Parameters hints tooltip do not spread inferred arguments (inferred arguments names not visible).

[guillaume86]

TypeScript Version: typescript@3.2.0-dev.20181025

Search Terms:

• spread infer arguments
• infer argument name
• ArgumentType

Code

type Fn = (...args: any[]) => any;
type ArgumentTypes<T extends Fn> = T extends (...args: infer U) => any ? U : never;

function buildActionCreator<
  T extends string,
  P extends (...args: any[]) => any
>(type: T, payloadCreator?: P) {
    return (...args: ArgumentTypes<P>) => {
        const payload = payloadCreator(...args) as ReturnType<P>;
        return {
            type,
            payload,
        };
    };
};

// if you hover here on setAge, the inferred arguments
// are spreaded and their names preserved: (age: number) => { ... }
const setAge = buildActionCreator("SET_AGE", (age: number) => ({ age }));

// the toolip showing function signatures when typing the open parenthesis
// do not spread arguments, the arguments names are lost: (...args: [number]) => { ... }  
// note that the hover on function tooltip still works ok
setAge(10);

Expected behavior:

When typing the opening parenthesis of setAge(, the tooltip showing function overloads should spread the inferred arguments and show the argument names like (age: number) => { ... }.

Actual behavior:

When typing the opening parenthesis of setAge(, the tooltip showing function overloads do not spread the inferred arguments and so the argument names are not visible (...args: [number]) => { ... }.

Playground Link:
http://www.typescriptlang.org/play/#src=type%20Fn%20%3D%20(...args%3A%20any%5B%5D)%20%3D%3E%20any%3B%0D%0Atype%20ArgumentTypes%3CT%20extends%20Fn%3E%20%3D%20T%20extends%20(...args%3A%20infer%20U)%20%3D%3E%20any%20%3F%20U%20%3A%20never%3B%0D%0A%0D%0Afunction%20buildActionCreator%3C%0D%0A%20%20T%20extends%20string%2C%0D%0A%20%20P%20extends%20(...args%3A%20any%5B%5D)%20%3D%3E%20any%0D%0A%3E(type%3A%20T%2C%20payloadCreator%3F%3A%20P)%20%7B%0D%0A%20%20%20%20return%20(...args%3A%20ArgumentTypes%3CP%3E)%20%3D%3E%20%7B%0D%0A%20%20%20%20%20%20%20%20const%20payload%20%3D%20payloadCreator(...args)%20as%20ReturnType%3CP%3E%3B%0D%0A%20%20%20%20%20%20%20%20return%20%7B%0D%0A%20%20%20%20%20%20%20%20%20%20%20%20type%2C%0D%0A%20%20%20%20%20%20%20%20%20%20%20%20payload%2C%0D%0A%20%20%20%20%20%20%20%20%7D%3B%0D%0A%20%20%20%20%7D%3B%0D%0A%7D%3B%0D%0A%0D%0A%2F%2F%20if%20you%20hover%20here%20on%20setAge%2C%20the%20inferred%20arguments%0D%0A%2F%2F%20are%20spreaded%20and%20their%20names%20preserved%3A%20(age%3A%20number)%20%3D%3E%20%7B%20...%20%7D%0D%0Aconst%20setAge%20%3D%20buildActionCreator(%22SET_AGE%22%2C%20(age%3A%20number)%20%3D%3E%20(%7B%20age%20%7D))%3B%0D%0A%0D%0A%2F%2F%20the%20toolip%20showing%20function%20signatures%20when%20typing%20the%20open%20parenthesis%0D%0A%2F%2F%20do%20not%20spread%20arguments%2C%20the%20arguments%20names%20are%20lost%3A%20(...args%3A%20%5Bnumber%5D)%20%3D%3E%20%7B%20...%20%7D%20%20%0D%0A%2F%2F%20note%20that%20the%20hover%20on%20function%20tooltip%20still%20works%20ok%0D%0AsetAge(10)%3B

Related Issues:

[AlexGalays]

Just hit this issue as well. It's not uncommon to just map the return type of a function while leaving the arguments untouched.

[dragomirtitian]

This seems like a duplicate of #26563

[guillaume86]

Thanks seems like it is, should close and mark as duplicate. I'll go upvote that one.