Error handling

Another way to handle it would be to instantiate a new NSError object that can be passed to the completion handler. Underneath the public constant for MissingHTTPResponseError create another constant:

public let MissingJsonAndErrorVariablesError: Int = 11

Now where the TODO comment is:

let userInfo = [NSLocalizedDescriptionKey: NSLocalizedString("Both JSON and Error Variables Are Set to Nil", comment: "")]
let newError = NSError(domain: TRENetworkingErrorDomain, code: MissingJsonAndErrorVariablesError, userInfo: userInfo)

completion(.Failure(newError))

Since Pasan said that with the way we structured our code in JSONTask(:completion:) method, there should be no way for the program to reach this line of code. So my proposal here is to simply call fatalError. If it so happens that the application we use this code in reaches this particular line, it would mean that we have made a logical error when modifying JSONTask(:completion:) method. And instead of trying to handle this case "gracefully", what we really should do is review that method.

That would be something like that:

fatalError("Both `json` and `error` variables are set to nil")

I'm a few months late, but what I decided to do is create a .failureWithoutError clause in the result enum, and I handled this error later like this in the view controller upon calling forecastClient.fetchCurrentWeather().

case.failureWithoutError:
                DispatchQueue.main.async {
                    self.showAlert(title: "Forecast Failed", message: "An unknown error has occurred. Try again.")
                    self.toggleRefreshIndicator(on: false)

                }

As for the DispatchQueue.main.async line, that's the way of running async on the main thread in Swift 3.

As for the answer above, while there is no way the code really should've / could've reached that location, it's just a simple networking error - no reason to crash the app if you don't get an error back while loading network data. The user can always try reloading again and it might work.