test(dtslint): add dtslint test for defer observable static function (#…

…4093) (#4417)

spec-dtslint/observables/defer-spec.ts

@@ -0,0 +1,13 @@
+import { of, defer } from 'rxjs';
+
+it('should enforce function parameter', () => {
+  const a = defer(); // $ExpectError
+});
+
+it('should infer correctly with function return observable', () => {
+  const a = defer(() => of(1, 2, 3)); // $ExpectType Observable<number>
+});
+
+it('should infer correctly with function return promise', () => {
+  const a = defer(() => Promise.resolve(5)); // $ExpectType Observable<number>
+});