Fix DateFormatter TimeZone Setter

Because of the use of 'timeZone' instead of 'newValue' in the setter, setting the timeZone of a DateFormatter doesn't actually work. Instead, it winds up re-setting the old value, which happens to be whatever was originally set, usually what we get from copying the default down in CoreFoundation.

Also adds a unit test to help prevent breaks like this again.

Author: Kaiede

Foundation/DateFormatter.swift

@@ -176,13 +176,13 @@ open class DateFormatter : Formatter {
     /*@NSCopying*/ internal var _timeZone: TimeZone? { willSet { _reset() } }
     open var timeZone: TimeZone! {
         get {
-            guard let timeZone = _timeZone else {
+            guard let tz = _timeZone else {
                 return (CFDateFormatterCopyProperty(_cfObject, kCFDateFormatterTimeZone) as! NSTimeZone)._swiftObject
             }
-            return timeZone
+            return tz
         }
         set {
-            _timeZone = timeZone
+            _timeZone = newValue
         }
     }
 

TestFoundation/TestDateFormatter.swift

@@ -24,6 +24,7 @@ class TestDateFormatter: XCTestCase {
             ("test_dateFormatString", test_dateFormatString),
             ("test_setLocaleToNil", test_setLocaleToNil),
             ("test_setTimeZoneToNil", test_setTimeZoneToNil),
+            ("test_setTimeZone", test_setTimeZone),
         ]
     }
 
@@ -356,4 +357,22 @@ class TestDateFormatter: XCTestCase {
         // Time zone should go back to the system one.
         XCTAssertEqual(f.timeZone, NSTimeZone.system)
     }
+
+    func test_setTimeZone() {
+        // Test two different time zones. Should ensure that if one
+        // happens to be TimeZone.current, we still get a valid test.
+        let newYork = TimeZone(identifier: "America/New_York")!
+        let losAngeles = TimeZone(identifier: "America/Los_Angeles")!
+
+        XCTAssertNotEqual(newYork, losAngeles)
+
+        // Case 1: New York
+        let f = DateFormatter()
+        f.timeZone = newYork
+        XCTAssertEqual(f.timeZone, newYork)
+
+        // Case 2: Los Angeles
+        f.timeZone = losAngeles
+        XCTAssertEqual(f.timeZone, losAngeles)
+    }
 }