Parser rejects async arrow function in export default position

[Shine-neko]

export default async (x) => x; fails to parse, while Node.js and other engines accept it.

Reproduction

// repro.mjs
export default async (x) => x;

SyntaxError: expected token 'function', got '(' in async function declaration at line 1, col 22

All parenthesised and unparenthesised forms fail:

export default async () => 1;         // fails
export default async x => x;          // fails
export default async (a, b) => a + b; // fails

Closely related forms that parse fine

This narrows the problem to export default combined with an async arrow:

export default async function (x) { return x; }  // ok
const f = async (x) => x; export default f;      // ok
export const g = async (x) => x;                 // ok

Node accepts every failing form above (verified with node --input-type=module).

Spec

export default takes an AssignmentExpression (ExportDeclaration), and AsyncArrowFunction is reachable from it, so these should parse.

Version

main @ 5718cc81111ceff12d6054ab948146a45bc6e114

Found while running a real-world CLI whose command handlers are written as export default async (options) => { … }.