Don't copy RUBY_FL_PROMOTED flag in rb_obj_setup

RUBY_FL_PROMOTED is used by the garbage collector to track when an
object becomes promoted to the old generation. rb_obj_setup must not
copy that flag over because then it may become out-of-sync with the age
of the object.

This fixes a bug in Method#clone where the cloned Method object may get
RUBY_FL_PROMOTED incorrectly set.

object.c

@@ -119,7 +119,8 @@ rb_obj_reveal(VALUE obj, VALUE klass)
 VALUE
 rb_obj_setup(VALUE obj, VALUE klass, VALUE type)
 {
-    RBASIC(obj)->flags = type;
+    VALUE ignored_flags = RUBY_FL_PROMOTED;
+    RBASIC(obj)->flags = (type & ~ignored_flags) | (RBASIC(obj)->flags & ignored_flags);
     RBASIC_SET_CLASS(obj, klass);
     return obj;
 }

test/ruby/test_method.rb

@@ -450,6 +450,17 @@ def m.bar; :bar; end
     assert_equal(:bar, m.clone.bar)
   end
 
+  def test_clone_under_gc_compact_stress
+    EnvUtil.under_gc_compact_stress do
+      o = Object.new
+      def o.foo; :foo; end
+      m = o.method(:foo)
+      def m.bar; :bar; end
+      assert_equal(:foo, m.clone.call)
+      assert_equal(:bar, m.clone.bar)
+    end
+  end
+
   def test_inspect
     o = Object.new
     def o.foo; end; line_no = __LINE__