Promise.then does not accept callback that returns `T | Promise<S>`

[rkirov]

TypeScript Version: 2.3.1

Code

declare let p: Promise<void>;
declare let p2: Promise<string>;

let p3 = p.then(() => Math.random() ? 0 : p2);

Expected behavior:
Code is accepted and type of p3 is Promise<string|number>.

Actual behavior:
Type Error: Argument of type '() => Promise<string> | 0' is not assignable to parameter of type '(value: void) => string | PromiseLike<string>'.

[RyanCavanaugh]

Promise<void> remains problematic 😢