typescript: extending interface

extend.js

import * as $ from "jquery";

function extend<T extends U, U>(itrf: T, part: U): T {
  return $.extend(itrf, part);
}

// --------

interface Test {
  poyo: string,
  nyan: number
};

let u: Test = {poyo: "piyo", nyan: 20};
let u2: Test = extend(u, {poyo: "hey"});

// Error
// let u3: Test = extend(u, {noname: "hey"});