Open Radar

 
Summary:
The documentation for the String struct in the Swift Standard Library Reference states that "String is bridged to Objective-C as NSString", which implies that the Swift convenience initializer String(format:_:) should behave the same as the Objective-C initializer -[NSString initWithFormat:...].  However, the behavior actually differs when the format string contains positional specifiers such as "%1$@" (which the String Programming Guide documents as supported by NSString in the section "String Format Specifiers").

Specifically, when running in an Xcode playground, the use of a positional specifier in the format string causes an EXC_BAD_INSTRUCTION exception.  

(When running equivalent code on the command line, the initializer returns a String object that represents a tuple containing the unprocessed format string and the argument(s) that would have been applied.  This bug is being reported separately to the Swift project.)

Steps to Reproduce:
In Xcode 7.3, create a playground with the following program:

import Foundation

let fmtstr = "%1$@ shall be the number thou shalt count, and the number of the counting shall be %1$@."
let stext = String(format: fmtstr, "THREE")
let nstext = NSString(format:fmtstr, "THREE")


Expected Results:
The variables stext and nstext should contain the same text, namely:

"THREE shall be the number thou shalt count, and the number of the counting shall be THREE."

Actual Results:
The line defining stext fails with an EXC_BAD_INSTRUCTION error.

Version:
Xcode Version 7.3.1 (7D1012)
(also reproducible in Xcode Version 7.3)

OS X Version 10.11.4 (15E65)

Notes:
As suggested by the program description, invoking the NSString initializer directly works around the problem on OS X when the Foundation framework is available; but this workaround is presumably not portable to Linux.