[nightly][regression] Mapped type over generic key unexpectedly makes key treated as optional about typescript HOT 6 CLOSED

  type requiredKeys<T extends object> = {
    [k in keyof T]: undefined extends T[k] ? never : k;
  }[keyof T];

  export type addQuestionMarks<
    T extends object,
    R extends keyof T = requiredKeys<T>
  > = Pick<Required<T>, R> & Partial<T>;

type Obj = {
  a: string;
  b: number;
};

type Obj2 = {
  b: number;
  c: boolean;
};

declare const mapped: {
  [K in keyof (Partial<Obj> & Required<Obj2>)]: number;
};

// displays the same way as `resolved` below!
mapped;
// ^? const mapped: { a?: number | undefined; b: number; c: number; }

const accessMapped = <K extends keyof Obj2>(key: K) => mapped[key].toString(); // error, a mystery to the user

declare const resolved: { a?: number | undefined; b: number; c: number };

const accessResolved = <K extends keyof Obj2>(key: K) => resolved[key].toString(); // since this is OK

type EmptyObject = {[K in never]?: never};  // This is just `{}` ?

type BadData = { id: string, prop: string } & EmptyObject;

function badExample<K extends keyof BadData>(data: Pick<Pick<BadData, K | 'id'>, K>): Pick<BadData, K> {
    return data;
    //     ^^^^Same error
}

type Foo = {
    prop: string;
}

function test<K extends keyof Foo>(obj: Pick<Required<Foo> & Partial<Foo>, K>, key: K) {
    obj[key].length;  // Error: Object is possibly 'undefined'
}