[ruby-core:122220] [Ruby Bug#21357] Crash in Hash#merge! with ruby-dev in rubocop-rspec test suite

Issue #21357 has been updated by Earlopain (Earlopain _).


I reduced it down significantly:

```rb
require "yaml"

YML = <<~YML
  foo:
    - bar: abc
      baz: def
      bat: ghi
YML
x = 1500 # lower numbers don't consistently crash

(0..x).each_with_object({}) do |_i, hash|
  hash.merge!(YAML.safe_load(YML)) do |_key, first, second|
    first.concat(second)
  end
end
```

This doesn't have much in common with the real code anymore, but the crash is the same.

----------------------------------------
Bug #21357: Crash in Hash#merge! with ruby-dev in rubocop-rspec test suite
https://bugs.ruby-lang.org/issues/21357#change-113368

* Author: Earlopain (Earlopain _)
* Status: Open
* ruby -v: ruby 3.5.0dev (2025-05-20T09:22:41Z master a82e7132df) +PRISM [x86_64-linux]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
I've encountered crahses on ruby-head in recent days, related to hash methods like `merge` and `merge!`. I can now reproduce it locally while running the rubocop-rspec test suite:

* Clone https://github.com/rubocop/rubocop-rspec
* Run `NO_COVERAGE=1 bundle exec rake spec`

I attached the crash report. I want to reduce this down but have no idea how. The crash is very consistent for me though, it doesn't take very long for it to happen (only a few seconds usually).

There were related(?) changes to hash like https://bugs.ruby-lang.org/issues/21333, maybe related.

---Files--------------------------------
test.log (224 KB)


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