fix(pipe): replace rest parameters overload

Replace the rest parameters overload with a signature that also includes
the A-I type parameters.

Closes ReactiveX#3841

src/internal/Observable.ts

@@ -296,7 +296,7 @@ export class Observable<T> implements Subscribable<T> {
   pipe<A, B, C, D, E, F, G>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>): Observable<G>;
   pipe<A, B, C, D, E, F, G, H>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>, op8: OperatorFunction<G, H>): Observable<H>;
   pipe<A, B, C, D, E, F, G, H, I>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>, op8: OperatorFunction<G, H>, op9: OperatorFunction<H, I>): Observable<I>;
-  pipe<R>(...operations: OperatorFunction<any, any>[]): Observable<R>;
+  pipe<A, B, C, D, E, F, G, H, I, R>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>, op8: OperatorFunction<G, H>, op9: OperatorFunction<H, I>, ...operations: OperatorFunction<any, any>[]): Observable<R>;
   /* tslint:enable:max-line-length */
 
   /**
@@ -318,7 +318,7 @@ export class Observable<T> implements Subscribable<T> {
    *   .subscribe(x => console.log(x))
    * ```
    */
-  pipe<R>(...operations: OperatorFunction<T, R>[]): Observable<R> {
+  pipe<R>(...operations: OperatorFunction<any, any>[]): Observable<R> {
     if (operations.length === 0) {
       return this as any;
     }

type-specs/Observable-spec.ts

@@ -0,0 +1,94 @@
+import { of, OperatorFunction } from 'rxjs';
+import { mapTo } from 'rxjs/operators';
+
+function a<I extends string, O extends string>(input: I, output: O): OperatorFunction<I, O>;
+function a<I, O extends string>(output: O): OperatorFunction<I, O>;
+function a<I, O extends string>(inputOrOutput: I | O, output?: O): OperatorFunction<I, O> {
+  return mapTo<I, O>(output === undefined ? inputOrOutput as O : output);
+}
+
+describe('pipe', () => {
+  it('should infer for no arguments', () => {
+    const o = of('foo').pipe(); // $ExpectType Observable<string>
+  });
+
+  it('should infer for 1 argument', () => {
+    const o = of('foo').pipe(a('1')); // $ExpectType Observable<"1">
+  });
+
+  it('should infer for 2 arguments', () => {
+    const o = of('foo').pipe(a('1'), a('2')); // $ExpectType Observable<"2">
+  });
+
+  it('should infer for 3 arguments', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3')); // $ExpectType Observable<"3">
+  });
+
+  it('should infer for 4 arguments', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4')); // $ExpectType Observable<"4">
+  });
+
+  it('should infer for 5 arguments', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5')); // $ExpectType Observable<"5">
+  });
+
+  it('should infer for 6 arguments', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('6')); // $ExpectType Observable<"6">
+  });
+
+  it('should infer for 7 arguments', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('6'), a('7')); // $ExpectType Observable<"7">
+  });
+
+  it('should infer for 8 arguments', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('6'), a('7'), a('8')); // $ExpectType Observable<"8">
+  });
+
+  it('should infer for 9 arguments', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('6'), a('7'), a('8'), a('9')); // $ExpectType Observable<"9">
+  });
+
+  it('should infer {} for more than 9 arguments', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('6'), a('7'), a('8'), a('9'), a('10')); // $ExpectType Observable<{}>
+  });
+
+  it('should enforce types for the 1st argument', () => {
+    const o = of('foo').pipe(a('#', '1')); // $ExpectError
+  });
+
+  it('should enforce types for the 2nd argument', () => {
+    const o = of('foo').pipe(a('1'), a('#', '2')); // $ExpectError
+  });
+
+  it('should enforce types for the 3rd argument', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('#', '3')); // $ExpectError
+  });
+
+  it('should enforce types for the 4th argument', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('#', '4')); // $ExpectError
+  });
+
+  it('should enforce types for the 5th argument', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('#', '5')); // $ExpectError
+  });
+
+  it('should enforce types for the 6th argument', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('#', '6')); // $ExpectError
+  });
+
+  it('should enforce types for the 7th argument', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('6'), a('#', '7')); // $ExpectError
+  });
+
+  it('should enforce types for the 8th argument', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('6'), a('7'), a('#', '8')); // $ExpectError
+  });
+
+  it('should enforce types for the 9th argument', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('6'), a('7'), a('8'), a('#', '9')); // $ExpectError
+  });
+
+  it('should not enforce types beyond the 9th argument', () => {
+    const o = of('foo').pipe(a('1'), a('2'), a('3'), a('4'), a('5'), a('6'), a('7'), a('8'), a('9'), a('#', '10')); // $ExpectType Observable<{}>
+  });
+});