util.promisifyにカスタムの型情報を与える

test.ts

function fn(a: number, b: number, done: (err: Error, sum: number) => void) {
  done(null, a + b);
}

declare namespace fn {
  function __promisify__(a: number, b: number): Promise<number>
}

import {promisify as p} from "util";

(async () => {
  var n = await p(fn)(1, 2);
})();