Revert "Remove valid_scope_name? check - use ruby"

This reverts commit f6db31ec.

Reason:
Scope names can very easily conflict, particularly when sharing Concerns
within the team, or using multiple gems that extend AR models.

It is true that Ruby has the ability to detect this with the -w option, but the
reality is that we are depending on too many gems that do not care about Ruby
warnings, therefore it might not be a realistic solution to turn this switch on
in our real-world apps.

activerecord/lib/active_record/scoping/named.rb

@@ -151,6 +151,7 @@ def scope(name, body, &block)
               "a class method with the same name."
           end
 
+          valid_scope_name?(name)
           extension = Module.new(&block) if block
 
           if body.respond_to?(:to_proc)
@@ -169,6 +170,15 @@ def scope(name, body, &block)
             end
           end
         end
+
+      protected
+
+        def valid_scope_name?(name)
+          if respond_to?(name, true)
+            logger.warn "Creating scope :#{name}. " \
+                        "Overwriting existing method #{self.name}.#{name}."
+          end
+        end
       end
     end
   end

activerecord/test/cases/scoping/named_scoping_test.rb

@@ -440,6 +440,25 @@ def test_table_names_for_chaining_scopes_with_and_without_table_name_included
     end
   end
 
+  def test_scopes_with_reserved_names
+    class << Topic
+      def public_method; end
+      public :public_method
+
+      def protected_method; end
+      protected :protected_method
+
+      def private_method; end
+      private :private_method
+    end
+
+    [:public_method, :protected_method, :private_method].each do |reserved_method|
+      assert Topic.respond_to?(reserved_method, true)
+      ActiveRecord::Base.logger.expects(:warn)
+      Topic.scope(reserved_method)
+    end
+  end
+
   def test_scopes_on_relations
     # Topic.replied
     approved_topics = Topic.all.approved.order('id DESC')