Ah right, I'm guessing it would be possible to make this work with some extra internal changes but there are better options if going that route. I definitely agree a case should be declared like that, but I think there's a case to have an available overload.
I'm guessing it would work if the Default Arguments section of Protocol Witness Matching Mini-Manifesto were added? (Something I've personally bumped up against other places so am hopeful to see added).