fix(scan): fixed declarations to properly support different return types

this solves an issue where if you would reduce over `T` you can get `T` out reliably, but you could
not get out `R` in anyway without manually typing the reduce method call

compat/operator/reduce.ts

@@ -2,9 +2,9 @@ import { Observable } from 'rxjs';
 import { reduce as higherOrderReduce } from 'rxjs/operators';
 
 /* tslint:disable:max-line-length */
-export function reduce<T>(this: Observable<T>, accumulator: (acc: T, value: T, index: number) => T, seed?: T): Observable<T>;
-export function reduce<T>(this: Observable<T>, accumulator: (acc: T[], value: T, index: number) => T[], seed: T[]): Observable<T[]>;
 export function reduce<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed: R): Observable<R>;
+export function reduce<T>(this: Observable<T>, accumulator: (acc: T, value: T, index: number) => T, seed?: T): Observable<T>;
+export function reduce<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R): Observable<R>;
 /* tslint:enable:max-line-length */
 
 /**
@@ -51,7 +51,7 @@ export function reduce<T, R>(this: Observable<T>, accumulator: (acc: R, value: T
  * @method reduce
  * @owner Observable
  */
-export function reduce<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index?: number) => R, seed?: R): Observable<R> {
+export function reduce<T, R>(this: Observable<T>, accumulator: (acc: T | R, value: T, index?: number) => (T | R), seed?: R): Observable<T | R> {
   // providing a seed of `undefined` *should* be valid and trigger
   // hasSeed! so don't use `seed !== undefined` checks!
   // For this reason, we have to check it here at the original call site
@@ -61,5 +61,5 @@ export function reduce<T, R>(this: Observable<T>, accumulator: (acc: R, value: T
     return higherOrderReduce(accumulator, seed)(this);
   }
 
-  return higherOrderReduce(accumulator)(this);
+  return higherOrderReduce<T, T | R>(accumulator)(this);
 }

compat/operator/scan.ts

@@ -3,9 +3,9 @@ import { Observable } from 'rxjs';
 import { scan as higherOrderScan } from 'rxjs/operators';
 
 /* tslint:disable:max-line-length */
+export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed: R): Observable<R>;
 export function scan<T>(this: Observable<T>, accumulator: (acc: T, value: T, index: number) => T, seed?: T): Observable<T>;
-export function scan<T>(this: Observable<T>, accumulator: (acc: T[], value: T, index: number) => T[], seed?: T[]): Observable<T[]>;
-export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed?: R): Observable<R>;
+export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R): Observable<R>;
 /* tslint:enable:max-line-length */
 
 /**
@@ -46,10 +46,10 @@ export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T,
  * @owner Observable
  */
 export function scan<T, R>(this: Observable<T>,
-                           accumulator: (acc: T | Array<T> | R, value: T, index: number) => T | Array<T> | R,
-                           seed?: T | R): Observable<R> {
+                           accumulator: (acc: T | R, value: T, index: number) => T | R,
+                           seed?: T | R): Observable<T | R> {
   if (arguments.length >= 2) {
-    return higherOrderScan(accumulator, seed)(this) as Observable<R>;
+    return higherOrderScan(accumulator, seed)(this);
   }
-  return higherOrderScan(accumulator)(this) as Observable<R>;
+  return higherOrderScan<T, T | R>(accumulator)(this);
 }

spec-dtslint/operators/reduce-spec.ts

@@ -15,5 +15,17 @@ it('should infer correctly for accumulator of type array', () => {
 
