[SR-1988] Ranges of String.UTF16View.Index have negative count

Author: DaveLiu888

Foundation/ExtraStringAPIs.swift

@@ -21,7 +21,7 @@ extension String.UTF16View.Index : Strideable {
     }
 
     public func distance(to other: String.UTF16View.Index) -> Int {
-        return other._offset.distance(to: _offset)
+        return _offset.distance(to: other._offset)
     }
 
     public func advanced(by n: Int) -> String.UTF16View.Index {

TestFoundation/TestNSString.swift

@@ -87,6 +87,7 @@ class TestNSString : XCTestCase {
             ("test_ExternalRepresentation", test_ExternalRepresentation),
             ("test_mutableStringConstructor", test_mutableStringConstructor),
             ("test_PrefixSuffix", test_PrefixSuffix),
+            ("test_utf16StringRangeCount", test_utf16StringRangeCount),
             ("test_reflection", { _ in test_reflection }),
         ]
     }
@@ -937,6 +938,22 @@ class TestNSString : XCTestCase {
         }
     }
 
+    //[SR-1988] Ranges of String.UTF16View.Index have negative count
+    func test_utf16StringRangeCount(){
+        let str = "How many elements do I contain?".utf16
+        let indices: Range = str.startIndex ..< str.endIndex
+        XCTAssertEqual(indices.count , 31, "Range should be 31")
+        
+        let blank = "".utf16
+        let blankindices: Range = blank.startIndex ..< blank.endIndex
+        XCTAssertTrue(blankindices.count >= 0, "Range should not be negative")
+        
+        let one = "1".utf16
+        let oneindices: Range = one.startIndex ..< one.endIndex
+        XCTAssertTrue(oneindices.count == 1, "Range should be 1")
+        
+    }
+    
     func test_mutableStringConstructor() {
         let mutableString = NSMutableString(string: "Test")
         XCTAssertEqual(mutableString, "Test")