[Bug]: incorrectly rejects `await` when following a function with an arrow in default parameters

💻

• Would you like to work on a fix?

How are you using Babel?

Programmatic API (babel.transform, babel.parse)

Input code

(async function () {
  function f(_=()=>null) {}
  await null;
});

or

(function* () {
  function f(_=()=>null) {}
  yield;
});

Configuration file name

package.json

Configuration

No response

Current and expected behavior

These are both valid programs but are rejected with

SyntaxError: ‘await’ is not allowed in async function parameters.

and

Yield expression is not allowed in formal parameters.

respectively

Environment

This is with @babel/parser: 7.15.8

Possible solution

No response

Additional context

I found this with a fuzzer, not in real code, so prioritize accordingly. That said, these do actually seem more plausible than many fuzzer-derived bugs.

[The-x-Theorist]

@azeez1776 I’m currently working on this give me some time please, If I won’t be able to take it forward I’ll let you know.

[JLHwung]

@The-x-Theorist If it is the first time you contribute to Babel, see Nicolò’s comment above on setting up the project.

This issue is on babel parser. If you are not familiar with the topic, which is absolutely okay, you can start by trying out https://astexplorer.net/ (Select JavaScript then @babel/parser) or reading tutorials about parsing and AST.

If you already had some experience on developing a parser, you can start by https://github.com/babel/babel/blob/fb7ddf4d389d3c9eff52567080f54995ba182031/packages/babel-parser/src/parser/expression.js#L2304 and scrutinize how ExpressionScope is handled when Babel is parsing arrow expression – because the OP’s error is thrown when an ExpressionScope is validated. For the definition of Expression Scope, see here.

For more questions, you can ask team members on Slack.