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Parse a switch guard that calls a function named async - #8935
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Why:
The async arrow lookahead also ran in guards, where the case's `=>`
follows the guard, so `if async(x) =>` was read as an async arrow.
Async arrows now skip the lookahead in the positions where other arrows
already do: guards and conditions. Fixes #8933.
AST modified (switch guard):
Example:
let async = x => x > 0
let f = v =>
switch v {
| Some(x) if async(x) => 1
| _ => 0
}
Before (diagnostic):
Did you forget a `=>` here?
After (expanded):
let f [arity:1]v =
match v with
| Some x when async x -> 1
| _ -> 0
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fc7LEuyEfynjaKCVEBPmTF
Signed-off-by: Christoph Knittel <ck@cca.io>
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #8935 +/- ##
=======================================
Coverage 81.13% 81.13%
=======================================
Files 456 456
Lines 63754 63754
=======================================
Hits 51728 51728
Misses 12026 12026
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cknitt
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October 21, 2026 06:34
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cknitt
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Oct 21, 2026
Why:
#8935 suppressed the async arrow lookahead in every `WhenExpr`
position, so an async arrow as a switch scrutinee or try body no
longer parsed. Only guards are followed by the case's `=>`.
Guards now have their own parsing context, in which both async and
plain arrows are suppressed; other `WhenExpr` positions suppress plain
arrows only, as before #8935.
AST modified (switch scrutinee):
Example:
let scrutinee = switch async () => 1 {
| _ => 0
}
Before (diagnostic):
Did you forget a `{` here?
After (expanded):
let scrutinee = match async fun [arity:1]() -> 1 with | _ -> 0
Claude-Session: https://claude.ai/code/session_01Fc7LEuyEfynjaKCVEBPmTF
Signed-off-by: Christoph Knittel <ck@cca.io>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
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Fixes #8933.
To tell an async arrow (
async (x) => …) apart from a call (async(x)), the parser looks ahead for=>after the parentheses. In a switch guard, the case's own=>follows the guard, sowas taken as the start of an async arrow and failed with "Did you forget a
=>here?".Ordinary arrows already skip this lookahead in guards and conditions (the
WhenExprcontext); async arrows now do the same. An arrow is never valid in those positions, since guards and conditions arebool.This matters beyond
async: the generator RFC (#8934) proposesgenas a contextual prefix that works the same way, andgenis a common function name.Test:
parsing/grammar/expressions/async.resgets a guard that callsasync(x)and async arrows as case bodies.🤖 Generated with Claude Code
https://claude.ai/code/session_01Fc7LEuyEfynjaKCVEBPmTF