diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index 34e643b2de..38a364ff88 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/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)
@@ -313,6 +314,13 @@ module ActiveRecord
       klass.current_scope = previous
     end
 
+    def _exec_scope(*args, &block) # :nodoc:
+      @delegate_to_klass = true
+      instance_exec(*args, &block) || self
+    ensure
+      @delegate_to_klass = false
+    end
+
     # Updates all records in the current relation with details given. This method constructs a single SQL UPDATE
     # statement and sends it straight to the database. It does not instantiate the involved models and it does not
     # trigger Active Record callbacks or validations. However, values passed to #update_all will still go through
@@ -446,6 +454,7 @@ module ActiveRecord
     end
 
     def reset
+      @delegate_to_klass = false
       @to_sql = @arel = @loaded = @should_eager_load = nil
       @records = [].freeze
       @offsets = {}
diff --git a/activerecord/lib/active_record/relation/delegation.rb b/activerecord/lib/active_record/relation/delegation.rb
index 4b16b49cdf..488f71cdde 100644
--- a/activerecord/lib/active_record/relation/delegation.rb
+++ b/activerecord/lib/active_record/relation/delegation.rb
@@ -82,6 +82,11 @@ module ActiveRecord
           if @klass.respond_to?(method)
             self.class.delegate_to_scoped_klass(method)
             scoping { @klass.public_send(method, *args, &block) }
+          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 \
               "Delegating #{method} to arel is deprecated and will be removed in Rails 6.0."
diff --git a/activerecord/lib/active_record/scoping/named.rb b/activerecord/lib/active_record/scoping/named.rb
index 752655aa05..a784001587 100644
--- a/activerecord/lib/active_record/scoping/named.rb
+++ b/activerecord/lib/active_record/scoping/named.rb
@@ -183,7 +183,7 @@ module ActiveRecord
           if body.respond_to?(:to_proc)
             singleton_class.send(:define_method, name) do |*args|
               scope = all
-              scope = scope.instance_exec(*args, &body) || scope
+              scope = scope._exec_scope(*args, &body)
               scope = scope.extending(extension) if extension
               scope
             end
diff --git a/activerecord/test/cases/scoping/named_scoping_test.rb b/activerecord/test/cases/scoping/named_scoping_test.rb
index ea71a5ce28..03f8a4f7c9 100644
--- a/activerecord/test/cases/scoping/named_scoping_test.rb
+++ b/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"
diff --git a/activerecord/test/models/topic.rb b/activerecord/test/models/topic.rb
index 8cd4dc352a..fa50eeb6a4 100644
--- a/activerecord/test/models/topic.rb
+++ b/activerecord/test/models/topic.rb
@@ -12,9 +12,17 @@ class Topic < ActiveRecord::Base
 
   scope :scope_with_lambda, lambda { all }
 
+  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 private_lifo
+        "lifo"
+      end
+  end
+
   scope "approved_as_string", -> { where(approved: true) }
   scope :anonymous_extension, -> {} do
     def one