SR-9439: Calendar.dateComponetns: Enable .nanoseconds

- Nanoseconds were originally disable in #1658 to fix SR-7011,
  however the CF/NSCalendar rewrite in #1755 fixed them to
  work correctly so they can now be re-enabled.

(cherry picked from commit d8f09b6)

Author: spevans

Foundation/NSCalendar.swift

@@ -643,9 +643,9 @@ open class NSCalendar : NSObject, NSCopying, NSSecureCoding {
 
     open func components(_ unitFlags: Unit, from startingDate: Date, to resultDate: Date, options opts: Options = []) -> DateComponents {
         let validUnitFlags: NSCalendar.Unit = [
-            .era, .year, .month, .day, .hour, .minute, .second, .weekOfYear, .weekOfMonth, .yearForWeekOfYear, .weekday, .weekdayOrdinal ]
+            .era, .year, .month, .day, .hour, .minute, .second, .nanosecond, .weekOfYear, .weekOfMonth, .yearForWeekOfYear, .weekday, .weekdayOrdinal ]
 
-        let invalidUnitFlags: NSCalendar.Unit = [ .quarter, .nanosecond, .timeZone, .calendar]
+        let invalidUnitFlags: NSCalendar.Unit = [ .quarter, .timeZone, .calendar]
 
         // Mask off the unsupported fields
         let newUnitFlags = Unit(rawValue: unitFlags.rawValue & validUnitFlags.rawValue)
@@ -665,10 +665,7 @@ open class NSCalendar : NSObject, NSCopying, NSSecureCoding {
             let emptyUnitFlags = Unit(rawValue: unitFlags.rawValue & invalidUnitFlags.rawValue)
             var components = _components(newUnitFlags, vector: ints, addIsLeapMonth: false)
 
-            // nanosecond and quarter always get set to zero if requested in the output
-            if emptyUnitFlags.contains(.nanosecond) {
-                components.nanosecond = 0
-            }
+            // quarter always gets set to zero if requested in the output
             if emptyUnitFlags.contains(.quarter) {
                 components.quarter = 0
             }

TestFoundation/TestCalendar.swift

@@ -215,6 +215,7 @@ class TestNSDateComponents: XCTestCase {
             ("test_hash", test_hash),
             ("test_copyNSDateComponents", test_copyNSDateComponents),
             ("test_dateDifferenceComponents", test_dateDifferenceComponents),
+            ("test_nanoseconds", test_nanoseconds),
         ]
     }
 
@@ -429,4 +430,36 @@ class TestNSDateComponents: XCTestCase {
         XCTAssertNil(diff9.calendar)
         XCTAssertNil(diff9.timeZone)
     }
+
+    func test_nanoseconds() throws {
+        // 1971-06-21 00:00:00
+        let date1 = Date(timeIntervalSince1970: 46310400)
+
+        // 1971-06-21 00:00:00.00123
+        let date2 = Date(timeIntervalSince1970: 46310400.00123)
+
+        // 1971-06-24 00:16:40:00123
+        let date3 = Date(timeIntervalSince1970: 46570600.45678)
+
+        var calendar = Calendar.current
+        calendar.timeZone = try TimeZone(abbreviation: "UTC").unwrapped()
+
+        let diff1 = calendar.dateComponents([.nanosecond], from: date1, to: date2)
+        XCTAssertEqual(diff1.nanosecond, 1230003)
+
+        let diff2 = calendar.dateComponents([.nanosecond], from: date1, to: date2)
+        XCTAssertEqual(diff2.nanosecond, 1230003)
+
+        let diff3 = calendar.dateComponents([.day, .minute, .second, .nanosecond], from: date2, to: date3)
+        XCTAssertEqual(diff3.day, 3)
+        XCTAssertEqual(diff3.minute, 16)
+        XCTAssertEqual(diff3.second, 40)
+        XCTAssertEqual(diff3.nanosecond, 455549949)
+
+        let diff4 = calendar.dateComponents([.day, .minute, .second, .nanosecond], from: date3, to: date2)
+        XCTAssertEqual(diff4.day, -3)
+        XCTAssertEqual(diff4.minute, -16)
+        XCTAssertEqual(diff4.second, -40)
+        XCTAssertEqual(diff4.nanosecond, -455549950)
+    }
 }