Added test to ensure that DescendantsTracker does not leak memory on singleton classes

activesupport/test/descendants_tracker_without_autoloading_test.rb

@@ -4,4 +4,14 @@
 
 class DescendantsTrackerWithoutAutoloadingTest < ActiveSupport::TestCase
   include DescendantsTrackerTestCases
+
+  def test_clear_without_autoloaded_singleton_parent
+    mark_as_autoloaded do
+      parent_instance = Parent.new
+      parent_instance.singleton_class.descendants   #adds singleton class in @@direct_descendants
+      ActiveSupport::DescendantsTracker.clear   #clear is supposed to remove singleton class keys so GC can remove them.
+      assert !ActiveSupport::DescendantsTracker.class_variable_get(:@@direct_descendants).keys.include?(parent_instance.singleton_class)
+    end
+  end
+
 end