[cknitt]

Follow-up to #8935, addressing its Codex review comment that came in after the merge.

#8935 skipped the async arrow lookahead in every WhenExpr position, but WhenExpr is also used for the switch scrutinee and the try body, which can be of any type. So

let scrutinee = switch async () => 1 {
| _ => 0
}

stopped parsing ("Did you forget a { here?"). Only a guard is followed by the case's own =>, so guards now get their own parsing context, GuardExpr:

• In guards, both async and plain arrows are skipped, so | Some(x) if async(x) => … and | Some(x) if (x) => … stay guards.
• In the other WhenExpr positions, only plain arrows are skipped, as before Parse a switch guard that calls a function named async #8935.

No changelog fragment: #8935's fragment is not released yet, and this only narrows that unreleased change.

Test: parsing/grammar/expressions/async.res gets an async arrow as switch scrutinee and a parenthesized guard.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Fc7LEuyEfynjaKCVEBPmTF

[github-actions]

Semantic diff: https://cdn.rescript-lang.org/semantic-diff/pr-8941/index.html?v=f8805a4bf

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.14%. Comparing base (38ba55a) to head (f8805a4).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8941   +/-   ##
=======================================
  Coverage   81.14%   81.14%           
=======================================
  Files         456      456           
  Lines       63760    63773   +13     
=======================================
+ Hits        51738    51751   +13     
  Misses      12022    12022           

Files with missing lines	Coverage Δ
compiler/syntax/src/res_core.ml	91.99% <100.00%> (+<0.01%)	⬆️

... and 1 file with indirect coverage changes

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commit: f8805a4

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8941

[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-21T12:54:01.155701Z	f8805a4	Manual request

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Nice work!

Reviewed commit: f8805a4bfc

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