Use of [[value]] internal slot on ECMAScript language type.

[ursi]

25.6.1.3.2 step 9 is

Let then be Get(resolution, "then")

Get returns an ECMAScript language type, not a Property Attribute. However, the algorithm goes on to use then.[[value]]. As far as I can tell, ECMAScript language types don't have a [[value]] internal slot.

[devsnek]

Get returns a Completion, which does have [[Value]].

[ursi]

uhg, the ? of course. I feel like I've trained myself to just filter those out. My bad.

Edit: Wait no. I don't think that's what does it. I'm just gonna read more.

[jmdyck]

No, you're right, the ? (on step 3 of Get) indicates that Get can return an abrupt completion. The Promise Resolve Function's step 10 checks to see if it did, and if so, 10.a extracts that abrupt completion's [[Value]] field.

[ursi]

@jmdyck the real culprit was 5.2.3(.1). I was unaware that Return implicitly wrapped its value in NormalCompletion.

[michaelficarra]

@ursi Yes, section 5.2.3.1, Implicit Completion Values, has been the source of much spec reader pain. Follow along at #497, #1572, #253, and others.