This doesn't seem to be about default implementations on protocols, I guess they do actually do virtual dispatch so you wouldn't hit this bug with default impls. The main issue is that we have an extension method on a type with the same name of a member of that type. So it can always be statically flagged whether or not the method is used anywhere else. This is easily reproducible in any language that supports extension methods (C#, for instance). I wonder if that could be useful at all.