Fix DateFormatter behaviour for nil locale and timeZone. (swiftlang#1681)

The locale and timeZone properties of DateFormatter seems to be "null
resetteable" in macOS, but the Swift Foundation behaviour differs.

Before this patch, setting locale to nil would segfault the program the
next time the formatter is used, while timeZone will incorrectly return
nil. After this patch a nil locale will simply reset to the current
locale and a nil timeZone will reset to the system one.

The tests check that the initial defaults are also correctly set.

(cherry picked from commit 1965003)

Author: drodriguez

Foundation/DateFormatter.swift

@@ -97,7 +97,7 @@ open class DateFormatter : Formatter {
 
     internal func _setFormatterAttributes(_ formatter: CFDateFormatter) {
         _setFormatterAttribute(formatter, attributeName: kCFDateFormatterIsLenient, value: isLenient._cfObject)
-        _setFormatterAttribute(formatter, attributeName: kCFDateFormatterTimeZone, value: timeZone?._cfObject)
+        _setFormatterAttribute(formatter, attributeName: kCFDateFormatterTimeZone, value: _timeZone?._cfObject)
         if let ident = _calendar?.identifier {
             _setFormatterAttribute(formatter, attributeName: kCFDateFormatterCalendarName, value: Calendar._toNSCalendarIdentifier(ident).rawValue._cfObject)
         } else {
@@ -166,11 +166,31 @@ open class DateFormatter : Formatter {
         }
     }
 
-    /*@NSCopying*/ open var locale: Locale! = .current { willSet { _reset() } }
+    /*@NSCopying*/ internal var _locale: Locale? { willSet { _reset() } }
+    open var locale: Locale! {
+        get {
+            guard let locale = _locale else { return .current }
+            return locale
+        }
+        set {
+            _locale = newValue
+        }
+    }
 
     open var generatesCalendarDates = false { willSet { _reset() } }
 
-    /*@NSCopying*/ open var timeZone: TimeZone! = NSTimeZone.system { willSet { _reset() } }
+    /*@NSCopying*/ internal var _timeZone: TimeZone? { willSet { _reset() } }
+    open var timeZone: TimeZone! {
+        get {
+            guard let timeZone = _timeZone else {
+                return (CFDateFormatterCopyProperty(_cfObject, kCFDateFormatterTimeZone) as! NSTimeZone)._swiftObject
+            }
+            return timeZone
+        }
+        set {
+            _timeZone = timeZone
+        }
+    }
 
     /*@NSCopying*/ internal var _calendar: Calendar! { willSet { _reset() } }
     open var calendar: Calendar! {

TestFoundation/TestDateFormatter.swift

@@ -22,6 +22,8 @@ class TestDateFormatter: XCTestCase {
             ("test_customDateFormat", test_customDateFormat),
             ("test_setLocalizedDateFormatFromTemplate", test_setLocalizedDateFormatFromTemplate),
             ("test_dateFormatString", test_dateFormatString),
+            ("test_setLocaleToNil", test_setLocaleToNil),
+            ("test_setTimeZoneToNil", test_setTimeZoneToNil),
             ("test_dateFrom", test_dateFrom),
         ]
     }
@@ -332,6 +334,30 @@ class TestDateFormatter: XCTestCase {
         }
     }
 
+    func test_setLocaleToNil() {
+        let f = DateFormatter()
+        // Locale should be the current one by default
+        XCTAssertEqual(f.locale, .current)
+
+        f.locale = nil
+
+        // Locale should go back to current.
+        XCTAssertEqual(f.locale, .current)
+
+        // A nil locale should not crash a subsequent operation
+        let result: String? = f.string(from: Date())
+        XCTAssertNotNil(result)
+    }
+
+    func test_setTimeZoneToNil() {
+        let f = DateFormatter()
+        // Time zone should be the system one by default.
+        XCTAssertEqual(f.timeZone, NSTimeZone.system)
+        f.timeZone = nil
+        // Time zone should go back to the system one.
+        XCTAssertEqual(f.timeZone, NSTimeZone.system)
+    }
+
     func test_dateFrom() throws {
         let formatter = DateFormatter()
         formatter.timeZone = TimeZone(identifier: "UTC")