Mapped type indexed access issue

[ahejlsberg]

Bug Report

An indexed access type { [P in K]: E }[X] should be assignable to an instantiation of E with X substituted for P.

This issue was originally reported here.

🔎 Search Terms

Mapped type indexed access.

🕗 Version & Regression Information

Nightly build

⏯ Playground Link

Playground link with relevant code

💻 Code

type ArgMap = { a: number, b: string };
type Func<K extends keyof ArgMap> = (x: ArgMap[K]) => void;
type Funcs = { [K in keyof ArgMap]: Func<K> };

function f1<K extends keyof ArgMap>(funcs: Funcs, key: K, arg: ArgMap[K]) {
    funcs[key](arg);
}

function f2<K extends keyof ArgMap>(funcs: Funcs, key: K, arg: ArgMap[K]) {
    const func = funcs[key];  // Type Funcs[K]
    func(arg);
}

function f3<K extends keyof ArgMap>(funcs: Funcs, key: K, arg: ArgMap[K]) {
    const func: Func<K> = funcs[key];  // Error, Funcs[K] not assignable to Func<K>
    func(arg);
}

🙁 Actual behavior

Unexpected error in example above.

🙂 Expected behavior

No errors in example above.