Await nullish async disposable

[JLHwung]

Q	A
Fixed Issues?	When an async-disposable is nullish, Babel does not await its disposing process (REPL)
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

In the REPL example,

(async function () {
  const log = [];

  function disposable(name) {
    return {
      async [Symbol.dispose || Symbol.for("Symbol.dispose")]() {
        log.push(name);
      },
    };
  }

  async function f() {
    using y = disposable("y");
    await using x = null;
    log.push("f body");
  }

  const promise = f();

  log.push("body");

  await promise;

  console.log(log)
})()

It should log "f body", "body", "y", but currently it logs "f body", "y", "body".

Per spec 7.5.7 Dispose, if hint is async-dispose, the result should be awaited, even if it is nullish. Therefore the disposing of the next resource y should follow log.push("body").

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/56669

[nicolo-ribaudo]

Can you explicitly set d to undefined, given that we read it later?

[JLHwung]

Or set it to false to save a few bytes?

[liuxingbaoyu]

We could probably use value here.

[nicolo-ribaudo]

Anything works for me as long as it's something :)

[JLHwung]

Aside: currently await using x = document.all will go to this branch, we probably should reject it. I will check the spec and open a new PR to fix that.

[nicolo-ribaudo]

There will be some changes related to this in the proposal: tc39/proposal-explicit-resource-management#219

We will have a conclusion at the end of the week.