fix(parser): reject optional class members in JavaScript (#28439)

Reject TypeScript optional class members in JavaScript with a diagnostic
on the question mark. The existing TypeScript compiler fixture covers
the rejection.

Validated with scoped Rust checks, full conformance, parser bindings,
allocation tracking.

AI-assisted (Codex).

crates/oxc_parser/src/diagnostics.rs

@@ -1186,6 +1186,11 @@ parser_diagnostics! {
             .with_label(span)
     };
 
+    optional_class_member_in_ts(span: Span) => {
+        OxcDiagnostic::error("Optional class members can only be used in TypeScript files.")
+            .with_label(span)
+    };
+
     parameter_modifiers_in_ts(modifier: Modifier, allowed: Option<ModifierKinds>) => {
         ts_error("8012", "Parameter modifiers can only be used in TypeScript files.")
             .with_label(modifier.span())

crates/oxc_parser/src/js/class.rs

@@ -629,6 +629,9 @@ impl<'a, C: Config> ParserImpl<'a, C> {
         let optional_span = (cur_token.kind() == Kind::Question).then(|| {
             let span = cur_token.span();
             self.bump_any();
+            if !self.is_ts {
+                self.error(diagnostics::optional_class_member_in_ts(span));
+            }
             span
         });
 

tasks/coverage/snapshots/parser_typescript.snap

@@ -3,7 +3,7 @@ commit: 8a62bbe7
 parser_typescript Summary:
 AST Parsed     : 9859/9868 (99.91%)
 Positive Passed: 9849/9868 (99.81%)
-Negative Passed: 1712/2807 (60.99%)
+Negative Passed: 1713/2807 (61.03%)
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/FunctionDeclaration3.ts
 
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/FunctionDeclaration4.ts
@@ -586,8 +586,6 @@ Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler
 
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/jsFileCompilationDuplicateFunctionImplementationFileOrderReversed.ts
 
-Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/jsFileCompilationOptionalClassElementSyntaxOfClass.ts
-
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/jsFunctionWithPrototypeNoErrorTruncationNoCrash.ts
 
 Expect Syntax Error: tasks/coverage/typescript/tsc/testdata/tests/cases/compiler/jsTypedefMergedWithModuleExportProperty.ts
@@ -9102,6 +9100,22 @@ Expect to Parse: tasks/coverage/typescript/tsc/testdata/tests/cases/conformance/
    ╰────
   help: Try inserting a semicolon here
 
+  × Optional class members can only be used in TypeScript files.
+   ╭─[typescript/tsc/testdata/tests/cases/compiler/jsFileCompilationOptionalClassElementSyntaxOfClass.ts:2:8]
+ 1 │ class C {
+ 2 │     foo?() {
+   ·        ─
+ 3 │     }
+   ╰────
+
+  × Optional class members can only be used in TypeScript files.
+   ╭─[typescript/tsc/testdata/tests/cases/compiler/jsFileCompilationOptionalClassElementSyntaxOfClass.ts:4:8]
+ 3 │     }
+ 4 │     bar? = 1; 
+   ·        ─
+ 5 │ }
+   ╰────
+
   × Expected `,` or `)` but found `?`
    ╭─[typescript/tsc/testdata/tests/cases/compiler/jsFileCompilationOptionalParameter.ts:1:13]
  1 │ function F(p?) { }