"it is not possible for a single platform object to implement both interfaces" about webidl

[bzbarsky]

The point here is that if two types are distinguishable, then given any object you have to be able to tell which of the two types it is.

This means that if there is an object which can claim to be of both types, they need to not be distinguishable. Therefore interpretation A is the correct one and we should adjust phrasing as needed to make that clear.

To determine if A and B are distinguishable, you have to look at all interfaces in the web platform

Yes, you do. That's necessary to achieve the "given any object" goal. I agree that this is rather annoying, but I don't see how the goal can be achieved otherwise.

And, is this actually implementable, or implemented by existing binding layers?

What Gecko's binding layer implements is the following:

Each interface has a set of interfaces associated with it. This set contains the interface itself, all interfaces inheriting from it (transitively, not directly), and all interfaces that have it as a "consequential interface" or inherit from an interface that has it as a consequential interface. So in your example above, the set associated with Baz just contains Baz itself. The set associated with Foo contains Foo and Baz. The set associated with Bar contains Bar and Baz. Two interfaces are distinguishable if their associated sets are disjoint (have no common members). I believe this is a correct implementation of your interpretation A.

[jyasskin]

Would it make sense to replace "It is not possible for a single platform object to implement both interfaces" with "No platform object implements both interfaces"? The non-local nature of the check is annoying, but at least this makes it clear.

[bzbarsky]

I think once we have proper mixins this might be a lot easier to spec, especially if we don't allow mixins to be used as types. Because at that point, the only way for two interfaces to not be distinguishable will be if one of them inherits from the other, I believe.