From 72d1a790cfe0e4a457db98c587f1acaa5e39f001 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@ruby-lang.org>
Date: Tue, 8 Aug 2023 19:03:38 +0900
Subject: [PATCH] [Bug #19833] Fix index underflow at superclasses of
 `BasicObject`

---
 object.c                | 4 ++++
 test/ruby/test_class.rb | 7 +++++++
 2 files changed, 11 insertions(+)

diff --git a/object.c b/object.c
index 823b8df2cf..45a571adac 100644
--- a/object.c
+++ b/object.c
@@ -2165,6 +2165,10 @@ rb_class_superclass(VALUE klass)
         if (klass == rb_cBasicObject) return Qnil;
         rb_raise(rb_eTypeError, "uninitialized class");
     }
+
+    if (!RCLASS_SUPERCLASS_DEPTH(klass)) {
+        return Qnil;
+    }
     else {
         super = RCLASS_SUPERCLASSES(klass)[RCLASS_SUPERCLASS_DEPTH(klass) - 1];
         RUBY_ASSERT(RB_TYPE_P(klass, T_CLASS));
diff --git a/test/ruby/test_class.rb b/test/ruby/test_class.rb
index 98acbaf67b..a8a019cee2 100644
--- a/test/ruby/test_class.rb
+++ b/test/ruby/test_class.rb
@@ -96,6 +96,13 @@ class TestClass < Test::Unit::TestCase
 
   def test_superclass_of_basicobject
     assert_equal(nil, BasicObject.superclass)
+
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      module Mod end
+      BasicObject.include(Mod)
+      assert_equal(nil, BasicObject.superclass)
+    end;
   end
 
   def test_module_function