Aw :( the other PR was a lot simpler, and people seemed enthusiastic about it in committee. Can you say more about why you went with this more complex design?

Hey @bakkot !

In terms of more complexity. Compared to #357, the difference is:

• a [[RecordData]] slot is used for storing the original primitive
• Object::toString checks for this slot
• Record.recordValue checks for this slot
• ToPrimitive checks for this slot

Comparing this to existing primitive types, e.g. Boolean

• a [[BooleanData]] slot is used for storing the original primitive
• Object::toString checks for this slot
• Boolean.prototype.valueOf checks for this slot

So the only major difference between record objects and boolean objects is the one line in ToPrimitive to address the lack of valueOf, toString or Symbol.toPrimitive methods on records. For implementers this could amount to the equivalent of an internal private WeakSet to 'tag' record objects and checking for this 'tag' in their ToPrimitive implementation.

In terms of why this PR was merged. Primarily because it was editorial while still benefiting from the spec simplification demonstrated by #357. Secondarily, because @ljharb was keen for us to retain the implicit type coercion when people use + for string concatenation of record objects. Thirdly, because we have discovered another way to still provide a brand-check for this slot (the core motivation for removing the slot was due to the difficulty in also providing a brand-check for it without compromising other APIs).

To my mind the major difference between this PR and #357 is that this PR includes an additional kind of object, "boxed records", which are observably different from normal objects.

I don't think it's worth having an additional kind of object just to make + for string concatenation of boxed record objects not throw. More generally I don't think it's worth any amount of additional complexity to make boxed record objects special, given how rarely they should come up in real code. So I would much prefer to go with #357 instead, and to remove Record[@@hasInstance].