fix(pluck): fix pluck's catch-all signature for better type safety (#5192)

* fix(pluck): fix pluck's catch-all signature for better type safety

Change pluck's catch-all signature that is causing troubles when the
passed string is not a key in the argument object.

Issue: #5188

* fix(pluck): fix signature and tests

Correct the signtaure to be accepted by tests and to return unknown in
general

* fix(pluck): add test case to check type inference

Author: Lonelind

spec-dtslint/operators/pluck-spec.ts

@@ -1,4 +1,4 @@
-import { of } from 'rxjs';
+import { of, Observable } from 'rxjs';
 import { pluck } from 'rxjs/operators';
 
 it('should infer correctly', () => {
@@ -41,6 +41,10 @@ it('should accept string only', () => {
   const a = of({ name: 'abc' }).pipe(pluck(1)); // $ExpectError
 });
 
+it('should not infer type from the variable if key doesn\'t exist', () => {
+  const a: Observable<number> = of({ name: 'abc' }).pipe(pluck('xyz')); // $ExpectError
+});
+
 it('should accept a spread of arguments', () => {
   const obj = {
     foo: {

src/internal/operators/pluck.ts

@@ -9,8 +9,8 @@ export function pluck<T, K1 extends keyof T, K2 extends keyof T[K1], K3 extends
 export function pluck<T, K1 extends keyof T, K2 extends keyof T[K1], K3 extends keyof T[K1][K2], K4 extends keyof T[K1][K2][K3]>(k1: K1, k2: K2, k3: K3, k4: K4): OperatorFunction<T, T[K1][K2][K3][K4]>;
 export function pluck<T, K1 extends keyof T, K2 extends keyof T[K1], K3 extends keyof T[K1][K2], K4 extends keyof T[K1][K2][K3], K5 extends keyof T[K1][K2][K3][K4]>(k1: K1, k2: K2, k3: K3, k4: K4, k5: K5): OperatorFunction<T, T[K1][K2][K3][K4][K5]>;
 export function pluck<T, K1 extends keyof T, K2 extends keyof T[K1], K3 extends keyof T[K1][K2], K4 extends keyof T[K1][K2][K3], K5 extends keyof T[K1][K2][K3][K4], K6 extends keyof T[K1][K2][K3][K4][K5]>(k1: K1, k2: K2, k3: K3, k4: K4, k5: K5, k6: K6): OperatorFunction<T, T[K1][K2][K3][K4][K5][K6]>;
-export function pluck<T, K1 extends keyof T, K2 extends keyof T[K1], K3 extends keyof T[K1][K2], K4 extends keyof T[K1][K2][K3], K5 extends keyof T[K1][K2][K3][K4], K6 extends keyof T[K1][K2][K3][K4][K5], R>(k1: K1, k2: K2, k3: K3, k4: K4, k5: K5, k6: K6, ...rest: string[]): OperatorFunction<T, R>;
-export function pluck<T, R= unknown>(...properties: string[]): OperatorFunction<T, R>;
+export function pluck<T, K1 extends keyof T, K2 extends keyof T[K1], K3 extends keyof T[K1][K2], K4 extends keyof T[K1][K2][K3], K5 extends keyof T[K1][K2][K3][K4], K6 extends keyof T[K1][K2][K3][K4][K5]>(k1: K1, k2: K2, k3: K3, k4: K4, k5: K5, k6: K6, ...rest: string[]): OperatorFunction<T, unknown>;
+export function pluck<T>(...properties: string[]): OperatorFunction<T, unknown>;
 /* tslint:enable:max-line-length */
 
 /**