Allow config-less DateTime, Duration, and Interval

DateTime, Duration, and Interval constructors now
return invalid when no config is supplied. This
should fix any code which relies on default new
while still keeping that invariant that configs
are supplied.

src/datetime.js

@@ -514,6 +514,10 @@ export default class DateTime {
    * @access private
    */
   constructor(config) {
+    if (!config) {
+      return DateTime.invalid("no config supplied");
+    }
+
     const zone = config.zone || Settings.defaultZone;
 
     let invalid =

src/duration.js

@@ -218,6 +218,10 @@ export default class Duration {
    * @private
    */
   constructor(config) {
+    if (!config) {
+      return Duration.invalid("no config supplied");
+    }
+
     const accurate = config.conversionAccuracy === "longterm" || false;
     let matrix = accurate ? accurateMatrix : casualMatrix;
 

src/interval.js

@@ -41,6 +41,10 @@ export default class Interval {
    * @private
    */
   constructor(config) {
+    if (!config) {
+      return Interval.invalid("no config supplied");
+    }
+
     /**
      * @access private
      */

test/datetime/create.test.js

@@ -9,6 +9,15 @@ const withDefaultLocale = Helpers.withDefaultLocale,
   withthrowOnInvalid = Helpers.setUnset("throwOnInvalid"),
   withDefaultZone = Helpers.withDefaultZone;
 
+//------
+// new
+//------
+test("Default constructor works and returns an invalid instance", () => {
+  const datetime = new DateTime();
+  expect(datetime.isValid).toBe(false);
+  expect(datetime.invalidReason).toBe("no config supplied");
+});
+
 //------
 // .now()
 //------

test/duration/create.test.js

@@ -2,6 +2,15 @@
 
 import { Duration } from "../../src/luxon";
 
+//------
+// new
+//------
+test("Default constructor works and returns an invalid instance", () => {
+  const duration = new Duration();
+  expect(duration.isValid).toBe(false);
+  expect(duration.invalidReason).toBe("no config supplied");
+});
+
 //------
 // .fromObject()
 //-------

test/interval/create.test.js

@@ -4,6 +4,15 @@ import Helpers from "../helpers";
 
 const withThrowOnInvalid = Helpers.setUnset("throwOnInvalid");
 
+//------
+// new
+//------
+test("Default constructor works and returns an invalid instance", () => {
+  const interval = new Interval();
+  expect(interval.isValid).toBe(false);
+  expect(interval.invalidReason).toBe("no config supplied");
+});
+
 //------
 // .fromObject()
 //-------