[ruby-core:104791] [Ruby master Bug#17806] Bad interaction between method cache, prepend, and refinements

Issue #17806 has been updated by alanwu (Alan Wu).


Hi @mk, from the description you gave, it's possible that you are running into #17725.
I would try running these two lines before requiring the test file that contains the | call:

```
iseq = RubyVM::InstructionSequence
iseq.compile_option = iseq.compile_option.merge(specialized_instruction: false)
```

If that fixes the issue I would try to run the test unmodified with a build of the master branch.

If the issue still exists on the master branch, I would try creating a reduced reproducer by logging `::Integer.ancestors` before each | call and refinement creation, assuming it's an issue related to lookup caching. 



----------------------------------------
Bug #17806: Bad interaction between method cache, prepend, and refinements
https://bugs.ruby-lang.org/issues/17806#change-93126

* Author: alanwu (Alan Wu)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [x86_64-darwin19]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: DONE
----------------------------------------
I'm running into a couple of issues with Ruby 3's new method cache and
refinements.

The first script raises `SystemStackError` unexpectedly:

```ruby
module R1
  refine Hash do
    def foo; :r1; end
  end
end
class Hash
  prepend(Module.new)
end
class Hash
  def foo; end
end
{}.method(:foo) # put it on pCMC
module R2
  refine Hash do
    def foo; :r2; end
  end
end
{}.foo # SystemStackError
```

The second script calls the wrong method:

```ruby
klass = Class.new { def foo; end }
_refinement = Module.new do
  refine(klass) { def foo; :refined; end }
end
klass.prepend(Module.new)
klass.new.foo # cache foo
klass.define_method(:foo) { :second }
p klass.new.foo # prints nil. False caching.
```

I submitted a GitHub PR to fix the issue: https://github.com/ruby/ruby/pull/4386



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