types inferring problem with scan operator

[ghetolay]

I'm having a problem when using scan with 2 different type for item and accumulator/seed on ts 2.5+ (didn't try older versions).
You can see it here : https://stackblitz.com/edit/ts-fn-overload-bug?file=test.ts

of('0').pipe(
  scan((acc, str) => acc + parseInt(str), 0)
);

gives the following error :

Argument of type 'UnaryFunction<Observable, Observable>' is not assignable to parameter of type 'UnaryFunction<Observable, Observable>'.
Types of parameters 'source' and 'source' are incompatible.
Type 'Observable' is not assignable to type 'Observable'.
Type 'string' is not assignable to type 'number'.

Even if scan has a signature matching that use case (the last one) :

function scan<T>(accumulator: (acc: T, value: T, index: number) => T, seed?: T): MonoTypeOperatorFunction<T>;
function scan<T>(accumulator: (acc: T[], value: T, index: number) => T[], seed?: T[]): OperatorFunction<T, T[]>;
function scan<T, R>(accumulator: (acc: R, value: T, index: number) => R, seed?: R): OperatorFunction<T, R>;

As it's shown on the stackblitz, order seems to matter so it's most likely a ts bug.
But looking closely on those types, @NaridaL pointed out that the last signature should be enough to cover all cases. Indeed R can totally be inferred with the same type as T or T[].

So the question is : is there any reason for having those subtype signatures ? especially since now it seems to create a bug ?

P.S: Couldn't find a corresponding ts issue but didn't do much digging
P.S2: I think the same applies for reduce.

[cartant]

Probably related to #2897. See discussion therein.

[ghetolay]

Oh sorry, I missed that.

But now there is a game changer as it's causing errors. I'll dig into typescripts issues see if I can find something.

[ghetolay]

I thought the problem was because I updated my ts but it's not, it's because I refactored my code to use pipe.
I can go as low as 2.0 with that code and I got the same error while the non-pipeable version, with this on the signature, works fine.

I found this new issue microsoft/TypeScript#20305 but I'm not sure it's exactly the same problem.

[benlesh]

This has been addressed to the best of our ability in RxJS 7.