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Wrong type inferred for indirect call of generic function #29638
Description
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").
Activity
weswigham commentedon Jan 29, 2019
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 commentedon Mar 5, 2019
Would this also be fixed by #30215?
Edit: Ah, I guess not, because that PR states:
Indeed this won't be supported yet with that change: #30215 (comment)
tp commentedon Mar 5, 2019
Ryan Cavanaugh (@RyanCavanaugh) Why did you close this if the reported issue still persists?
RyanCavanaugh commentedon Mar 5, 2019
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.
ahejlsberg commentedon Mar 6, 2019
Actually, this issue is fixed with the latest commits in #30215.