fix(parser): reject async modifiers on class fields (#26486)

`class C { async x; }` was accepted in JavaScript and TypeScript, silently discarding `async` from the resulting field. Reject the modifier after parsing a class field, once fields have been distinguished from methods.

Async methods, fields named `async`, and newline-separated fields remain valid.

crates/oxc_parser/src/js/class.rs

@@ -712,6 +712,14 @@ impl<'a, C: Config> ParserImpl<'a, C> {
             return self.fatal_error(error);
         }
 
+        // `async` is valid on methods, but not fields.
+        self.verify_modifiers(
+            modifiers,
+            ModifierKinds::all_except([ModifierKind::Async]),
+            false,
+            diagnostics::modifier_cannot_be_used_here,
+        );
+
         let r#abstract = modifiers.contains(ModifierKind::Abstract);
         let r#type = if r#abstract {
             PropertyDefinitionType::TSAbstractPropertyDefinition

tasks/coverage/misc/fail/async-field.js

@@ -0,0 +1 @@
+class C { async x; }

tasks/coverage/misc/fail/async-field.ts

@@ -0,0 +1 @@
+class C { async x; }

tasks/coverage/misc/pass/async-field-newline.js

@@ -0,0 +1,7 @@
+class C {
+  async
+  x;
+
+  static async
+  y;
+}

tasks/coverage/misc/pass/async-field-newline.ts

@@ -0,0 +1,7 @@
+class C {
+  async
+  x: number;
+
+  static async
+  y: number;
+}

tasks/coverage/snapshots/codegen_misc.snap

@@ -1,3 +1,3 @@
 codegen_misc Summary:
-AST Parsed     : 91/91 (100.00%)
-Positive Passed: 91/91 (100.00%)
+AST Parsed     : 93/93 (100.00%)
+Positive Passed: 93/93 (100.00%)

tasks/coverage/snapshots/formatter_misc.snap

@@ -1,3 +1,3 @@
 formatter_misc Summary:
-AST Parsed     : 91/91 (100.00%)
-Positive Passed: 91/91 (100.00%)
+AST Parsed     : 93/93 (100.00%)
+Positive Passed: 93/93 (100.00%)

tasks/coverage/snapshots/lexer_misc.snap

@@ -1,3 +1,3 @@
 lexer_misc Summary:
-AST Parsed     : 286/286 (100.00%)
-Positive Passed: 286/286 (100.00%)
+AST Parsed     : 290/290 (100.00%)
+Positive Passed: 290/290 (100.00%)

tasks/coverage/snapshots/parser_misc.snap

@@ -1,7 +1,7 @@
 parser_misc Summary:
-AST Parsed     : 91/91 (100.00%)
-Positive Passed: 91/91 (100.00%)
-Negative Passed: 195/195 (100.00%)
+AST Parsed     : 93/93 (100.00%)
+Positive Passed: 93/93 (100.00%)
+Negative Passed: 197/197 (100.00%)
 
   × Cannot assign to 'arguments' in strict mode
    ╭─[misc/fail/arguments-eval-ambient.ts:2:13]
@@ -190,6 +190,18 @@ Negative Passed: 195/195 (100.00%)
    ·                                         ─────
    ╰────
 
+  × 'async' modifier cannot be used here.
+   ╭─[misc/fail/async-field.js:1:11]
+ 1 │ class C { async x; }
+   ·           ─────
+   ╰────
+
+  × 'async' modifier cannot be used here.
+   ╭─[misc/fail/async-field.ts:1:11]
+ 1 │ class C { async x; }
+   ·           ─────
+   ╰────
+
   × `await` is only allowed within async functions and at the top levels of modules
    ╭─[misc/fail/auto-accessor-initializer-await.js:3:18]
  2 │   class C {

tasks/coverage/snapshots/semantic_misc.snap

@@ -1,6 +1,6 @@
 semantic_misc Summary:
-AST Parsed     : 91/91 (100.00%)
-Positive Passed: 87/91 (95.60%)
+AST Parsed     : 93/93 (100.00%)
+Positive Passed: 89/93 (95.70%)
 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     : 91/91 (100.00%)
-Positive Passed: 91/91 (100.00%)
+AST Parsed     : 93/93 (100.00%)
+Positive Passed: 93/93 (100.00%)