Compiler Crash: Must be my Type Specification

protocol Identifies : Hashable {}

// Something is Identifiable if it has an Identifier
protocol Identifiable : Hashable {
  typealias Identifier : Identifies
  var identifier : Identifier { get }
}

extension String : Identifies {}

class Person : Identifiable {
  var identifier : String = "avoid init for now"
}

protocol Service {
  typealias Object : Identifiable
  func lookup (id : Object.Identifier) -> Object?
}

class PersonService <P:Person> : Service {  // crashes here...
  func lookup(id: P.Identifier) -> P? {
    return nil
  }
}

The crash is avoidable with

P:Person where P:Identifiable

Why?

Also, assume I want to avoid all type inferencing of the protocol typealiases. How would I modify the code to be as explicit as possible regarding type relationships?