With the following example:

import { RequireAtLeastOne } from "type-fest";

type T = RequireAtLeastOne<{ a: string; b: string }>;

const getResult = (value: T) => value.a ?? value.b;

getResult has a return type of string | undefined, but I would expect it to have a return type of string, as if value.a is undefined, value.b should have a type of string? Would this be possible to achieve?

Note that I have tested this with the latest version of typescript at the time of writing (4.3.5).