Re: [whatwg/webidl] ObservableArray types in namespace (#1000)

> > prevents OAs from being nullable
> 
> A nullable OA meant you’d hit undefined behavior in the the getter/setter steps for OA attributes (which would seemingly need to do something novel with the backing list?).

The attribute setter steps covers the new value, V, to `sequence<T>`, see step 4.5.10.1 of https://webidl.spec.whatwg.org/#dfn-attribute-setter. If a `null` is passed, https://webidl.spec.whatwg.org/#es-sequence throws a `TypeError`.

> 
> > or put into another OA
> 
> It seems like there’s a whole lot that prevents OA<OA> from being coherent currently. Instances are 1:1 with platform objects, allegedly created during [define the attributes](https://webidl.spec.whatwg.org/#define-the-attributes) (but see below*). The getter/setter behaviors of the attributes depend on the backing list / “backing observable array exotic object” (@_@) associations and that accessor behavior would seemingly need to become the outer OA’s own behavior for index-key properties.

Yeah, BOAEO is created 1:1 with OA attribute, what should accessing the inner OA of OA<OA> behave is undefined. 



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