Singleton classes returns parent's methods with instance_methods(false) and this makes remove_method in Module#delegate fail. Add a test case and fix the bug.

activesupport/lib/active_support/core_ext/module/delegation.rb

@@ -1,3 +1,5 @@
+require "active_support/core_ext/module/remove_method"
+
 class Module
   # Provides a delegate class method to easily expose contained objects' methods
   # as your own. Pass one or more methods (specified as symbols or strings)
@@ -121,7 +123,7 @@ class Module
 
       module_eval(<<-EOS, file, line)
         if instance_methods(false).map(&:to_s).include?("#{prefix}#{method}")
-          remove_method("#{prefix}#{method}")
+          remove_possible_method("#{prefix}#{method}")
         end
 
         def #{prefix}#{method}(*args, &block)               # def customer_name(*args, &block)

activesupport/test/core_ext/module_test.rb

@@ -141,6 +141,20 @@ class ModuleTest < Test::Unit::TestCase
     assert_equal 0.0, nil_project.to_f
   end
 
+  def test_delegation_does_not_raise_error_when_removing_singleton_instance_methods
+    parent = Class.new do
+      def self.parent_method; end
+    end
+
+    assert_nothing_raised do
+      child = Class.new(parent) do
+        class << self
+          delegate :parent_method, :to => :superclass
+        end
+      end
+    end
+  end
+
   def test_parent
     assert_equal Yz::Zy, Yz::Zy::Cd.parent
     assert_equal Yz, Yz::Zy.parent