Thanks to Ole Begemann (who closed my bug report), I found out that Nate Cook 
submitted a fix <https://github.com/apple/swift/pull/4896> for this issue a few 
day ago. The fix will hopefully make it to the next maintenance release of 
Swift.

The documentation of range subscript (which is the main API for creating 
subsequences) already specifies that SubSequences (slices) should use the same 
indices, but I think we need to better document the specifications of 
SubSequence associated type and make sure that any API that returns a 
SubSequence links to SubSequence documentation.

> On Oct 16, 2016, at 3:33 PM, Dave Abrahams via swift-users 
> <swift-users@swift.org> wrote:
> 
> 
> on Thu Oct 13 2016, Tim Vermeulen <swift-users-AT-swift.org> wrote:
> 
>> Is it a requirement that collections share indices with its
>> subsequence? 
> 
> Yes.
> 
>> 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). 
> 
> That's a bug.
> 
>> 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?
> 
> The latter.
> 
> -- 
> -Dave
> 
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