Parse a switch guard that calls a function named async

changes/async-guard-lookahead.fix.md

@@ -0,0 +1 @@
+- Parse a switch guard that calls a function named `async`.

compiler/syntax/src/res_core.ml

@@ -2321,11 +2321,14 @@ and parse_operand_expr ~context p =
     (* we need to be careful when we're in a ternary true branch:
        `condition ? ternary-true-branch : false-branch`
        Arrow expressions could be of the form: `async (): int => stuff()`
-       But if we're in a ternary, the `:` of the ternary takes precedence
+       But if we're in a ternary, the `:` of the ternary takes precedence.
+       Like other arrows, none starts in a guard or a condition, where
+       `async(x) =>` is a call followed by the case's `=>`.
     *)
-      when is_es6_arrow_expression
-             ~in_ternary:(context = TernaryTrueBranchExpr)
-             p ->
+      when context != WhenExpr
+           && is_es6_arrow_expression
+                ~in_ternary:(context = TernaryTrueBranchExpr)
+                p ->
       let arrow_attrs = !attrs in
       let () = attrs := [] in
       parse_async_arrow_expression ~arrow_attrs p

tests/syntax_tests/data/parsing/grammar/expressions/async.res

@@ -42,3 +42,10 @@ let attr2 =  @a async (type a) => (type b c, x) => 3
 let attr3 =  @a  (type a) => async (type b c, x) => 3
 let attr4 =  @a  (type a) => @b async (type b c, x) => 3
 let attr5 : int =  @a @b async (type a, type b c) => (x:a) => x
+
+// A guard calling a function named async is a call, not an async arrow
+let guarded = v =>
+  switch v {
+  | Some(x) if async(x) => async () => x
+  | _ => async () => 0
+  }

tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt

@@ -36,4 +36,8 @@ let attr4 =
 let (attr5 : int) =
   ((async fun (type a) (type b) (type c) [arity:1]() ->
       fun [arity:1](x : a) -> x)
-  [@a ][@b ])
+  [@a ][@b ])
+let guarded [arity:1]v =
+  match v with
+  | Some x when async x -> (async fun [arity:1]() -> x)
+  | _ -> (async fun [arity:1]() -> 0)