Interfaces with named getters inheriting from interfaces with named getters

[domenic]

The Web IDL spec says

If an object implements more than one interface that defines a given special operation, then it is undefined which (if any) special operation is invoked for that operation.

However we have on the platform

[Exposed=Window]
interface HTMLFormControlsCollection : HTMLCollection {
  // inherits length and item()
  getter (RadioNodeList or Element)? namedItem(DOMString name); // shadows inherited namedItem()
};

[Exposed=Window, LegacyUnenumerableNamedProperties]
interface HTMLCollection {
  readonly attribute unsigned long length;
  getter Element? item(unsigned long index);
  getter Element? namedItem(DOMString name);
};

Thus instances of HTMLFormControlsCollection implement two interface that both defined named getters, and we're in some kind of undefined situation.

We should presumably instead update the Web IDL spec to use the most-derived operation.

The normative text in e.g. LegacyPlatformObjectGetOwnProperty is pretty unclear right now, talking a lot about running steps but not specifying what interface to run them for.

[bzbarsky]

We should presumably instead update the Web IDL spec to use the most-derived operation.

I agree. That's what browsers implement in practice, I expect. Certainly it's what Gecko implements.