fix(parser): recognize annotated empty arrows in conditionals (#26537)

Recognize bare `():` as a definite TypeScript arrow head so conditional parsing does not reject valid annotated empty-parameter arrows:

```ts
const f: number | ((x: number) => any) = a ? (b) : x => x ? c : (): any => b;
```

Keep `async():` ambiguous because `async()` can be a call on the true side of a conditional, as in `a ? async() : 0`.

crates/oxc_parser/src/js/arrow.rs

@@ -75,7 +75,8 @@ impl<'a, C: Config> ParserImpl<'a, C> {
     }
 
     fn is_parenthesized_arrow_function_expression_worker(&mut self) -> Tristate {
-        if self.eat(Kind::Async) {
+        let is_async = self.eat(Kind::Async);
+        if is_async {
             if self.cur_token().is_on_new_line() {
                 return Tristate::False;
             }
@@ -100,7 +101,11 @@ impl<'a, C: Config> ParserImpl<'a, C> {
                         self.bump_any();
                         let third = self.cur_kind();
                         match third {
-                            Kind::Colon if self.is_ts => Tristate::Maybe,
+                            // `async()` can be a call on the true side of a conditional.
+                            Kind::Colon if self.is_ts && is_async => Tristate::Maybe,
+                            // Bare `()` cannot be a parenthesized expression, so the colon
+                            // must start a return type, even inside a conditional expression.
+                            Kind::Colon if self.is_ts => Tristate::True,
                             Kind::Arrow | Kind::LCurly => Tristate::True,
                             _ => Tristate::False,
                         }

tasks/coverage/misc/pass/conditional-empty-arrow-return-type.ts

@@ -0,0 +1,12 @@
+const a = true, b = 1, c = 2;
+
+const original: number | ((x: number) => any) = a ? (b) : x => x ? c : (): any => b;
+const unparenthesized = a ? b : x => x ? c : (): any => b;
+const grouped = a ? (b) : (x => x ? c : (): any => b);
+const unannotated = a ? (b) : x => x ? c : () => b;
+const parameter = a ? (b) : x => x ? c : (y: number): any => b;
+
+function async(): number { return b; }
+const asyncCall = a ? async() : 0;
+const asyncCallWithArrowAlternate = a ? async() : x => x;
+const asyncArrow = a ? async (): Promise<number> => b : c;

tasks/coverage/snapshots/codegen_misc.snap

@@ -1,3 +1,3 @@
 codegen_misc Summary:
-AST Parsed     : 106/106 (100.00%)
-Positive Passed: 106/106 (100.00%)
+AST Parsed     : 107/107 (100.00%)
+Positive Passed: 107/107 (100.00%)

tasks/coverage/snapshots/formatter_misc.snap

@@ -1,3 +1,3 @@
 formatter_misc Summary:
-AST Parsed     : 106/106 (100.00%)
-Positive Passed: 106/106 (100.00%)
+AST Parsed     : 107/107 (100.00%)
+Positive Passed: 107/107 (100.00%)

tasks/coverage/snapshots/lexer_misc.snap

@@ -1,3 +1,3 @@
 lexer_misc Summary:
-AST Parsed     : 321/321 (100.00%)
-Positive Passed: 321/321 (100.00%)
+AST Parsed     : 322/322 (100.00%)
+Positive Passed: 322/322 (100.00%)

tasks/coverage/snapshots/parser_misc.snap

@@ -1,6 +1,6 @@
 parser_misc Summary:
-AST Parsed     : 106/106 (100.00%)
-Positive Passed: 106/106 (100.00%)
+AST Parsed     : 107/107 (100.00%)
+Positive Passed: 107/107 (100.00%)
 Negative Passed: 215/215 (100.00%)
 
   × Cannot assign to 'arguments' in strict mode

tasks/coverage/snapshots/parser_typescript.snap

@@ -27046,11 +27046,12 @@ Expect Syntax Error: tasks/coverage/typescript/tests/cases/conformance/types/wit
  4 │ }
    ╰────
 
-  × Empty parenthesized expression
-   ╭─[typescript/tests/cases/conformance/parser/ecmascript5/RegressionTests/parser509669.ts:2:9]
+  × Expected `=>` but found `{`
+   ╭─[typescript/tests/cases/conformance/parser/ecmascript5/RegressionTests/parser509669.ts:2:17]
  1 │ function foo():any {
  2 │  return ():void {};
-   ·         ──
+   ·                 ┬
+   ·                 ╰── `=>` expected
  3 │ }
    ╰────
 

tasks/coverage/snapshots/semantic_misc.snap

@@ -1,6 +1,6 @@
 semantic_misc Summary:
-AST Parsed     : 106/106 (100.00%)
-Positive Passed: 102/106 (96.23%)
+AST Parsed     : 107/107 (100.00%)
+Positive Passed: 103/107 (96.26%)
 semantic Error: tasks/coverage/misc/pass/declare-let-private.ts
 Bindings mismatch:
 after transform: ScopeId(0): ["private"]

tasks/coverage/snapshots/transformer_misc.snap

@@ -1,3 +1,3 @@
 transformer_misc Summary:
-AST Parsed     : 106/106 (100.00%)
-Positive Passed: 106/106 (100.00%)
+AST Parsed     : 107/107 (100.00%)
+Positive Passed: 107/107 (100.00%)