Fix for a crash in String.substring(with:) [SR-2483]

Foundation/NSString.swift

@@ -346,7 +346,49 @@ extension NSString {
             let start = _storage.utf16.startIndex
             let min = start.advanced(by: range.location)
             let max = start.advanced(by: range.location + range.length)
-            return String(_storage.utf16[min..<max])!
+            if let substr = String(_storage.utf16[min..<max]) {
+                return substr
+            }
+            //If we come here, then the range has created unpaired surrogates on either end.
+            //An unpaired surrogate is replaced by OXFFFD - the Unicode Replacement Character.
+            //The CRLF ("\r\n") sequence is also treated like a surrogate pair, but its constinuent
+            //characters "\r" and "\n" can exist outside the pair!
+
+            let replacementCharacter = String(describing: UnicodeScalar(0xFFFD)!)
+            let CR: UInt16 = 13  //carriage return
+            let LF: UInt16 = 10  //new line
+
+            //make sure the range is of non-zero length
+            guard range.length > 0 else { return "" }
+
+            //if the range is pointing to a single unpaired surrogate
+            if range.length == 1 {
+                switch _storage.utf16[min] {
+                case CR: return "\r"
+                case LF: return "\n"
+                default: return replacementCharacter
+                }
+            }
+
+            //set the prefix and suffix characters
+            let prefix = _storage.utf16[min] == LF ? "\n" : replacementCharacter
+            let suffix = _storage.utf16[max.advanced(by: -1)] == CR ? "\r" : replacementCharacter
+
+            //if the range breaks a surrogate pair at the beginning of the string
+            if let substrSuffix = String(_storage.utf16[min.advanced(by: 1)..<max]) {
+                return prefix + substrSuffix
+            }
+
+            //if the range breaks a surrogate pair at the end of the string
+            if let substrPrefix = String(_storage.utf16[min..<max.advanced(by: -1)]) {
+                return substrPrefix + suffix
+            }
+
+            //the range probably breaks surrogate pairs at both the ends
+            guard min.advanced(by: 1) <= max.advanced(by: -1) else { return prefix + suffix }
+
+            let substr =  String(_storage.utf16[min.advanced(by: 1)..<max.advanced(by: -1)])!
+            return prefix + substr + suffix
         } else {
             let buff = UnsafeMutablePointer<unichar>.allocate(capacity: range.length)
             getCharacters(buff, range: range)

TestFoundation/TestNSString.swift

@@ -96,6 +96,7 @@ class TestNSString : XCTestCase {
             ("test_reflection", { _ in test_reflection }),
             ("test_replacingOccurrences", test_replacingOccurrences),
             ("test_getLineStart", test_getLineStart),
+            ("test_substringWithRange", test_substringWithRange),
         ]
     }
 
@@ -1063,6 +1064,43 @@ class TestNSString : XCTestCase {
         XCTAssertTrue(testString.hasPrefix(""))
         XCTAssertTrue(testString.hasSuffix(""))
     }
+
+    func test_substringWithRange() {
+        let trivial = NSString(string: "swift.org")
+        XCTAssertEqual(trivial.substring(with: NSMakeRange(0, 5)), "swift")
+
+        let surrogatePairSuffix = NSString(string: "Hurray🎉")
+        XCTAssertEqual(surrogatePairSuffix.substring(with: NSMakeRange(0, 7)), "Hurray�")
+
+        let surrogatePairPrefix = NSString(string: "🐱Cat")
+        XCTAssertEqual(surrogatePairPrefix.substring(with: NSMakeRange(1, 4)), "�Cat")
+
+        let singleChar = NSString(string: "😹")
+        XCTAssertEqual(singleChar.substring(with: NSMakeRange(0,1)), "�")
+
+        let crlf = NSString(string: "\r\n")
+        XCTAssertEqual(crlf.substring(with: NSMakeRange(0,1)), "\r")
+        XCTAssertEqual(crlf.substring(with: NSMakeRange(1,1)), "\n")
+        XCTAssertEqual(crlf.substring(with: NSMakeRange(1,0)), "")
+
+        let bothEnds1 = NSString(string: "😺😺")
+        XCTAssertEqual(bothEnds1.substring(with: NSMakeRange(1,2)), "��") 
+
+        let s1 = NSString(string: "😺\r\n")
+        XCTAssertEqual(s1.substring(with: NSMakeRange(1,2)), "�\r")
+
+        let s2 = NSString(string: "\r\n😺")
+        XCTAssertEqual(s2.substring(with: NSMakeRange(1,2)), "\n�")
+
+        let s3 = NSString(string: "😺cats😺")
+        XCTAssertEqual(s3.substring(with: NSMakeRange(1,6)), "�cats�")
+
+        let s4 = NSString(string: "😺cats\r\n")
+        XCTAssertEqual(s4.substring(with: NSMakeRange(1,6)), "�cats\r")
+
+        let s5 = NSString(string: "\r\ncats😺")
+        XCTAssertEqual(s5.substring(with: NSMakeRange(1,6)), "\ncats�")
+    }
 }
 
 struct ComparisonTest {