Wrong type inferred for indirect call of generic function

[tp]

TypeScript Version: 3.2.2

Search Terms: Generics, lookahead, type inference

Code

function call<P1, R>(f: (p1: P1) => R, p1: P1): R {
    return f(p1);
}

function identity<T>(o: T): T {
    return o;
}

const x = call(identity, 1);


// When the parameters are reversed, it works:

function callReversed<P1, R>(p1: P1, f: (p1: P1) => R): R {
    return f(p1);
}

const y = callReversed(1, identity);

Expected behavior:

x is of type number

Actual behavior:

x is of type {}
only y is of type number

Playground Link: Playground link

Related Issues: I didn't find anything related, but maybe I have been using the wrong search term ("lookahead").

[weswigham]

We don't do full unification - we match parameters from left to right (which is why reversing the order fixes the issue). It's a known design limitation - we've talked about improving it before (you can probably find related discussions by searching for topics about higher order functions).

[tp]

Would this also be fixed by #30215?

Edit: Ah, I guess not, because that PR states:

In particular, it only delivers the desired outcome when types flow from left to right.

Indeed this won't be supported yet with that change: #30215 (comment)

[tp]

@RyanCavanaugh Why did you close this if the reported issue still persists?

[RyanCavanaugh]

@tp Issues are closed if there isn't an identifiable next set of work to do. In this case, we believe the only next step would be a near-complete rewrite of generic inference, which is already tracked at #30134.

[ahejlsberg]

Actually, this issue is fixed with the latest commits in #30215.