如果NSURLSession＃dataTaskWithRequest返回nil是什么意思？

Question

Starting in iOS 9.0, NSURLSession#dataTaskWithRequest may return NSURLSessionDataTask? where as before it always returned a NSURLSessionDataTask . The Swift migration simply suggests just adding a ! to force unwrap the task. But this still left me wondering why this change was made. What does it actually mean if nil was returned? Under what conditions with a task fail to be constructed? What's the best way to handle these errors in an application?

let task = NSURLSession.sharedSession().dataTaskWithRequest(request)
// Swift migrator suggests force unwrapping the optional
let task = NSURLSession.sharedSession().dataTaskWithRequest(request)!

iOS 8.3

- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request
- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request completionHandler:(void (^)(NSData *data, NSURLResponse *response, NSError *error))completionHandler

iOS 9.0

- (NSURLSessionDataTask * nullable)dataTaskWithRequest:(NSURLRequest * nonnull)request
- (NSURLSessionDataTask * nullable)dataTaskWithRequest:(NSURLRequest * nonnull)request completionHandler:(void (^ nonnull)(NSData * nullable data, NSURLResponse * nullable response, NSError * nullable error))completionHandler

https://developer.apple.com/library/prerelease/ios/releasenotes/General/iOS90APIDiffs/frameworks/Foundation.html

Answer 1

Looks like this was an error in the API. As of now the diff linked in the question shows

- (NSURLSessionDataTask * _Nonnull)dataTaskWithRequest:(NSURLRequest * _Nonnull)request