Don't panic when export default interface starts an expression or labeled statement

src/js_parser/parse/parse_stmt.rs

@@ -1052,7 +1052,26 @@ impl<'a, const TYPESCRIPT: bool, const SCAN_ONLY: bool> P<'a, TYPESCRIPT, SCAN_O
                                     };
                                 }
                             }
-                            _ => {}
+                            // "interface" turned out not to start an interface
+                            // declaration: the nested statement came back as an
+                            // expression statement ("export default interface = 2",
+                            // "export default interface => 1") or a labeled statement
+                            // ("export default interface: 0"). None of these can be a
+                            // default export value, so report a syntax error instead of
+                            // building an S.ExportDefault that the visit and print
+                            // passes don't support.
+                            _ => {
+                                let r = js_lexer::range_of_identifier(p.source, stmt.loc);
+                                p.log().add_range_error_fmt(
+                                    Some(p.source),
+                                    r,
+                                    format_args!(
+                                        "Unexpected \"{}\"",
+                                        bstr::BStr::new(p.source.text_for_range(r))
+                                    ),
+                                );
+                                return Err(err!("SyntaxError"));
+                            }
                         }
 
                         p.create_default_name(default_loc).expect("unreachable")

test/bundler/transpiler/transpiler.test.js

@@ -209,6 +209,33 @@ describe("Bun.Transpiler", () => {
       err("module = (t) => 0 Foo { () => () => 0 }", 'Expected ";" but found "Foo"');
     });
 
+    it("export default interface that is not an interface declaration does not crash", () => {
+      const exp = ts.expectPrinted_;
+      const err = ts.expectParseError;
+      const unexpected = 'Unexpected "interface"';
+
+      // "interface" turns out to start an expression or a labeled statement, not an
+      // interface declaration. None of these can be a default export value.
+      err("export default interface=2", unexpected);
+      err("export default interface + 1", unexpected);
+      err("export default interface.foo()", unexpected);
+      err("export default interface => 1", unexpected);
+      err("export default interface: 2", unexpected);
+
+      // The exact fuzz repro: tsx loader, no trailing newline.
+      expect(() => transpiler.transformSync("export default interface=2")).toThrow(unexpected);
+
+      // Same shapes through the plain JavaScript loader must not crash either.
+      const js = new Bun.Transpiler({ loader: "js" });
+      expect(() => js.transformSync("export default interface=2")).toThrow(unexpected);
+      expect(() => js.transformSync("export default interface => 1")).toThrow(unexpected);
+      expect(() => js.transformSync("export default interface: 2")).toThrow(unexpected);
+
+      // Real interface declarations still parse and get erased.
+      exp("export default interface Foo {}", "");
+      exp("export default interface Foo { bar(): void }\nexport const x = 1;", "export const x = 1;\n");
+    });
+
     it("should parse empty type parameters", () => {
       const exp = ts.expectPrinted_;
       const err = ts.expectParseError;