fix(TS): fix type inference for defaultIfEmpty.

spec-dtslint/operators/defaultIfEmpty-spec.ts

@@ -1,5 +1,5 @@
 import { of } from 'rxjs';
-import { defaultIfEmpty } from 'rxjs/operators';
+import { defaultIfEmpty, map } from 'rxjs/operators';
 
 it('should infer correctly', () => {
   const o = of(1, 2, 3).pipe(defaultIfEmpty()); // $ExpectType Observable<number>
@@ -13,6 +13,10 @@ 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>
 });
 
+it('should infer correctly with a subtype passed through parameters', () => {
+  const o = of(true, false).pipe(map(p => p), defaultIfEmpty(true)); // $ExpectType Observable<boolean>
+});
+
 it('should enforce types', () => {
   const o = of(1, 2, 3).pipe(defaultIfEmpty(4, 5)); // $ExpectError
 });

src/internal/operators/defaultIfEmpty.ts

@@ -4,8 +4,7 @@ import { Subscriber } from '../Subscriber';
 import { OperatorFunction, MonoTypeOperatorFunction } from '../types';
 
 /* tslint:disable:max-line-length */
-export function defaultIfEmpty<T>(defaultValue?: T): MonoTypeOperatorFunction<T>;
-export function defaultIfEmpty<T, R>(defaultValue?: R): OperatorFunction<T, T | R>;
+export function defaultIfEmpty<T, R = T>(defaultValue?: R): OperatorFunction<T, T | R>;
 /* tslint:enable:max-line-length */
 
 /**

src/internal/operators/first.ts

@@ -84,6 +84,6 @@ export function first<T, D>(
   return (source: Observable<T>) => source.pipe(
     predicate ? filter((v, i) => predicate(v, i, source)) : identity,
     take(1),
-    hasDefaultValue ? defaultIfEmpty<T | D>(defaultValue) : throwIfEmpty(() => new EmptyError()),
+    hasDefaultValue ? defaultIfEmpty<T, D>(defaultValue) : throwIfEmpty(() => new EmptyError()),
   );
 }

src/internal/operators/last.ts

@@ -47,6 +47,6 @@ export function last<T, D>(
   return (source: Observable<T>) => source.pipe(
     predicate ? filter((v, i) => predicate(v, i, source)) : identity,
     takeLast(1),
-    hasDefaultValue ? defaultIfEmpty<T | D>(defaultValue) : throwIfEmpty(() => new EmptyError()),
+    hasDefaultValue ? defaultIfEmpty<T, D>(defaultValue) : throwIfEmpty(() => new EmptyError()),
   );
 }