test: adding async optional chaining tests

[bcoe]

CC: @leobalter, @rkirsling

see: #2263

[bcoe]

@leobalter I've updated the tests, I'm not following this one production however:

  Promise.prototype.y = 43;
  var res = await Promise.reject(undefined)?.y;
  assert.sameValue(res, 43, 'await unwraps the evaluation of the whole optional chaining expression #2');

I believe this would result in an exception being thrown, not 43 being resolved; can we just drop it?

[leobalter]

@leobalter I've updated the tests, I'm not following this one production however:

  Promise.prototype.y = 43;
  var res = await Promise.reject(undefined)?.y;
  assert.sameValue(res, 43, 'await unwraps the evaluation of the whole optional chaining expression #2');

I believe this would result in an exception being thrown, not 43 being resolved; can we just drop it?

await does not unwrap the rejected promise, that's the tricky part. The optional chaining will evaluate the Promise.reject(undefined) to an instance of Promise, than it gets the y property. the await expression will never see the rejected promise there.

This might illustrate better the same resolution:

var prom = Promise.reject(undefined);
var res = await (prom?.y);

// or

var evaluation = prom?.y;
var res = await evaluation;

The test case is interesting as it verifies the order for runtime evaluation. We can add this in a follow up too.

[leobalter]

applying the mentioned "reject to chain" part in a follow up.

It's applied already. No need to follow up.

[bcoe]

@leobalter my concern was, at least in Node.js:

var res = await Promise.reject(undefined)?.y;

bubbles to an unhandled rejection and fails the test.

[leobalter]

that doesn't make the test incorrect, right? We can't block tests because a specific host has a non specific behavior, and I believe Node might have some flag to ignore non handled rejections. In this case, Node is even doing the correct evaluation but the warning is triggering a test failure in some test262 runner.

It's important to have these tests, and I believe this is not gonna be the first test here with this rejection swallowing.

[bcoe]

@leobalter I trust you 😄, was mainly concerned I was unable to validate in the test runner I had pulled together.