[SR-10079] Iterating over a reversed substring can crash

[martinr448]

Previous ID	SR-10079
Radar	None
Original Reporter	@martinr448
Type	Bug

Environment

Tested with Xcode 10.1 (Swift 4.2) and Xcode 10.2 beta 3 (Swift 5).

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Standard Library | |Labels | Bug, RunTimeCrash | |Assignee | None | |Priority | Medium | md5: 9ba13aeb1c246a44632281938881173b

Issue Description:

Iterating over a ReversedCollection<String.SubSequence> can crash if the substring starts within an extended grapheme cluster:

let string = "abc\r\nxyz"
let idx = string.range(of: "\n")!.lowerBound
let substring = string[idx...]
let rev = substring.reversed()

let revString = String(rev) // Fatal error: Out of bounds: index < startIndex

The same crash happens with any of the following:

let arr = Array(rev)
for c in rev { print(c) }
print(rev.last!)

[theblixguy]

Hmm this seems to crash in `public subscript(i: Index) -> Character {}` in `Substring.swift`. When I print the start and end indices of `rev`, then I get empty indices. Printing random elements works but fails when printing the newline character with the same crash. Reversing `rev` and creating a String works as well.

[belkadan]

range(of:) is implemented in terms of NSString and thus doesn't understand grapheme clusters. :-( cc @milseman