I think protocol Alpha<B> is implicitly protocol Alpha<B: Copyable>. IIUC, the compiler put Copyable in everywhere you don't explicitly make it ~Copyable.
Edit: Nope. I wasn't considering it's a primary associated type. Something more subtle.
ah. i hadn't realized that was the case. then there's not some logic about Copyability that i was missing, it's just an (as yet) unimplemented compiler feature?