Re: [heycam/webidl] Figure-out what supporting Array-subclassing implies (#345)

> Presumably it would deopt into the same speed as any normal method call, yes? 

Normal DOM method call, as opposed to normal JS function call, right?  It .. depends.  I can only speak for SpiderMonkey at this level of specificity, and there is a bit more overhead here in SpiderMonkey than a normal DOM call, but not a lot more.

Note that a lot of things that are DOM calls are not normal DOM calls, in both SpiderMonkey and other engines, in a variety of ways.  But even the "slot path" vmcall really isn't _that_ slow.  As in, I'd need to see a use case to see whether the performance matters.  Chances are it does not.

Your description of what you consider reasonable semantics sounds _very_ much like what typed arrays do.

@domenic, @annevk thoughts on having a way for things with indexed getters/setters to opt into that sort of behavior?  Honestly, we could just have `iterable<V>` on a thing with indexed getters induce all that stuff in addition to the things it induces now; we'd need to check whether we can define these operations in a way that works for existing consumers.

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