Using ... as a concrete type conforming to protocol ... is not supported

This is great, thanks, way better than how I “fixed” it … by inlining my desired implementation of `indexOf` wherever I needed it.

I wonder if anyone knows whether there are any plans in Swift to provide any better solutions to the heterogeneous collection problem, whether by having protocols conform to themselves, or by some other means…or maybe this is a question better asked on swift-evolution?