Union of Observables cannot have piped operators

RxJS version: I tested on both v5.5 and v6.0

Code to reproduce:

(foo as Observable<number>).pipe(filter(_=>true));
(foo as Observable<string>).pipe(filter(_=>true));
(foo as Observable<number|string>).pipe(filter(_=>true));
(foo as Observable<number>|Observable<string>).pipe(filter(_=>true));

Expected behavior: All build or all fail.

Actual behavior: Line 1-3 build, line 4 fails with “TS2554: Expected 0 arguments, but got 1.”

Additional information:

[cartant]

@bowenni The issue isn’t an RxJS problem. And I doubt that it’s a TypeScript problem, either; it’s just how the resolution of overload signatures works for union types.

To see what’s going on, have a look at the overload signatures for pipe. For Observable<number>, pipe will have signatures like:

pipe(): Observable<number>
pipe<A>(op1: OperatorFunction<number, A>): Observable<A>
...

And for Observable<string>, pipe will have signatures like:

pipe(): Observable<string>
pipe<A>(op1: OperatorFunction<string, A>): Observable<A>
...

For the union type - Observabe<number> | Observable<string> - the only pipe signature that has parameters common to both Observabe<number> and Observable<string> is the first - the signature with no parameters - so that’s the only one that’s available.

[lock[bot]]

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