title: @Injectable() decorator and providers array
body: <p>Does a service that is provided in "root" within the @Injectable() decorator still have to be in the providers array of the module?</p>
  
  <p>The <a href="https://angular.io/guide/architecture-services" rel="nofollow noreferrer">Angular documentation</a> doesn't really give me an answer or I don't quite understand it.</p>
  
  <p>Inside my core folder I have a authentication service which is provided in root. I wan't to import my core module inside the app module in order to use all the services and components provided.</p>
  
  <p>Do I have to additionally set up the service in the providers array of the module, or is it enough that it is already provided at root level using the decorator?</p>