// Literal version
type U1 = {
    foo: (arg: "foo") => void;
    bar: (arg: "bar") => void;
}

// Mapped type version
type U2 = { [K in 'foo' | 'bar']: (arg: K) => void }
//   ^? type U2 = { foo: (arg: "foo") => void; bar: (arg: "bar") => void; }

type Is_U2_equivalent_to_U1 = U1 extends U2 ? U2 extends U1 ? true : false : false;
//   ^? type Is_U2_equivalent_to_U1 = true

function fn1<T extends 'foo' | 'bar'>(cb: {[K in T]: U1[K]}[T], arg: T): void {
    cb(arg); // correct error
}

function fn2<T extends 'foo' | 'bar'>(cb: {[K in T]: U2[K]}[T], arg: T): void {
    cb(arg); // // correct error before 4.6, compiles after 4.6
}

function fn3<T extends 'foo' | 'bar'>(cb: {[K in T]: (arg: K) => void}[T], arg: T): void {
    cb(arg); // should error but compiles
}

function fn4<T extends 'foo' | 'bar'>(cb: T extends unknown ? (arg: T) => void : never, arg: T): void {
    cb(arg); // correct error
}