From: "kddnewton (Kevin Newton) via ruby-core" <ruby-core@...>
Date: 2025-09-17T16:45:01+00:00
Subject: [ruby-core:123283] [Ruby Feature#21572] Make illegal variable in alternation pattern a syntax error

Issue #21572 has been updated by kddnewton (Kevin Newton).


I agree, but there's precedence here of some things only being detectable at run-time, like illegal `break` usage. I just want to make sure this change goes through the proper channels to make sure everyone is onboard.

----------------------------------------
Feature #21572: Make illegal variable in alternation pattern a syntax error
https://bugs.ruby-lang.org/issues/21572#change-114654

* Author: kddnewton (Kevin Newton)
* Status: Open
----------------------------------------
Currently when you have an illegal variable in an alternation pattern, you get a compile error, but no syntax error.

```ruby
case foo
in {a: } | Array
  "matched: #{a}"
end
```

yields

```
$ ruby -cv test.rb
ruby 3.5.0dev (2025-04-10T10:21:51Z master 756479324f) +PRISM [x86_64-linux]
Syntax OK
```

but

```
$ ruby test.rb
test.rb:2: illegal variable in alternative pattern (a)
test.rb: compile error (SyntaxError)
```

Looking at the compiler, since illegal jumps (like break, next, etc.) are now syntax errors, the only other actual compiler errors are related to builtins. I think this should be a proper syntax error, so that editors and other tools can report on it and users can find it in development sooner.



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