microsoft / microsoft/TypeScript

Unable to infer method property but able to infer arrow function property

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#43,948 2 comments 0 reactions 1 assignee View on GitHub

@weswigham is already working on this.

Since May 13, 2021.

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Description

Bug Report

🔎 Search Terms

This issue came from StackOverflow question

🕗 Version & Regression Information

Since Object.fromEntries

⏯ Playground Link

Playground link with relevant code

💻 Code
const id = <T,>(x: T): T => x

const keys: string[] = []

/**
 * TS is unable to infer T generic argument of  fromEntries
 */
const fails = Object.fromEntries(keys.map(k => [
    k,
    id({ test() { } }) // <------ ERROR
]))

🙁 Actual behavior
const result: {
    [k: string]: T;
}
🙂 Expected behavior
const result: {
    [k: string]: {
        test(): void;
    };
}

I thought that it has smth to do with excess property checking, but it works if you replace id({ test() { } }) with id({ test:()=> { } })
Full list of workarounds:


/**
 * WORK
 */
const iter = keys.map(k => [
    k,
    id({ test() { } }) // ok
])
const obj1 = Object.fromEntries(iter)


const idResult = id({ test() { } })
const obj2 = Object.fromEntries(keys.map(k => [
    k,
    idResult
]))


const obj3 = Object.fromEntries(keys.map(k => [
    k,
    id({ test: () => { } }) // ok
]))


const obj4 = Object.fromEntries(
    keys.map<[string, { test(): void }]>((k) => [k, id({ test() { } })])
);


{
    interface Method {
        (): void;
    }
    const id = <T extends { [prop: string]: Method }>(x: T) => x;

    const keys: string[] = [];

    const obj = Object.fromEntries(keys.map((k) => [k, id({ test() { } })])); // ok

}

{
    type ArrowProp = () => any;
    const id = <T extends { [prop: string]: ArrowProp }>(x: T) => x;


    const obj = Object.fromEntries(keys.map((k) => [k, id({ test() { } })]));
}

{
    const id_ = <Prop, T extends Record<string, Prop>>(x: T) => x;
    // OR
    const id = <T extends object>(x: T) => x;
    const obj2 = Object.fromEntries(keys.map((k) => [k, id({ test() { } })]));
}

Playground

If I'm wrong and it is not a bug, could you please explain why TS is unable to infer method property?

I know that methods are bivariant, but I'm not sure if it has smth to do with current case

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