoselic <kristijan@glooko.com>
Date: Tue, 9 Jun 2015 19:00:24 +0200
Subject: [PATCH] Fix for #20489 - ActiveSupport::Concern#class_methods affects
 parent classes

---
 activesupport/lib/active_support/concern.rb |  2 +-
 activesupport/test/concern_test.rb          | 25 +++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/activesupport/lib/active_support/concern.rb b/activesupport/lib/active_support/concern.rb
index 4082d2d464..0403eb70ca 100644
--- a/activesupport/lib/active_support/concern.rb
+++ b/activesupport/lib/active_support/concern.rb
@@ -132,7 +132,7 @@ module ActiveSupport
     end
 
     def class_methods(&class_methods_module_definition)
-      mod = const_defined?(:ClassMethods) ?
+      mod = const_defined?(:ClassMethods, false) ?
         const_get(:ClassMethods) :
         const_set(:ClassMethods, Module.new)
 
diff --git a/activesupport/test/concern_test.rb b/activesupport/test/concern_test.rb
index 253c1adc23..8ea701cfb7 100644
--- a/activesupport/test/concern_test.rb
+++ b/activesupport/test/concern_test.rb
@@ -54,6 +54,11 @@ class ConcernTest < ActiveSupport::TestCase
     include Bar, Baz
   end
 
+  module Qux
+    module ClassMethods
+    end
+  end
+
   def setup
     @klass = Class.new
   end
@@ -70,6 +75,26 @@ class ConcernTest < ActiveSupport::TestCase
     assert_equal ConcernTest::Baz::ClassMethods, (class << @klass; self.included_modules; end)[0]
   end
 
+  def test_class_methods_are_extended_only_on_expected_objects
+    ::Object.__send__(:include, Qux)
+    Object.extend(Qux::ClassMethods)
+    # module needs to be created after Qux is included in Object or bug won't
+    # be triggered
+    test_module = Module.new do
+      extend ActiveSupport::Concern
+
+      class_methods do
+        def test
+        end
+      end
+    end
+    @klass.include test_module
+    assert_equal false, Object.respond_to?(:test)
+    Qux.class_eval do
+      remove_const :ClassMethods
+    end
+  end
+
   def test_included_block_is_ran
     @klass.include(Baz)
     assert_equal true, @klass.included_ran