[ruby-core:120264] [Ruby master Bug#20956] Nested alternation pattern matching bug in 3.4.0-rc1

Issue #20956 has been updated by alanwu (Alan Wu).

Assignee set to prism

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Bug #20956: Nested alternation pattern matching bug in 3.4.0-rc1
https://bugs.ruby-lang.org/issues/20956#change-111031

* Author: nevans (Nicholas Evans)
* Status: Open
* Assignee: prism
* ruby -v: 3.4.0-rc1
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
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I think I've found a pattern matching bug in ruby 3.4.0-rc1.

```ruby
[123, ["foo"]] in [_, /\Afoo\b/i | [/\Afoo\z/i, *]]
```
I also tested this in ruby 2.7.8, with:
```ruby
case [123, ["foo"]]; in [_, /\Afoo\b/i | [/\Afoo\z/i, *]] then true; else false end
```

These return `true` in ruby 2.7.8, 3.0.7, 3.1.6, 3.2.6, and 3.3.6.
But they return false in 3.4.0-rc1.


I'm not sure if this is the best example, but it is a simplification of how I came across the bug.

Here are some other similar cases, which give the same result in 3.4.0-rc1 and 3.3.6:
```irb
irb(main):001> ["foo"] in /\Afoo\b/i | [/\Afoo\z/i, *]
=> true
irb(main):002> [123, ["foo"]] in [_, [/\Afoo\z/i, *]]
=> true
irb(main):003> [["foo"]] in [[/\Afoo\z/i, *]]
=> true
```

But this case exhibits the bug again:
```irb
irb(main):004> [["foo"]] in [/\Afoo\b/i | [/\Afoo\z/i, *]]
=> false
```



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