fix(types): add Boolean signature to filter

spec-dtslint/operators/filter-spec.ts

@@ -1,5 +1,5 @@
-import { of } from 'rxjs';
-import { filter } from 'rxjs/operators';
+import { Observable, of } from 'rxjs';
+import { filter, map } from 'rxjs/operators';
 
 it('should support a predicate', () => {
   const o = of(1, 2, 3).pipe(filter(value => value < 3)); // $ExpectType Observable<number>
@@ -38,3 +38,22 @@ it('should enforce user-defined type guard types', () => {
   const o = of(1, 2, 3).pipe(filter((value: string): value is '1' => value < '3')); // $ExpectError
   const p = of(1, 2, 3).pipe(filter((value: number, index): value is 1 => index < '3')); // $ExpectError
 });
+
+it('should support Boolean as a predicate', () => {
+  const o = of(1, 2, 3).pipe(filter(Boolean)); // $ExpectType Observable<number>
+  const p = of(1, null, undefined).pipe(filter(Boolean)); // $ExpectType Observable<number>
+  const q = of(null, undefined).pipe(filter(Boolean)); // $ExpectType Observable<never>
+});
+
+// I've not been able to effect a failing dtslint test for this situation and a
+// conventional test won't fail because the TypeScript configuration isn't
+// sufficiently strict:
+// https://github.com/ReactiveX/rxjs/issues/4959#issuecomment-520629091
+it('should support inference from a return type with Boolean as a predicate', () => {
+  interface I {
+    a: string | null;
+  }
+
+  const i$: Observable<I> = of();
+  const s$: Observable<string> = i$.pipe(map(i => i.a), filter(Boolean)); // $ExpectType Observable<string>
+});

spec/operators/filter-spec.ts

@@ -330,4 +330,13 @@ describe('filter operator', () => {
 
     // tslint:disable enable
   });
+
+  it('should support Boolean as a predicate', () => {
+    const source = hot('-t--f--^-t-f-t-f--t-f--f--|', { t: 1, f: 0 });
+    const subs =              '^                  !';
+    const expected =          '--t---t----t-------|';
+
+    expectObservable(source.pipe(filter(Boolean))).toBe(expected, { t: 1, f: 0 });
+    expectSubscriptions(source.subscriptions).toBe(subs);
+  });
 });

src/internal/operators/filter.ts

@@ -4,6 +4,7 @@ import { Observable } from '../Observable';
 import { OperatorFunction, MonoTypeOperatorFunction, TeardownLogic } from '../types';
 
 /* tslint:disable:max-line-length */
+export function filter<T>(predicate: BooleanConstructor): OperatorFunction<T, NonNullable<T>>;
 export function filter<T, S extends T>(predicate: (value: T, index: number) => value is S,
                                        thisArg?: any): OperatorFunction<T, S>;
 export function filter<T>(predicate: (value: T, index: number) => boolean,