This is a bug reported multiple times in different forms. My version of it is: 
SR-1487 <>. 

It remains open because it is not easy to fix with the existing design of 
String. Apparently core standard library team are working on an overhaul of 
String to address this and other usability and performance issues.

> On Oct 13, 2016, at 5:12 PM, Tim Vermeulen via swift-users 
> <> wrote:
> Is it a requirement that collections share indices with its subsequence? 
> Array and ArraySlice do share indices, which is why ArraySlice isn’t 
> zero-based, and I think this is convenient. But String.CharacterView doesn’t 
> seem to share indices with its subsequence (which is String.CharacterView as 
> well). Consider this example:
> let foo = "foobar".characters
> let index = foo.index(foo.startIndex, offsetBy: 3)
> let bar = foo.suffix(from: index)                   // "bar"
> foo[index]                                          // "b" :)
> foo[bar.startIndex]                                 // "f" :(
> So does this mean that we can’t assume that collections and their 
> subsequences share their indices (which could be very handy),  or is this 
> just a bug related to String.CharacterView?
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