[ruby-core:123241] [Ruby Feature#21572] Make illegal variable in alternation pattern a syntax error

["kddnewton (Kevin Newton) via ruby-core" <ruby-core@ml.ruby-lang.org>]

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

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

* 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.



-- 
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