From: "kddnewton (Kevin Newton) via ruby-core" <ruby-core@...>
Date: 2026-06-01T19:51:00+00:00
Subject: [ruby-core:125602] [Ruby Bug#22089] Prism accepts statement modifiers in if/unless/elsif conditions that parse.y rejects

Issue #22089 has been updated by kddnewton (Kevin Newton).

Status changed from Open to Closed

Closed by 9e8ad9ac0489d8390bc11e5b18f4af065d60898c

----------------------------------------
Bug #22089: Prism accepts statement modifiers in if/unless/elsif conditions that parse.y rejects
https://bugs.ruby-lang.org/issues/22089#change-117449

* Author: Anonymous
* Status: Closed
* ruby -v: ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [arm64-darwin25]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------

The following is accepted by Prism but rejected by parse.y:

```
$ ruby -cve "if a if b then end"
ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [arm64-darwin25]
Syntax OK

$ ruby --parser=parse.y -cve "if a if b then end"
ruby 4.0.0 (2025-12-25 revision 553f1675f3) [arm64-darwin25]
-e:1: syntax error, unexpected 'if' modifier, expecting 'then' or '\n' or ';'
if a if b then end
     ^~
ruby: compile error (SyntaxError)
```

The same applies to other statement modifiers (`unless`/`while`/`until`) and to `unless`/`elsif` conditions. For example, all of the following are accepted by Prism but rejected by parse.y:

```ruby
if a if b then end
if a unless b then end
if a while b then end
if a until b then end
unless a until b then end
if x then elsif a if b then end
```

I can also reproduce this on Prism's main branch (1.9.0).

Prism already treats the same syntax as a syntax error in `while`/`until` conditions, so it looks like `if`/`unless`/`elsif` conditions should reject it as well, matching both parse.y and Prism's own handling of `while`/`until` conditions.

I have a draft patch here: https://github.com/ruby/prism/pull/4120



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