https://git.ruby-lang.org/ruby.git/commit/?id=9e0a91d064

From 9e0a91d0640600f2dfd7fc1d5fae6667019c9ca5 Mon Sep 17 00:00:00 2001
From: Jeremy Evans <code@j...>
Date: Tue, 4 Jan 2022 09:13:19 -0800
Subject: Don't segfault if Warning.warn is undefined

Check that there is a method entry for the method before passing
it to rb_method_entry_arity.

Fixes [Bug #18458]
---
 error.c                     | 3 ++-
 test/ruby/test_exception.rb | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/error.c b/error.c
index 6a60919d5c7..c7c5159927f 100644
--- a/error.c
+++ b/error.c
@@ -317,7 +317,8 @@ rb_warning_warn(VALUE mod, VALUE str) https://github.com/ruby/ruby/blob/trunk/error.c#L317
 static int
 rb_warning_warn_arity(void)
 {
-    return rb_method_entry_arity(rb_method_entry(rb_singleton_class(rb_mWarning), id_warn));
+    const rb_method_entry_t *me = rb_method_entry(rb_singleton_class(rb_mWarning), id_warn);
+    return me ? rb_method_entry_arity(me) : 1;
 }
 
 static VALUE
diff --git a/test/ruby/test_exception.rb b/test/ruby/test_exception.rb
index 2985f75fd60..3be9b9aee0e 100644
--- a/test/ruby/test_exception.rb
+++ b/test/ruby/test_exception.rb
@@ -1192,6 +1192,14 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status| https://github.com/ruby/ruby/blob/trunk/test/ruby/test_exception.rb#L1192
     assert_empty warning
   end
 
+  def test_undef_Warning_warn
+    assert_separately([], "#{<<-"begin;"}\n#{<<-"end;"}")
+    begin;
+      Warning.undef_method(:warn)
+      assert_raise(NoMethodError) { warn "" }
+    end;
+  end
+
   def test_undefined_backtrace
     assert_separately([], "#{<<-"begin;"}\n#{<<-"end;"}")
     begin;
-- 
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