[SR-6077] Character ends up accepting invalid emoji flag sequences.

[YOCKOW]

Previous ID	SR-6077
Radar	None
Original Reporter	@YOCKOW
Type	Bug
Status	Resolved
Resolution	Won't Do

Environment

• Swift 4.0

• OS: macOS, Ubuntu 16.04

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug
Assignee	None
Priority	Medium

md5: c82e0168fd3a0f487ee734ba7518dfc4

relates to:

• SR-9206 Should literal Characters precondition on single-grapheme check?

Issue Description:

The Swift code below will be compiled without any errors, although it does not conform to Unicode segmentation algorithm.

[code]

let a: Character = "\u{1F1E6}" // REGIONAL INDICATOR SYMBOL LETTER A
let abc: Character = "\u{1F1E6}\u{1F1E7}\u{1F1E8}" //REGIONAL INDICATOR SYMBOL LETTER A-C

[Expected Result]
An error is raised such as "error: cannot convert value of type 'String' to specified type 'Character'".

[References]

• UAX #29
  > Do not break within emoji flag sequences. That is, do not break between regional indicator (RI) symbols if there is an odd number of RI characters before the break point.
  > GB12 sot (RI RI)* RI × RI
  > GB13 RI (RI RI)* RI × RI

• UTS #51
  > emoji flag sequence — A sequence of two Regional Indicator characters, where the corresponding ASCII characters are valid region sequences
  > A singleton Regional Indicator character is called an ill-formed emoji flag sequence.

[belkadan]

I suspect we're not going to do this simply because the Unicode standard changes and we wouldn't want existing code to stop compiling. This would have been invalid for a different reason in Unicode 9.

cc @airspeedswift

[airspeedswift]

Right, we generally think the attempt to compile-time-validate graphemes previously to be a bit of a misadventure that leads to odd behavior when the compiler and OS differ. So we now only do it on a best-efforts basis (similar to how we catch some literal overflow issues but not others).

[YOCKOW]

Thank you for your responses.
I was just concerned about the difference from the failure that `let rnr: Character = "\r\n\r"` could not be compiled.