Inferred type does not work for generic reduce function with when not used as a lambda

[Roaders]

TypeScript Version: 3.1

Search Terms: type inferrence

Code

function reduceToPromise<T extends string>(promise: Promise<T>, items: string[]){
 return promise
}

const initialPromise: Promise<string> = {} as any;
const items: string[][] = [];

const reducedOne = items.reduce(reduceToPromise, initialPromise);
const reducedTwo = items.reduce((promise, items) => reduceToPromise(promise, items), initialPromise);

Playground

Expected behavior:
Both reducedOne and reducedTwo should be typed as Promise<string>

Actual behavior:
reducedOne is typed as {}

This issue only occurs when the function has a generic type. This works fine:

function reduceToPromise(promise: Promise<string>, items: string[]){
 return promise
}

const initialPromise: Promise<string> = {} as any;
const items: string[][] = [];

const reduced = items.reduce(reduceToPromise, initialPromise);

Playground

Simplified example:

function id<T>(x: T): T { return x; }
function callIt<T, U>(f: (t: T) => U, x: T): U { return f(x); }
const x = callIt(id, 0); // Infers `callIt<{}, {}>` so `x` is `{}`. No error if order of parameters is reversed.

[weswigham]

I think in the past we've called this a design limitation and told people to invert their parameter order; but because of what we've been looking at recently w.r.t. proper unification, I'll mark this as a bug instead and ping Anders Hejlsberg (@ahejlsberg) for input. You think this is worth reconsidering?

[ahejlsberg]

I'd love to reconsider, but it is a hard problem. An inference scheme based on unification would presumably get this example right, but we know from experiments that unification surfaces hard problems for which we don't have an answer. See #24626 and other issues linked in that thread.

We could consider tweaks to the existing algorithm to defer inference from arguments that have a generic function type (similar to what we do for function expressions with contextually typed parameters), but that would only work for simple generic function arguments not for structures containing generic functions values. So, not sure that's worthwhile.

[DanielRosenwasser]

Closing in favor of #9366.

[typescript-bot]

This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed for automatic house-keeping purposes.