Fix another IUO that appears in a type.

rudkx · rudkx · commit e185e3869e89 · 2017-11-07T14:27:16.000-08:00
Another case that SE-0054 disallows.
diff --git a/Foundation/NSString.swift b/Foundation/NSString.swift
@@ -1191,7 +1191,7 @@ extension NSString {
     }
     
     public convenience init(format: NSString, _ args: CVarArg...) {
-        let str = withVaList(args) { (vaPtr) -> CFString! in
+        let str = withVaList(args) { (vaPtr) -> CFString? in
             CFStringCreateWithFormatAndArguments(kCFAllocatorSystemDefault, nil, format._cfObject, vaPtr)
         }!
         self.init(str._swiftObject)

Original file line number	Diff line number	Diff line change
`@@ -1191,7 +1191,7 @@ extension NSString {`
`1191`	`1191`	`}`
`1192`	`1192`
`1193`	`1193`	`public convenience init(format: NSString, _ args: CVarArg...) {`
`1194`		`- let str = withVaList(args) { (vaPtr) -> CFString! in`
	`1194`	`+ let str = withVaList(args) { (vaPtr) -> CFString? in`
`1195`	`1195`	`CFStringCreateWithFormatAndArguments(kCFAllocatorSystemDefault, nil, format._cfObject, vaPtr)`
`1196`	`1196`	`}!`
`1197`	`1197`	`self.init(str._swiftObject)`