I believe this is deliberate; you're allowed to shadow things from other modules in your own module, which is important if someone introduces a new decl in their library but you want to continue using yours. This also works:

import A

extension P {
  var test: Int? {
    return nil
  }
}

Arguably, though, we could have a warning or something at first, and then you have to add an attribute to say "yes, I know I'm shadowing this".

Spent an hour trying to figure out why my code wasn’t working and this was the culprit. It would be really useful to have warning diagnostic when you have an optional protocol requirement satisfied by a default implementation and the conforming type’s implementation is non-optional. An attribute to silence the warning sounds reasonable, maybe something like “@shadowImplementation” (open to suggestions).