Re: [whatwg/webidl] ObservableArray's "set the length"'s behavior with empty backing lists (Issue #1049)

With my implementer hat on, I guess what's confusing is the (potential) signed/unsigned mixture in the algorithm (the current Blink implementation special-cases the empty [[BackingList]] case for this reason).

From the spec it's possible to know `uint32Len`'s type/range, but in `oldLen`'s case it's not that clear -- it represents a list size, which I thought would be unsigned, but if `indexToDelete` were to become -1 then I guess it's not the case. Also, if it's a mathematical number in the ECMAScript sense, isn't `indexToDelete >= uint32Len` applying an operator to mixed-type operands (mathematical value and Number, respectively), which is not allowed?

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