4.2 regression: TS2345: argument is not assignable to a parameter of a generic type

[Igorbek]

Bug Report

Mapped type's element is not assignable to type T | undefined where T is unconstrained

🔎 Search Terms

mapped types, TS2345, generic, not assignable

🕗 Version & Regression Information

Regression in TS 4.2.3

⏯ Playground Link

Playground link with relevant code

💻 Code

type MyMap<M extends object> = {
    [K in keyof M]: {
        x: number
    }
}

declare function g<T>(value?: T): void;

function f<M extends object>(mymap: MyMap<M>, k: keyof any) {
    const elemofM = mymap[k as keyof M]
    g(elemofM)
    //~~~~~~~ TS2345: Argument of type '{ x: number; }' is not assignable to parameter of type 'MyMap<M>[keyof M]'.
}

🙁 Actual behavior

error TS2345: Argument of type '{ x: number; }' is not assignable to parameter of type 'MyMap[keyof M]'.

🙂 Expected behavior

No error (it was not in TS 4.1)

[ahejlsberg]

Repro that fails in both 4.1 and 4.2:

function f<M extends object>(mymap: MyMap<M>, k: keyof M, z: { x: number }) {
    const q1: MyMap<M>[keyof M] = z;  // Ok
    const q2: MyMap<M>[keyof M] | undefined = z;  // Error
    const q3: MyMap<M>[keyof M] | string = z;  // Ok
}

It only fails for unions with undefined or null. The issue is a missing normalization step in code that eliminates undefined and null from target unions in order to improve the error reporting experience.