I'm in favor of the feature and I think the proposal covers enough ground to be a useful addition to the language.
However, the vague syntax suggestions in the "Future Directions" section concern me. I'm worried that the -> some P syntax may be adequate for this proposal's purposes, but may not extend cleanly to uses we envision in the future. Maybe there is some larger scheme we could fit the some syntax into; if so, I'd like to see a sketch of that scheme before we commit to it here. (More thoughts on that in the Protocol<.AssocType == T> thread.)
Otherwise, I support it.