Editorial: FlattenIntoArray: add assertions

[ljharb]

Add some assertions to clarify the intended usage of this abstract operation.

I wanted to assert that target is an Array exotic object, but I think because of ArraySpeciesCreate I can't.

[jmdyck]

I wanted to assert that _target_ is an Array exotic object, but I think because of ArraySpeciesCreate I can't.

I think you're right. That is, the result of ArraySpeciesCreate (ignoring abrupts) isn't guaranteed to be an Array (despite what its preamble says), because a subclass of Array could override its @@species property to return a non-Array constructor?

[jmdyck]

When A.p.flat invokes this operation, the value passed to _depth_ can be any normal return from ToInteger, which includes negative integers and also some odd values like +/- zero and +/- infinity. The infinities fail the "integer" part of this assertion, and the negative integers fail the ≥ 0 part. Maybe change to:

Assert: _depth_ is an integer Number or *+∞* or *-∞*.

Alternatively, A.p.flat could at least map all negatives to (positive) zero. Then you could say:

Assert: _depth_ is either *+∞* or an integer Number ≥ 0.

(Tangent: I was going to complain about _depth_ - 1 when _depth_ is +infinity, but I guess it became well-defined with the merge of PR #1135 ("an operation with no subscript is interpreted to be a Number operation"). However, it might be worth a Note.)

[michaelficarra]

Alternatively, A.p.flat could at least map all negatives to (positive) zero.

I like this suggestion a lot.

[jmdyck]

Looks okay to me.

per #1620 (review)