Not only that, if you have `trait Foo: Iterator<Item = u8>` and `trait Bar: Iterator<Item = i32>` how could `!` implement both of them? It would simply make the language incoherent.

> What would <! as Iterator>::Item be?

Another ! makes sense to me here. Are there any cases where it doesn't work to auto-assign ! to all associated types of a ! trait impl? Associated constants might require some mechanism similar to `compile_error!()`.