fix(parser): reject TypeScript default exports in JavaScript (#28440)

Restrict default-exported interfaces and abstract classes to TypeScript
sources.

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

AI-assisted (Codex).

crates/oxc_parser/src/js/module.rs

@@ -753,7 +753,7 @@ impl<'a, C: Config> ParserImpl<'a, C> {
             let next = self.lexer.peek_token();
             if !next.is_on_new_line() {
                 // export default abstract class C {}
-                if kind == Kind::Abstract && next.kind() == Kind::Class {
+                if self.is_ts && kind == Kind::Abstract && next.kind() == Kind::Class {
                     self.bump_any();
                     let modifiers = Modifiers::new_single(ModifierKind::Abstract, modifier_start);
                     return ExportDefaultDeclarationKind::ClassDeclaration(
@@ -778,7 +778,7 @@ impl<'a, C: Config> ParserImpl<'a, C> {
                     return ExportDefaultDeclarationKind::FunctionDeclaration(func);
                 }
                 // export default interface I {}
-                if kind == Kind::Interface {
+                if self.is_ts && kind == Kind::Interface {
                     self.bump_any();
                     for decorator in &decorators {
                         self.error(diagnostics::decorators_are_not_valid_here(decorator.span));

tasks/coverage/misc/fail/js-export-default-abstract-class.js

@@ -0,0 +1 @@
+export default abstract class C {}

tasks/coverage/misc/fail/js-export-default-interface.js

@@ -0,0 +1 @@
+export default interface I {}

tasks/coverage/snapshots/lexer_misc.snap

@@ -1,3 +1,3 @@
 lexer_misc Summary:
-AST Parsed     : 393/393 (100.00%)
-Positive Passed: 393/393 (100.00%)
+AST Parsed     : 395/395 (100.00%)
+Positive Passed: 395/395 (100.00%)

tasks/coverage/snapshots/parser_misc.snap

@@ -1,7 +1,7 @@
 parser_misc Summary:
 AST Parsed     : 130/130 (100.00%)
 Positive Passed: 130/130 (100.00%)
-Negative Passed: 265/265 (100.00%)
+Negative Passed: 267/267 (100.00%)
 
   × TS(1275): 'accessor' modifier can only appear on a property declaration.
    ╭─[misc/fail/accessor-modifier-abstract-method.ts:1:29]
@@ -1192,6 +1192,20 @@ Negative Passed: 265/265 (100.00%)
    ·                             ──────
    ╰────
 
+  × Expected a semicolon or an implicit semicolon after a statement, but found none
+   ╭─[misc/fail/js-export-default-abstract-class.js:1:24]
+ 1 │ export default abstract class C {}
+   ·                        ▲
+   ╰────
+  help: Try inserting a semicolon here
+
+  × Expected a semicolon or an implicit semicolon after a statement, but found none
+   ╭─[misc/fail/js-export-default-interface.js:1:25]
+ 1 │ export default interface I {}
+   ·                         ▲
+   ╰────
+  help: Try inserting a semicolon here
+
   × Adjacent JSX elements must be wrapped in an enclosing tag.
    ╭─[misc/fail/jsx-adjacent-elements.jsx:3:3]
  2 │   <div />