Can we cut down on `Object.assign` overloads?

We can convert unions into intersections (thanks @jcalz!), and we can model variadic args pretty well, so I figured I'd try to model Object.assign with a single overload. Here's what I came up with:

// Maps elements of a tuple to contravariant inference sites.
type MapContravariant<T> = {
    [K in keyof T]: (x: T[K]) => void
}

type TupletoIntersection<T, Temp = MapContravariant<T>> =
    // Ensure we can index with a number.
    Temp extends Record<number, unknown>
        // Infer from every element now.
        ? Temp[number] extends (x: infer U) => unknown ? U : never
        : never;

declare function assign<T extends object, Rest extends object[]>(
    x: T,
    ...xs: Rest
): T & TupletoIntersection<Rest>;

Unfortunately this doesn't quite give the right results on the following:

let asdf = assign({x: "hello"}, Math.random() ? {x: "hello"} : {z: true });

Currently the type of asdf is:

| ({ x: string; } & { x: string; z?: undefined; })
| ({ x: string; } & { z: boolean; x?: undefined; })

What a beautiful type! Unfortunately you can see that the second element of the union tries to intersect types with conflicting properties for x which is nonsense.

Additionally, @weswigham pointed out that this really won't work for the case where you started off with just a plain array whose elements are eventually spread into Object.assign.

I do wonder if there's anything better we can do here.

この issue にはファイル、テスト、エントリポイントが記載されていません。まず issue にある提案された assign の型付けと、union および spread-array の例を再現し、その後、TypeScript の型チェッカーと標準ライブラリの宣言における既存の Object.assign のモデル化を追跡してください。これらのケースを競合する intersection なしで扱える、レビュー済みのアプローチができれば完了です。