fix(pluck): key union type strictness (#4585)

Author: imcotton

compat/operator/pluck.ts

@@ -28,5 +28,6 @@ import { pluck as higherOrder } from 'rxjs/operators';
  * @owner Observable
  */
 export function pluck<T, R>(this: Observable<T>, ...properties: string[]): Observable<R> {
+  // @ts-ignore
   return higherOrder(...properties)(this) as Observable<R>;
 }

spec-dtslint/operators/pluck-spec.ts

@@ -29,12 +29,12 @@ it('should support nested object of more than 6 layer depth', () => {
   const a = of({ a: { b: { c: { d: { e: { f: { name: 'abc' } } } } } } }).pipe(pluck('a', 'b', 'c', 'd', 'e', 'f', 'name')); // $ExpectType Observable<{}>
 });
 
-it('should infer empty interface for non-existance key', () => {
-  const a = of({ name: 'abc' }).pipe(pluck('xyz')); // $ExpectType Observable<{}>
+it('should accept existing keys only', () => {
+  const a = of({ name: 'abc' }).pipe(pluck('xyz')); // $ExpectError
 });
 
-it('should infer empty interface for empty parameter', () => {
-  const a = of({ name: 'abc' }).pipe(pluck()); // $ExpectType Observable<{}>
+it('should not accept empty parameter', () => {
+  const a = of({ name: 'abc' }).pipe(pluck()); // $ExpectError
 });
 
 it('should accept string only', () => {

spec/operators/pluck-spec.ts

@@ -82,13 +82,15 @@ describe('pluck operator', () => {
     const expected = '--r-x--y-z---w-|';
     const values: { [key: string]: number | undefined } = {r: 1, x: undefined, y: undefined, z: undefined, w: 5};
 
+    // @ts-ignore
     const r = a.pipe(pluck('a', 'b', 'c'));
     expectObservable(r).toBe(expected, values);
     expectSubscriptions(a.subscriptions).toBe(asubs);
   });
 
   it('should throw an error if not property is passed', () => {
     expect(() => {
+      // @ts-ignore
       of({prop: 1}, {prop: 2}).pipe(pluck());
     }).to.throw(Error, 'list of properties cannot be empty.');
   });
@@ -98,6 +100,7 @@ describe('pluck operator', () => {
     const asubs =    '(^!)';
     const expected = '#';
 
+    // @ts-ignore
     const r = a.pipe(pluck('whatever'));
     expectObservable(r).toBe(expected);
     expectSubscriptions(a.subscriptions).toBe(asubs);
@@ -120,6 +123,7 @@ describe('pluck operator', () => {
 
     const invoked = 0;
     const r = a.pipe(
+      // @ts-ignore
       pluck('whatever'),
       tap(null, null, () => {
         expect(invoked).to.equal(0);
@@ -169,7 +173,7 @@ describe('pluck operator', () => {
 
     const r = a.pipe(
       mergeMap((x: { prop: string }) => of(x)),
-      pluck<{ prop: string }, string>('prop'),
+      pluck('prop'),
       mergeMap((x: string) => of(x))
     );
 

src/internal/operators/pluck.ts

@@ -9,7 +9,7 @@ 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, R>(...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], R>(k1: K1, k2: K2, k3: K3, k4: K4, k5: K5, k6: K6, ...rest: string[]): OperatorFunction<T, R>;
 /* tslint:enable:max-line-length */
 
 /**