1 files changed, 23 insertions, 7 deletions
diff --git a/activesupport/lib/active_support/concern.rb b/activesupport/lib/active_support/concern.rb
index eeeba60839..342d3a9d52 100644
--- a/activesupport/lib/active_support/concern.rb
+++ b/activesupport/lib/active_support/concern.rb
@@ -26,7 +26,7 @@ module ActiveSupport
   #       scope :disabled, -> { where(disabled: true) }
   #     end
   #
-  #     module ClassMethods
+  #     class_methods do
   #       ...
   #     end
   #   end
@@ -95,32 +95,48 @@ module ActiveSupport
   #   end
   #
   #   class Host
-  #     include Bar # works, Bar takes care now of its dependencies
+  #     include Bar # It works, now Bar takes care of its dependencies
   #   end
   module Concern
+    class MultipleIncludedBlocks < StandardError #:nodoc:
+      def initialize
+        super "Cannot define multiple 'included' blocks for a Concern"
+      end
+    end
+
     def self.extended(base) #:nodoc:
-      base.instance_variable_set("@_dependencies", [])
+      base.instance_variable_set(:@_dependencies, [])
     end
 
     def append_features(base)
-      if base.instance_variable_defined?("@_dependencies")
-        base.instance_variable_get("@_dependencies") << self
+      if base.instance_variable_defined?(:@_dependencies)
+        base.instance_variable_get(:@_dependencies) << self
         return false
       else
         return false if base < self
         @_dependencies.each { |dep| base.send(:include, dep) }
         super
-        base.extend const_get("ClassMethods") if const_defined?("ClassMethods")
-        base.class_eval(&@_included_block) if instance_variable_defined?("@_included_block")
+        base.extend const_get(:ClassMethods) if const_defined?(:ClassMethods)
+        base.class_eval(&@_included_block) if instance_variable_defined?(:@_included_block)
       end
     end
 
     def included(base = nil, &block)
       if base.nil?
+        raise MultipleIncludedBlocks if instance_variable_defined?(:@_included_block)
+
         @_included_block = block
       else
         super
       end
     end
+
+    def class_methods(&class_methods_module_definition)
+      mod = const_defined?(:ClassMethods) ?
+        const_get(:ClassMethods) :
+        const_set(:ClassMethods, Module.new)
+
+      mod.module_eval(&class_methods_module_definition)
+    end
   end
 end