 it('should accept seed parameter of the same type', () => {
   const a = of(1, 2, 3).pipe(reduce((x, y, z) => x + 1, 5)); // $ExpectType Observable<number>
-  const b = of(1, 2, 3).pipe(reduce((x, y, z) => x + 1, '5')); // $ExpectError
+  const b = of(1, 2, 3).pipe(reduce((x, y, z) => x + 1, [])); // $ExpectError
+});
+
+it('should accept seed parameter of the seed array type', () => {
+  const a = of(1, 2, 3).pipe(reduce((x, y, z) => { x.push(y); return x; }, [4])); // $ExpectType Observable<number[]>
+  // Array must be typed...
+  const b = of(1, 2, 3).pipe(reduce((x, y, z) => { x.push(y); return x; }, [])); // $ExpectError
+});
+
+it('should accept seed parameter of a different type', () => {
+  const a = of(1, 2, 3).pipe(reduce((x, y, z) => x + '1', '5')); // $ExpectType Observable<string>
+  const bv: { [key: string]: string } = {};
+  const b = of(1, 2, 3).pipe(reduce((x, y, z) => ({ ...x, [y]: y.toString() }), bv)); // $ExpectType Observable<{ [key: string]: string; }>
 });

spec-dtslint/operators/scan-spec.ts

@@ -0,0 +1,31 @@
+import { of, Observable } from 'rxjs';
+import { scan } from 'rxjs/operators';
+
+it('should enforce parameter', () => {
+  const a = of(1, 2, 3).pipe(scan()); // $ExpectError
+});
+
+it('should infer correctly ', () => {
+  const a = of(1, 2, 3).pipe(scan((x, y, z) => x + 1)); // $ExpectType Observable<number>
+});
+
+it('should infer correctly for accumulator of type array', () => {
+  const a = of(1, 2, 3).pipe(scan((x: number[], y: number, i: number) => x, [])); // $ExpectType Observable<number[]>
+});
+
+it('should accept seed parameter of the same type', () => {
+  const a = of(1, 2, 3).pipe(scan((x, y, z) => x + 1, 5)); // $ExpectType Observable<number>
+  const b = of(1, 2, 3).pipe(scan((x, y, z) => x + 1, [])); // $ExpectError
+});
+
+it('should accept seed parameter of the seed array type', () => {
+  const a = of(1, 2, 3).pipe(scan((x, y, z) => { x.push(y); return x; }, [4])); // $ExpectType Observable<number[]>
+  // Array must be typed...
+  const b = of(1, 2, 3).pipe(scan((x, y, z) => { x.push(y); return x; }, [])); // $ExpectError
+});
+
+it('should accept seed parameter of a different type', () => {
+  const a = of(1, 2, 3).pipe(scan((x, y, z) => x + '1', '5')); // $ExpectType Observable<string>
+  const bv: { [key: string]: string } = {};
+  const b = of(1, 2, 3).pipe(scan((x, y, z) => ({ ...x, [y]: y.toString() }), bv)); // $ExpectType Observable<{ [key: string]: string; }>
+});

spec/operators/reduce-spec.ts

@@ -292,7 +292,7 @@ describe('reduce operator', () => {
 
   type('should accept array typed reducers', () => {
     let a: Observable<{ a: number; b: string }>;
-    a.pipe(reduce<{ a: number; b: string }>((acc, value) => acc.concat(value), []));
+    a.pipe(reduce((acc, value) => acc.concat(value), []));
   });
 
   type('should accept T typed reducers', () => {
@@ -322,7 +322,7 @@ describe('reduce operator', () => {
 
   type('should accept R typed reduces when R is an array of T', () => {
     let a: Observable<number>;
-    const reduced = a.pipe(reduce<number>((acc, value) => {
+    const reduced = a.pipe(reduce((acc, value) => {
       acc.push(value);
       return acc;
     }, []));

spec/operators/scan-spec.ts

@@ -230,12 +230,12 @@ describe('scan operator', () => {
 
   type('should accept array typed reducers', () => {
     let a: Observable<{ a: number; b: string }>;
-    a.pipe(reduce<{ a: number; b: string }>((acc, value) => acc.concat(value), []));
+    a.pipe(scan((acc, value) => acc.concat(value), []));
   });
 
