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

Author: Pushkar Kulkarni

Foundation/NSString.swift

@@ -346,7 +346,23 @@ 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
+            } else {
+                //a broken surrogate pair is replaced by OXFFFD - the Unicode Replacement Character
+                let replacementCharacter = String(describing: UnicodeScalar(0xFFFD)!)
+
+                //is min breaking a surrogate pair?
+                guard min.advanced(by: 1) < max else { return "" }
+                if let substrSuffix = String(_storage.utf16[min.advanced(by: 1)..<max]) {
+                    return replacementCharacter + substrSuffix
+                }
+
+                //max is breaking a surrogate pair
+                guard min < max.advanced(by: -1) else { return "" }
+                let substrPrefix = String(_storage.utf16[min..<max.advanced(by: -1)])!
+                return substrPrefix + replacementCharacter
+            }
         } else {
             let buff = UnsafeMutablePointer<unichar>.allocate(capacity: range.length)
             getCharacters(buff, range: range)

TestFoundation/TestNSString.swift

@@ -89,6 +89,7 @@ class TestNSString : XCTestCase {
             ("test_PrefixSuffix", test_PrefixSuffix),
             ("test_utf16StringRangeCount", test_StringUTF16ViewIndexStrideableRange),
             ("test_reflection", { _ in test_reflection }),
+            ("test_substringWithRange", test_substringWithRange),
         ]
     }
 
@@ -970,6 +971,21 @@ class TestNSString : XCTestCase {
         let mutableString = NSMutableString(string: "Test")
         XCTAssertEqual(mutableString, "Test")
     }
+
+    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 crlf = NSString(string: "\r\n")
+        XCTAssertEqual(crlf.substring(with: NSMakeRange(0,1)), "")
+        XCTAssertEqual(crlf.substring(with: NSMakeRange(1,0)), "")
+    }
 }
 
 struct ComparisonTest {