DefaultProvidable.swift

protocol DefaultProvidable {
   static var defaultValue: Self {get}
}
extension Bool: DefaultProvidable {
    static let defaultValue = false
}
extension Optional: DefaultProvidable {
    static var defaultValue: Optional<Wrapped> {return nil}
}
...
func niceStub<I,O: DefaultProvidable>(of: (I) -> (O)) -> (I) -> (O){
  return { _ in O.defaultValue }
}
class DatabaseStub: Database {    
  lazy var addUserAction = niceStub(of: addUser) // returns false
  func addUser(name: String) -> Bool {     
    return addUserAction(name)    
  }
}