function isObject<T>(it: T): it is T & Record<string, unknown> {
    return Object.prototype.toString.call(it) === "[object Object]";
}

declare function test(arg: Record<string, any>): void

declare const arg: (string | number | { a: "b" });

if (isObject(arg)) {
    // arg is:
    // - TS 4.7: {a: "b"}
    // - TS 4.8: (string | number | { a: "b" }) & Record<string, unknown>
    test({ ...arg }) // ERROR: Spread types may only be created from object types.(2698)
}