From f3c77bd480834f2835fe6fef5c0475336248dbde Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@ruby-lang.org>
Date: Mon, 17 Jan 2022 01:56:31 +0900
Subject: [PATCH] Fix the placeholder subclass entry skipping [Bug #18489]

---
 class.c                  | 3 +--
 test/ruby/test_module.rb | 9 +++++++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/class.c b/class.c
index 9739aadc73..280ca8e97d 100644
--- a/class.c
+++ b/class.c
@@ -1054,8 +1054,7 @@ rb_include_module(VALUE klass, VALUE module)
     if (RB_TYPE_P(klass, T_MODULE)) {
         rb_subclass_entry_t *iclass = RCLASS_SUBCLASSES(klass);
         // skip the placeholder subclass entry at the head of the list
-        if (iclass && iclass->next) {
-            RUBY_ASSERT(!iclass->klass);
+        if (iclass && !iclass->klass) {
             iclass = iclass->next;
         }
 
diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index 0a6959c5e6..e744121718 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -475,6 +475,15 @@ class TestModule < Test::Unit::TestCase
     assert_not_include(mod.ancestor_list, BasicObject)
   end
 
+  def test_module_collected_extended_object
+    m1 = labeled_module("m1")
+    m2 = labeled_module("m2")
+    Object.new.extend(m1)
+    GC.start
+    m1.include(m2)
+    assert_equal([m1, m2], m1.ancestors)
+  end
+
   def test_dup
     OtherSetup.call