Handles when ClassMethods is used inside a class

rubocop/cop/prefer_class_methods_over_module.rb

@@ -48,7 +48,7 @@ module RuboCop
       private
 
       def extends_activesupport_concern?(node)
-        container_module = container_module_of(node.parent)
+        container_module = container_module_of(node)
         return false unless container_module
 
         container_module.descendants.any? do |descendant|
@@ -57,7 +57,9 @@ module RuboCop
       end
 
       def container_module_of(node)
-        node = node.parent until node.type == :module
+        while node = node.parent
+          break if node.type == :module
+        end
 
         node
       end

spec/rubocop/cop/prefer_class_methods_over_module_spec.rb

@@ -48,6 +48,17 @@ describe RuboCop::Cop::PreferClassMethodsOverModule do
     RUBY
   end
 
+  it "doesn't flag violation when ClassMethods is used inside a class" do
+    expect_no_offenses(<<~RUBY)
+      class Foo
+        module ClassMethods
+          def a_class_method
+          end
+        end
+      end
+    RUBY
+  end
+
   it "doesn't flag violation when not using either class_methods or ClassMethods" do
     expect_no_offenses(<<~RUBY)
       module Foo