[SR-367] Regional Indicator letters & flags: prefix and suffix

[glessard]

Previous ID	SR-367
Radar	None
Original Reporter	@glessard
Type	Bug
Status	Closed
Resolution	Done

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Standard Library | |Labels | Bug | |Assignee | None | |Priority | Medium | md5: 42c209e3a14e2350b7530adf1f1fb8ba

Issue Description:

In the presence of Objective-C, hasPrefix and hasSuffix have the wrong behaviour.

let b1 = "\u{1F1E7}" // Regional indicator letter B
let b2 = b1+b1       // flag of Barbados

b2.hasPrefix(b1) // returns true
b2.hasSuffix(b1) // returns true

According to Unicode rules, the grapheme cluster from string b1 is not present in string b2, as there is should be no grapheme cluster break between the two instances of \u{1F1E7} in b2. (http://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundary_Rules).
Both functions should return false in the example above.

[dabrahams]

This problem is being fixed in the new String design, FWIW.

[airspeedswift]

Should be resolved by https://github.com/apple/swift-corelibs-foundation/pull/1459/

[glessard]

This is fixed in Swift 4.2, as expected