Wrong type inferred for indirect call of generic function

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").

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).

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)

Ryan Cavanaugh (@RyanCavanaugh)

Ryan Cavanaugh (@RyanCavanaugh) Why did you close this if the reported issue still persists?

Timm Preetz (@tp)

Timm Preetz (@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.

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

Navigation Menu

Repository navigation

Wrong type inferred for indirect call of generic function #29638

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Wrong type inferred for indirect call of generic function #29638

Description

Activity

weswigham commented on Jan 29, 2019

tp commented on Mar 5, 2019

tp commented on Mar 5, 2019

RyanCavanaugh commented on Mar 5, 2019

ahejlsberg commented on Mar 6, 2019

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions