Bug #22190: Class#subclasses does not include clones of classes that include modules

• Status: Closed
• Assignee: jhawthorn (John Hawthorn)
• Target version: 4.0
• ruby -v: ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [x86_64-linux]
• Backport: 3.3: DONTNEED, 3.4: DONTNEED, 4.0: REQUIRED

[shugo (Shugo Maeda)]

Class#subclasses does not return clones of classes that include (or prepend) a module, while clones of classes without modules are returned. This is a regression first released in Ruby 4.0.4.

module M; end

class X; end

class C < X
  include M
end

class D < X; end

clone_c = C.clone
clone_d = D.clone

p clone_c.superclass              #=> X
p X.subclasses.include?(C)        #=> true
p X.subclasses.include?(clone_d)  #=> true  (D includes no module)
p X.subclasses.include?(clone_c)  #=> false (expected: true)

The same happens with prepend instead of include. Ruby 4.0.3 and earlier return true for all of the above.

Analysis

Since a2531ba293 ("Simplify subclasses list, remove from Box"), class_associate_super() maintains subclass lists only for direct T_CLASS -> T_CLASS links:

// class.c (master)
// Only maintain subclass lists for T_CLASS→T_CLASS relationships.
// Include/prepend inserts ICLASSes into the super chain, but T_CLASS
// subclass lists should track only the immutable T_CLASS→T_CLASS link.
if (RB_TYPE_P(klass, T_CLASS) && RB_TYPE_P(super, T_CLASS)) {

For normally created classes this is fine because the T_CLASS -> T_CLASS link is registered at class creation time, before any module is included. However, rb_mod_init_copy() builds the clone's superclass chain differently:

• When the original has no origin, the clone shares the original's ICLASS chain: rb_class_set_super(clone, RCLASS_SUPER(orig)) (class.c:983), where RCLASS_SUPER(orig) is a T_ICLASS. The condition above is false, so the clone is never added to the real superclass's subclass list.
• When the original has an origin (prepend), the tail is connected via rb_class_set_super(clone_origin, RCLASS_SUPER(orig_origin)) (class.c:1031), where clone_origin is a T_ICLASS, so this link is not registered either.

Before a2531ba293, subclass entries were reparented onto ICLASSes and the Class#subclasses walk descended through non-T_CLASS entries (see class_descendants_recursive in 4.0.3), so a clone registered under the shared ICLASS was still found from the superclass.

Method lookup and method cache invalidation do not seem to be affected (redefining a method on the superclass after cloning is observed correctly through the clone); the visible breakage is in the reflection API. Other users of the subclass lists may want checking, though (for example the allocator propagation added in 111215de27 also iterates subclass lists).

[shugo (Shugo Maeda)]

• Assignee set to jhawthorn (John Hawthorn)
• Backport changed from 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN to 3.3: DONTNEED, 3.4: DONTNEED, 4.0: REQUIRED

I've created a pull request: https://github.com/ruby/ruby/pull/17799/changes

[shugo (Shugo Maeda)]

• Status changed from Open to Closed

Merged: https://github.com/ruby/ruby/pull/17799/changes