   type('should accept T typed reducers', () => {
     let a: Observable<{ a?: number; b?: string }>;
-    a.pipe(reduce((acc, value) => {
+    a.pipe(scan((acc, value) => {
       value.a = acc.a;
       value.b = acc.b;
       return acc;
@@ -244,7 +244,7 @@ describe('scan operator', () => {
 
   type('should accept R typed reducers', () => {
     let a: Observable<{ a: number; b: string }>;
-    a.pipe(reduce<{ a?: number; b?: string }>((acc, value) => {
+    a.pipe(scan<{ a?: number; b?: string }>((acc, value) => {
       value.a = acc.a;
       value.b = acc.b;
       return acc;

src/internal/operators/reduce.ts

@@ -6,9 +6,9 @@ import { OperatorFunction, MonoTypeOperatorFunction } from '../types';
 import { pipe } from '../util/pipe';
 
 /* tslint:disable:max-line-length */
+export function reduce<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed: R): OperatorFunction<T, R>;
 export function reduce<T>(accumulator: (acc: T, value: T, index: number) => T, seed?: T): MonoTypeOperatorFunction<T>;
-export function reduce<T>(accumulator: (acc: T[], value: T, index: number) => T[], seed: T[]): OperatorFunction<T, T[]>;
-export function reduce<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed?: R): OperatorFunction<T, R>;
+export function reduce<T, R>(accumulator: (acc: R, value: T, index: number) => R): OperatorFunction<T, R>;
 /* tslint:enable:max-line-length */
 
 /**
@@ -62,20 +62,20 @@ export function reduce<T, R>(accumulator: (acc: R, value: T, index: number) => R
  * @method reduce
  * @owner Observable
  */
-export function reduce<T, R>(accumulator: (acc: R, value: T, index?: number) => R, seed?: R): OperatorFunction<T, R> {
+export function reduce<T, R>(accumulator: (acc: T | R, value: T, index?: number) => T | R, seed?: T | R): OperatorFunction<T, T | R> {
   // providing a seed of `undefined` *should* be valid and trigger
   // hasSeed! so don't use `seed !== undefined` checks!
   // For this reason, we have to check it here at the original call site
   // otherwise inside Operator/Subscriber we won't know if `undefined`
   // means they didn't provide anything or if they literally provided `undefined`
   if (arguments.length >= 2) {
-    return function reduceOperatorFunctionWithSeed(source: Observable<T>): Observable<R> {
+    return function reduceOperatorFunctionWithSeed(source: Observable<T>): Observable<T | R> {
       return pipe(scan(accumulator, seed), takeLast(1), defaultIfEmpty(seed))(source);
     };
   }
-  return function reduceOperatorFunction(source: Observable<T>): Observable<R> {
+  return function reduceOperatorFunction(source: Observable<T>): Observable<T | R> {
     return pipe(
-      scan((acc: R, value: T, index: number): R => accumulator(acc, value, index + 1)),
+      scan<T, T | R>((acc, value, index) => accumulator(acc, value, index + 1)),
       takeLast(1),
     )(source);
   };

src/internal/operators/scan.ts

@@ -4,9 +4,9 @@ import { Subscriber } from '../Subscriber';
 import { OperatorFunction, MonoTypeOperatorFunction } from '../types';
 
 /* tslint:disable:max-line-length */
+export function scan<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed: R): OperatorFunction<T, R>;
 export function scan<T>(accumulator: (acc: T, value: T, index: number) => T, seed?: T): MonoTypeOperatorFunction<T>;
-export function scan<T>(accumulator: (acc: T[], value: T, index: number) => T[], seed?: T[]): OperatorFunction<T, T[]>;
-export function scan<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed?: R): OperatorFunction<T, R>;
+export function scan<T, R>(accumulator: (acc: R, value: T, index: number) => R): OperatorFunction<T, R>;
 /* tslint:enable:max-line-length */
 
 /**