title: Cannot convert value of type '()' to expected argument type 'Data'
body: <p>Here is my problem : </p>
  
  <p>A Json object is returned by the function fetch() and I would like to parse it with structures, Moreover the 'let welcome' does not accept my json object :</p>
  
  <p>error message : Cannot convert value of type '()' to expected argument type 'Data'</p>
  
  <p>There may be something wrong with the JSON Decoder function ?</p>
  
  <pre><code>import Foundation
  import Alamofire
  
  func fetch() {
  AF.request("https://shopping.yahooapis.jp/ShoppingWebService/V1/json/itemSearch?appid=XXXXXX&amp;store_id=ejapan&amp;jan=4901330197711").responseJSON { response in
          debugPrint(response.value!)
          return
      }
  }
  
  let jsonData = fetch()
  
  let welcome = try? newJSONDecoder().decode(Welcome.self, from: jsonData)
  
  
  
  struct Welcome: Codable {
      var resultSet: ResultSet
  
      enum CodingKeys: String, CodingKey {
          case resultSet = "ResultSet"
      }
  }
  </code></pre>