From: "k0kubun (Takashi Kokubun) via ruby-core" <ruby-core@...>
Date: 2026-09-14T20:16:55+00:00
Subject: [ruby-core:126714] [Ruby Bug#22257] Prepending a module to an already-included module leaves stale super caches

Issue #22257 has been updated by k0kubun (Takashi Kokubun).

Backport changed from 3.3: WONTFIX, 3.4: REQUIRED, 4.0: REQUIRED to 3.3: WONTFIX, 3.4: REQUIRED, 4.0: DONE

ruby_4_0 commit:06611602a958ebe1126be3a5cb151f3c2c8bffcc.

----------------------------------------
Bug #22257: Prepending a module to an already-included module leaves stale super caches
https://bugs.ruby-lang.org/issues/22257#change-119012

* Author: eightbitraptor (Matt V-H)
* Status: Closed
* Target version: 4.1
* Backport: 3.3: WONTFIX, 3.4: REQUIRED, 4.0: DONE
----------------------------------------
When a module is prepended to a module that already has includers, super call sites inside the prepended module keep calling the old method entry after the method is redefined.

[**Fix implemented in this PR**](https://github.com/ruby/ruby/pull/18421)

<pre>
module M; def foo; :m; end; end
class D; include M; end
M.prepend(Module.new { def foo; super; end })
D.new.foo                          # prime the super call-site cache
M.send(:define_method, :foo) { :hooked }
p D.new.foo
</pre>

I found this while working on an unrelated Ractor issue. But this problem manifests itself for Ractors as follows:

<pre>
Ractor.new {}
require 'set'
Kernel.send(:define_method, :require) { |f| $hook = f }
require 'set'
p $hook # => nil, the hook never runs
</pre>


This is because creating the first Ractor prepends an internal `RactorRequire` wrapper onto Kernel. After the first require primes the cache in the wrapper, redefining Kernel#require silently does nothing.

The linked PR fixes this by registering the backfilled iclass in the module's subclasses list, after the includer walk finishes. 



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