[ruby-core:125398] [Ruby Bug#22022] Refinement zsuper problems when referenced method changes

4 May 2026

      Issue #22022 has been reported by jeremyevans0 (Jeremy Evans).

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Bug #22022: Refinement zsuper problems when referenced method changes
https://bugs.ruby-lang.org/issues/22022

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* ruby -v: ruby 4.1.0dev (2026-05-03T23:57:21Z master d8d2ed5dc9) +PRISM [x86_64-openbsd7.8]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
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I've found many problems with refinement zsuper methods. Here's one example:

```ruby
module M
  private def a = :a
  alias a a
end

class A
  include M
end

class B < A
end

module R
  refine B do
    public :a
  end
end
using R
B.new.a
# false() function is unimplemented on this machine (NotImplementedError)
```

Here's another:

```ruby
class A
  private def a = :a
end

module R
  refine A do
    public :a
  end
end
using R
A.new.a
# => :a

module M
  def a = :b
end
A.prepend M
A.new.a
# SystemStackError: stack level too deep
```

The problematic cases are in general can be broken down into:

* Method being redefined (in same or closer ancestor)
* Method being removed
* Method being undef-ed (in same or closer ancestor)
* Method being overridden by method in included module
* Method being overridden by method in prepended module
* Method being defined in an included/prepended module at time of refinement

In addition to the `NotImplementedError` and `SystemStackError`, there are other cases where incorrect results are given.

One possible approach for fixing this is to turn the zsuper methods into regular methods that call super . This fixes all cases that I found. This is not a perfect solution, due to mildly worse performance (I'm guessing) as well as arity/parameters for the method not as helpful.

It may possible to avoid these issues by clearing method caches in more cases. However, I think that would require a lot of extra work, since you cannot just clear the method cache for the current class. You would need to clear it for all subclasses, if this is a module, do the same for all classes that include/prepend the module, as well as any subclasses of those classes.

Considering the need for refinement zsuper methods is very rare, the performance and arity/parameters issues seem acceptable (to me).

Pull request: https://github.com/ruby/ruby/pull/16844

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