Issue #22089 has been reported by Impotency6272 (kai matsudate). ---------------------------------------- Bug #22089: Prism accepts statement modifiers in if/unless/elsif conditions that parse.y rejects https://bugs.ruby-lang.org/issues/22089 * Author: Impotency6272 (kai matsudate) * Status: Open * 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 -- https://bugs.ruby-lang.org/