String literal function overloads not working as expected

[geoffreak]

I ran into an issue today with string literal function overloads using Typescript 2.0.3 while trying to write a library for Sift Science. I was attempting to create a function with stricter object checking when called with a certain string first. I was a bit surprised this doesn't work in a way I had expected.

async function addEvent(eventType: '$login', email: string | undefined, data: LoginEvent): Promise<void | Error>;
async function addEvent(eventType: '$logout', email: string, data?: LogoutEvent): Promise<void | Error>;
async function addEvent(eventType: string, email: string | undefined, data: Object = {}): Promise<void | Error> {
  // ...
}

interface SiftScienceBaseEvent {
  $ip?: string;
  $time?: number;
}
interface LoginEvent extends SiftScienceBaseEvent {
  $session_id: string;
  $login_status: '$success' | '$failure';
}
interface LogoutEvent extends SiftScienceBaseEvent {
  // This page intentionally left blank
}

let sid: string | undefined;
addEvent('$login', undefined, {
  $login_status: '$success',
  $session_id: sid,
});

The error I expected to see: (and you can indeed see by commenting out the $logout overload)

Types of property $session_id are incompatible

The error I see instead:

Argument of type '"$login"' is not assignable to parameter of type '"$logout"'

I had assumed that the string literal as the first parameter would specify which overload to use, but it seems that only the object types really work.

[DanielRosenwasser]

What is the shape of LoginEvent?

[geoffreak]

Okay, I've isolated the issue and rewritten the first post. Hopefully that shows why I'm seeing this as unexpected.

[mhegazy]

This is a duplicate of #10523

[mhegazy]

I should add that the issue here is the overload resolution algorithm decided that the first signature does not match, and then fell through to the next one. we should have better handling in cases of literals, to say that clearly the first one was the correct match, but the arguments were not correct.