Deprecate accessibility of private/protected class methods in named scope

activerecord/lib/active_record/relation.rb

@@ -30,6 +30,7 @@ module ActiveRecord
       @offsets = {}
       @loaded = false
       @predicate_builder = predicate_builder
+      @delegate_to_klass = false
     end
 
     def initialize_copy(other)
@@ -453,6 +454,7 @@ module ActiveRecord
     end
 
     def reset
+      @delegate_to_klass = false
       @to_sql = @arel = @loaded = @should_eager_load = nil
       @records = [].freeze
       @offsets = {}

activerecord/lib/active_record/relation/delegation.rb

@@ -82,8 +82,10 @@ module ActiveRecord
           if @klass.respond_to?(method)
             self.class.delegate_to_scoped_klass(method)
             scoping { @klass.public_send(method, *args, &block) }
-          elsif defined?(@delegate_to_klass) &&
-            @delegate_to_klass && @klass.respond_to?(method, true)
+          elsif @delegate_to_klass && @klass.respond_to?(method, true)
+            ActiveSupport::Deprecation.warn \
+              "Delegating missing #{method} method to #{@klass}. " \
+              "Accessibility of private/protected class methods in :scope is deprecated and will be removed in Rails 6.0."
             @klass.send(method, *args, &block)
           elsif arel.respond_to?(method)
             ActiveSupport::Deprecation.warn \

activerecord/test/cases/scoping/named_scoping_test.rb

@@ -303,6 +303,13 @@ class NamedScopingTest < ActiveRecord::TestCase
     assert_equal "lifo", topic.author_name
   end
 
+  def test_deprecated_delegating_private_method
+    assert_deprecated do
+      scope = Topic.all.by_private_lifo
+      assert_not scope.instance_variable_get(:@delegate_to_klass)
+    end
+  end
+
   def test_reserved_scope_names
     klass = Class.new(ActiveRecord::Base) do
       self.table_name = "topics"

activerecord/test/models/topic.rb

@@ -12,12 +12,13 @@ class Topic < ActiveRecord::Base
 
   scope :scope_with_lambda, lambda { all }
 
-  scope :by_lifo, -> { where(author_name: author_name) }
+  scope :by_private_lifo, -> { where(author_name: private_lifo) }
+  scope :by_lifo, -> { where(author_name: "lifo") }
   scope :replied, -> { where "replies_count > 0" }
 
   class << self
     private
-      def author_name
+      def private_lifo
         "lifo"
       end
   end