Issue #15720 has been updated by jeremyevans0 (Jeremy Evans).

Assignee set to ko1 (Koichi Sasada)
Status changed from Open to Assigned
File sse-unused-refinement.patch added

I can confirm this issue.  I believe it stems from the fact that in `vm_call_method_each_type`, `cc->me` is getting overwritten by `ref_me` even if they already have the same `def` but a different `defined_class`, which results in infinite recursion.  The attached patch should fix your example, but my understanding of this code is limited.  ko1 or shugo should probably review this patch and determine if it is the proper way to fix this issue.  This could possibly be related to #14068.

----------------------------------------
Bug #15720: SystemStackError when referencing a refinement in a module that isn't used
https://bugs.ruby-lang.org/issues/15720#change-78424

* Author: chocolateboy (Chocolate Boy)
* Status: Assigned
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Target version: 
* ruby -v: ruby 2.7.0dev (2019-03-21 trunk 67332) [x86_64-linux]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
```ruby
module M1
  refine Kernel do
    def foo
      'foo called!'
    end
  end
end

module M2
  refine Kernel do
    def bar
      'bar called!'
    end
  end
end

using M1

puts foo
puts bar
```

Expected:

```
foo called!
Traceback (most recent call last):
bug.rb:20:in `<main>': undefined local variable or method `bar' for main:Object (NameError)
```

Actual:

```
foo called!
Traceback (most recent call last):
bug.rb:20:in `<main>': stack level too deep (SystemStackError)
```

Platform: Linux (Arch)

ruby -v:

- ruby 2.7.0dev (2019-03-21 trunk 67332) [x86_64-linux]
- ruby 2.6.2p47 (2019-03-13 revision 67232) [x86_64-linux]
- ruby 2.5.5p157 (2019-03-15 revision 67260) [x86_64-linux]

---Files--------------------------------
bug.rb (188 Bytes)
sse-unused-refinement.patch (1.85 KB)


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