试图找出下面第一行为什么不能编译的圈子。这只是一个游乐场，但如果它是应用程序的一部分同样问题并不重要。 Xcode 7beta5 / Swift。
错误是：'Index' does not have a member named 'advancedBy'
"Why doesn't this work".startIndex.advancedBy(3) // Won't compile
"This does work".startIndex.successor() // Works
"So does this".endIndex.predecessor() // Works