title: Use of undeclared type "Decodable"/"Codable" in Swift 3
body: <p>So I'm following along with Apple's <a href="https://developer.apple.com/documentation/foundation/archives_and_serialization/using_json_with_custom_types" rel="nofollow noreferrer">Using JSON with Custom Types</a> tutorial, via the storyboard, and I've run into an error trying the following:</p>
  
  <pre><code>struct GroceryStoreService: Decodable {
      let name: String
      let aisles: [Aisle]
  
      struct Aisle: Decodable {
          let name: String
          let shelves: [Shelf]
  
          struct Shelf: Decodable {
              let name: String
              let product: GroceryStore.Product
          }
      }
  }
  </code></pre>
  
  <p>Only I'm getting the error:</p>
  
  <p><code>Use of undeclared type Decodable</code> - and the same with Codable.</p>
  
  <p>I've checked other SO questions, trying cleaning and building, poking through the build phases and so far no luck on getting that sorted. </p>