Cannot use Parameters in generic function

[akwodkiewicz]

I cannot use generic fn's Parameters when calling fn.

TypeScript Version: 3.9.2

Search Terms:
wrong generic function infer parameters

Code

const functionDict = {
    A: (a: any) => ({ a }),
    B: (a: any, b: string) => ({ a, b }),
    C: (a: any, b: string, c: string) => ({ a, b, c }),
};

type ValueOf<T> = T[keyof T];

export function foo<F extends ValueOf<typeof functionDict>>(
    fn: F,
    pp: Parameters<F>,
): void {
    fn(...pp);
}

Expected behavior:
No error.

Actual behavior:
Error: Expected 3 arguments, but got 0 or more.(2556)
input.ts(4, 9): An argument for 'a' was not provided

Playground Link: https://www.typescriptlang.org/play/#code/MYewdgzgLgBAZgVzMKBLcARVKYF4YDeAsAFAzkwCCAXDABQCGtDYAngJR4B89BMDMAL7sANKQowAQrUbM2ImACNa0AE6owAc064edPgwWKho8RQDCMpv3lKVUdVoXB7j7d178jzk2JKCAblJSKFYABwBTGAA1BgAbBAiAeTgAHgAVHnx0gG0AawjWEDgYdIBdIJJSCIAPMJBVWERkNHB4EBBUgDEYWqgIsAATCBj4xJTU0Mji+CQUdDAsFC4uOjNyODBaLr8JMLDaAAUGVQYAWwj+1Qhurj92WgA3EFRBwnX4MDoAOl-99kqglIQA

Related Issues:

[jcalz]

The generic constraint T extends X | Y does not mean "T extends X or T extends Y". It could be the case that T is X | Y, which extends neither X nor Y in general. Without a feature like #27808 there is no way in TypeScript to express a constraint of the form "T extends exactly_one_member_of X | Y".

In foo() the generic type F could turn out to be, for example, typeof functionDict.A | typeof functionDict.B:

const fn = Math.random() < 0.5 ?
    (a: any) => ({ a: 123 }) :
    (a: any, b: string) => ({ a: 123, b: b.toUpperCase() });
foo(fn, ["a"]); // 50% chance of "b is undefined" error

So it is technically unsound to allow fn(...pp) inside the function implementation without error. Of course, with some refactoring you could get the compiler to accept what you're doing inside the implementation:

function goo<A extends any[]>(fn: Extract<ValueOf<typeof functionDict>, (...a: A) => any>, pp: A) {
    fn(...pp); // okay
}

goo(fn, ["a"]); // error at compile time now

but that's not exactly what you're looking for, I gather.

Playground link to code

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.