[ruby-core:124412] [Ruby Bug#21817] Ruby::Box crashes with refinement + bind_call + Symbol#to_proc combination

31 Dec 2025

      Issue #21817 has been reported by ahogappa (sho hirano).

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Bug #21817: Ruby::Box crashes with refinement + bind_call + Symbol#to_proc combination
https://bugs.ruby-lang.org/issues/21817

* Author: ahogappa (sho hirano)
* Status: Open
* ruby -v: ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [arm64-darwin25]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
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## Description
  When running with `RUBY_BOX=1`, a combination of refinement on Binding class, `bind_call`, and `Symbol#to_proc` causes a crash with the error: `[BUG] BUG: Local ep without cme/box, flags: 66660087`

  ## Reproduction Script

 ```ruby
  # Run with: RUBY_BOX=1 ruby repro.rb

  using Module.new {
    refine ::Binding do
      def eval_methods
        ::Kernel.instance_method(:methods).bind_call(receiver)
      end
    end
  }

  p binding.eval_methods.map(&:to_s)
```

  ## Expected Behavior

  The script should return an array of method names as strings.

  ## Actual Behavior

  Ruby crashes with `[BUG] BUG: Local ep without cme/box, flags: 66660087`

  ## Workaround

  Using block form instead of Symbol#to_proc does not crash:

  ```ruby
binding.eval_methods.map { it.to_s }  # This works
```

-- 
https://bugs.ruby-lang.org/