fix(defaultIfEmpty): Allow undefined as an argument, require an arg…

…ument

BREAKING CHANGE: `defaultIfEmpty` requires a value be passed. Will no longer convert `undefined` to `null` for no good reason.

Resolves #6064

api_guard/dist/types/operators/index.d.ts

@@ -68,7 +68,7 @@ export declare function debounce<T>(durationSelector: (value: T) => ObservableIn
 
 export declare function debounceTime<T>(dueTime: number, scheduler?: SchedulerLike): MonoTypeOperatorFunction<T>;
 
-export declare function defaultIfEmpty<T, R = T>(defaultValue?: R): OperatorFunction<T, T | R>;
+export declare function defaultIfEmpty<T, R>(defaultValue: R): OperatorFunction<T, T | R>;
 
 export declare function delay<T>(due: number | Date, scheduler?: SchedulerLike): MonoTypeOperatorFunction<T>;
 
@@ -86,7 +86,8 @@ export declare function distinctUntilChanged<T, K>(comparator: (previous: K, cur
 export declare function distinctUntilKeyChanged<T>(key: keyof T): MonoTypeOperatorFunction<T>;
 export declare function distinctUntilKeyChanged<T, K extends keyof T>(key: K, compare: (x: T[K], y: T[K]) => boolean): MonoTypeOperatorFunction<T>;
 
-export declare function elementAt<T>(index: number, defaultValue?: T): MonoTypeOperatorFunction<T>;
+export declare function elementAt<T, D>(index: number): MonoTypeOperatorFunction<T>;
+export declare function elementAt<T, D>(index: number, defaultValue: D): OperatorFunction<T, T | D>;
 
 export declare function endWith<T>(scheduler: SchedulerLike): MonoTypeOperatorFunction<T>;
 export declare function endWith<T, A>(v1: A, scheduler: SchedulerLike): OperatorFunction<T, T | A>;

spec-dtslint/operators/defaultIfEmpty-spec.ts

@@ -1,8 +1,9 @@
-import { of } from 'rxjs';
+import { EMPTY, of } from 'rxjs';
 import { defaultIfEmpty, map } from 'rxjs/operators';
 
 it('should infer correctly', () => {
-  const o = of(1, 2, 3).pipe(defaultIfEmpty()); // $ExpectType Observable<number>
+  const o = of(1, 2, 3).pipe(defaultIfEmpty()); // $ExpectError
+  const o2 = of(undefined).pipe(defaultIfEmpty(undefined)); // $ExpectType Observable<undefined>
 });
 
 it('should infer correctly with a defaultValue', () => {
@@ -11,6 +12,7 @@ it('should infer correctly with a defaultValue', () => {
 
 it('should infer correctly with a different type of defaultValue', () => {
   const o = of(1, 2, 3).pipe(defaultIfEmpty<number, string>('carbonara')); // $ExpectType Observable<string | number>
+  const o2 = of(1, 2, 3).pipe(defaultIfEmpty('carbonara')); // $ExpectType Observable<string | number>
 });
 
 it('should infer correctly with a subtype passed through parameters', () => {
@@ -20,3 +22,7 @@ it('should infer correctly with a subtype passed through parameters', () => {
 it('should enforce types', () => {
   const o = of(1, 2, 3).pipe(defaultIfEmpty(4, 5)); // $ExpectError
 });
+
+it('should handle Observable<never> appropriately', () => {
+  const o = EMPTY.pipe(defaultIfEmpty('blah')); // $ExpectType Observable<string>
+})

spec/operators/defaultIfEmpty-spec.ts

@@ -35,17 +35,6 @@ describe('defaultIfEmpty', () => {
     });
   });
 
-  it('should return null if the Observable is empty and no arguments', () => {
-    testScheduler.run(({ cold, expectObservable, expectSubscriptions }) => {
-      const e1 = cold(' |');
-      const e1subs = '  (^!)';
-      const expected = '(x|)';
-
-      expectObservable(e1.pipe(defaultIfEmpty())).toBe(expected, { x: null });
-      expectSubscriptions(e1.subscriptions).toBe(e1subs);
-    });
-  });
-
   it('should return the Observable if not empty with a default value', () => {
     testScheduler.run(({ hot, expectObservable, expectSubscriptions }) => {
       const e1 = hot('  --a--b--|');
@@ -57,13 +46,13 @@ describe('defaultIfEmpty', () => {
     });
   });
 
-  it('should return the Observable if not empty with no default value', () => {
+  it('should allow undefined as a default value', () => {
     testScheduler.run(({ hot, expectObservable, expectSubscriptions }) => {
-      const e1 = hot('  --a--b--|');
+      const e1 = hot('  --------|');
       const e1subs = '  ^-------!';
-      const expected = '--a--b--|';
+      const expected = '--------(U|)';
 
-      expectObservable(e1.pipe(defaultIfEmpty())).toBe(expected);
+      expectObservable(e1.pipe(defaultIfEmpty(undefined))).toBe(expected, { U: undefined });
       expectSubscriptions(e1.subscriptions).toBe(e1subs);
     });
   });

src/internal/operators/defaultIfEmpty.ts

@@ -2,10 +2,6 @@ import { OperatorFunction } from '../types';
 import { operate } from '../util/lift';
 import { OperatorSubscriber } from './OperatorSubscriber';
 
-/* tslint:disable:max-line-length */
-export function defaultIfEmpty<T, R = T>(defaultValue?: R): OperatorFunction<T, T | R>;
-/* tslint:enable:max-line-length */
-
 /**
  * Emits a given value if the source Observable completes without emitting any
  * `next` value, otherwise mirrors the source Observable.
@@ -34,13 +30,13 @@ export function defaultIfEmpty<T, R = T>(defaultValue?: R): OperatorFunction<T,
  * @see {@link empty}
  * @see {@link last}
  *
- * @param {any} [defaultValue=null] The default value used if the source
+ * @param defaultValue The default value used if the source
  * Observable is empty.
- * @return {Observable} An Observable that emits either the specified
+ * @return An Observable that emits either the specified
  * `defaultValue` if the source Observable emits no items, or the values emitted
  * by the source Observable.
  */
-export function defaultIfEmpty<T, R>(defaultValue: R | null = null): OperatorFunction<T, T | R> {
+export function defaultIfEmpty<T, R>(defaultValue: R): OperatorFunction<T, T | R> {
   return operate((source, subscriber) => {
     let hasValue = false;
     source.subscribe(