feat(groupBy): Add typeguards support for groupBy (#5441)

* feat(groupBy): Add typeguards support for groupBy

* fix: use T for type guard value

* test: add dtslint tests for groupBy type guard sig

Co-authored-by: Moshe Kolodny <kolodny@github.com>
Co-authored-by: Nicholas Jamieson <nicholas@cartant.com>

spec-dtslint/operators/groupBy-spec.ts

@@ -1,5 +1,5 @@
 import { of, Subject, GroupedObservable } from 'rxjs';
-import { groupBy } from 'rxjs/operators';
+import { groupBy, mergeMap } from 'rxjs/operators';
 
 it('should infer correctly', () => {
   const o = of(1, 2, 3).pipe(groupBy(value => value.toString())); // $ExpectType Observable<GroupedObservable<string, number>>
@@ -47,3 +47,38 @@ it('should enforce types of subject selector', () => {
   const o = of(1, 2, 3).pipe(groupBy(value => value.toString(), undefined, undefined, () => 'nope')); // $ExpectError
   const p = of(1, 2, 3).pipe(groupBy(value => value.toString(), undefined, undefined, (value) => new Subject<string>())); // $ExpectError
 });
+
+it('should support a user-defined type guard', () => {
+  function isNumber(value: string | number): value is number {
+    return typeof value === 'number';
+  }
+  const o = of('a', 1, 'b', 2).pipe(
+    groupBy(isNumber),
+    mergeMap((group) => {
+      if (group.key) {
+        const inferred = group; // $ExpectType GroupedObservable<true, number>
+        return inferred;
+      } else {
+        const inferred = group; // $ExpectType GroupedObservable<false, string>
+        return inferred;
+      }
+    })
+  );
+  const inferred = o; // $ExpectType Observable<string | number>
+});
+
+it('should support an inline user-defined type guard', () => {
+  const o = of('a', 1, 'b', 2).pipe(
+    groupBy((value): value is number => typeof value === 'number'),
+    mergeMap((group) => {
+      if (group.key) {
+        const inferred = group; // $ExpectType GroupedObservable<true, number>
+        return inferred;
+      } else {
+        const inferred = group; // $ExpectType GroupedObservable<false, string>
+        return inferred;
+      }
+    })
+  );
+  const inferred = o; // $ExpectType Observable<string | number>
+});

src/internal/operators/groupBy.ts

@@ -6,6 +6,7 @@ import { Subject } from '../Subject';
 import { OperatorFunction } from '../types';
 
 /* tslint:disable:max-line-length */
+export function groupBy<T, K extends T>(keySelector: (value: T) => value is K): OperatorFunction<T, GroupedObservable<true, K> | GroupedObservable<false, Exclude<T, K>>>;
 export function groupBy<T, K>(keySelector: (value: T) => K): OperatorFunction<T, GroupedObservable<K, T>>;
 export function groupBy<T, K>(keySelector: (value: T) => K, elementSelector: void, durationSelector: (grouped: GroupedObservable<K, T>) => Observable<any>): OperatorFunction<T, GroupedObservable<K, T>>;
 export function groupBy<T, K, R>(keySelector: (value: T) => K, elementSelector?: (value: T) => R, durationSelector?: (grouped: GroupedObservable<K, R>) => Observable<any>): OperatorFunction<T, GroupedObservable<K, R>>;