Incorrect overload selected for `Array.prototype.map(...)`

Bug Report

🔎 Search Terms

overload, array.prototype.map

🕗 Version & Regression Information

• This is the behavior in every version I tried.

⏯ Playground Link

Playground link with relevant code

💻 Code

declare type Obj = {a: true};
declare const arr: Array<string | Obj>;
declare function func(url: string | Obj): string;
declare function func<T extends Obj | string>(url: T): string | null;
const fails: string[] = arr.map(func);
// ^^^ Type '(string | null)[]' is not assignable to type 'string[]'.
const works: string[] = arr.map((v) => func(v));



interface CustomArray<T> {
    map<U>(callbackfn: (value: T) => U): U[];
}
declare const arr2: CustomArray<string | Obj>;
const fails2: string[] = arr2.map(func);
// ^^^ Type '(string | null)[]' is not assignable to type 'string[]'.
const works2: string[] = arr2.map((v) => func(v));

🙁 Actual behavior

It fails to select the correct overloard.

🙂 Expected behavior

It is supposed to select the correct overload.

[oleg-slapdash]

@RyanCavanaugh, this is an approximation. Basically, this is a helper function that gets a string property of an object or returns a string if a string is passed instead of an object.

type ObjWithFoo = {foo: string};

declare function getFoo(foo: string | ObjWithFoo): string;
declare function getFoo(url: ObjWithFoo | string | null | undefined): string | null | undefined;

declare const arr: Array<string | ObjWithFoo>;

export const works: string[] = arr.map((v) => getFoo(v));
export const fails: string[] = arr.map(getFoo);
// Err: ^^^ Type '(string | null | undefined)[]' is not assignable to type 'string[]'

[maple5233]

Conditional types will get instantiated, so I’d recommend this:

type ObjWithFoo = {foo: string};

declare function getFoo<T extends ObjWithFoo | string | null | undefined>(url: T): T extends undefined | null ? T : string;

declare const arr: Array<string | ObjWithFoo>;

export const works: string[] = arr.map((v) => getFoo(v));
export const alsoWorks: string[] = arr.map(getFoo);

declare const arrU: Array<string | ObjWithFoo | undefined>;
export const chk = arrU.map(getFoo); // Array<string | undefined>

Will this bug be fixed? The way using the conditional types looks like a work-around.