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

Author: Pushkar Kulkarni

Foundation/NSString.swift

@@ -346,7 +346,14 @@ 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])!
+            guard let substr = String(_storage.utf16[min..<max]) else {
+                let newMax = max.advanced(by: -1)
+                guard newMax > min else { return "" }
+                let substr1 = NSMutableString(string: String(_storage.utf16[min..<newMax])!)
+                substr1.append(String(describing: [_storage.utf16[max]]))
+                return String(describing: substr1)
+            }
+            return substr
         } 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 +971,18 @@ 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 endWithSurrogatePair = NSString(string: "Hurray🎉")
+        XCTAssertEqual(endWithSurrogatePair.substring(with: NSMakeRange(0, 7)), "Hurray[57225]")
+
+        let crlf = NSString(string: "\r\n")
+        XCTAssertEqual(crlf.substring(with: NSMakeRange(0,1)), "")
+        XCTAssertEqual(crlf.substring(with: NSMakeRange(1,0)), "")
+    }
 }
 
 struct ComparisonTest {