Feature #21572: Make illegal variable in alternation pattern a syntax error

• Status: Open
• Assignee: -
• Target version: -

[kddnewton (Kevin Newton)]

Currently when you have an illegal variable in an alternation pattern, you get a compile error, but no syntax error.

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.

[Earlopain (Earlopain _)]

Thanks for opening this.

It seems just like a normal bug though, no? I understand that this doesn't deviate from what parse.y does but it is a syntax error and right now consumers of prism simply accept such code. I guess there are tests in ruby/ruby that very specifically assert against this somehow? Other than that, is seems like an implementation detail.

[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.

[ktsj (Kazuki Tsujimoto)]

I agree with you, @kddnewton (Kevin Newton).
Your proposal makes sense.

[kddnewton (Kevin Newton)]

Thank you @ktsj (Kazuki Tsujimoto) — I will start working on a patch for Prism.

[nobu (Nobuyoshi Nakada)]

https://github.com/ruby/ruby/pull/14923