title: failed to produce diagnostic for expression, Xcode 11.4, Moya
body: <p>After updating to Xcode 11.4, I started to get this error</p>
  <blockquote>
  <p>Failed to produce diagnostic for expression please file a bug report</p>
  </blockquote>
  <p>On:</p>
  <pre class="lang-swift prettyprint-override"><code>let provider = MoyaProvider&lt;EndPoint&gt;(requestClosure: requestClosure)
  </code></pre>
  <p>I updated Moya but it doesn't look like the issue. Runs fine on previous Xcode versions.</p>
  <p>full code:</p>
  <pre class="lang-swift prettyprint-override"><code> let requestClosure: MoyaProvider.RequestClosure = {
              [unowned self] (endpoint: Endpoint, done: @escaping MoyaProvider.RequestResultClosure) in
  
         guard let request = try? endpoint.urlRequest() else { return }
  
         self.authenticator.authenticate(request, done: { (request) in
                  done(.success(request))
         })
   }
   provider = MoyaProvider&lt;EndPoint&gt;(requestClosure: requestClosure
  </code></pre>