Why do type guards behave differently when using Partial mapped types in TypeScript

Question

Compilation of the following code using tsc --strictNullChecks fails with error TS2339: Property 'name' does not exist on type '{}'.

type Obj = {} | undefined;

type User = {
  email: string;
  password: string;
  name: string;
};

type PartialUser = Partial<User>;

function isUser(obj: Obj): obj is PartialUser {
  return true;
}

function getUserName(obj: Obj) {
  if (isUser(obj)) {
    return obj.name;
  }

  return '';
}

However, if I replace type PartialUser = Partial<User>; with

type PartialUser = {
  email?: string;
  password?: string;
  name?: string;
};

everything is hunky-dory.

There are a few workarounds available, but I'm curious why this would be the case. Shouldn't these two definitions of PartialUser be functionally equivalent? I'm on Version 3.1.3

Answer 1

The difference here should be fixed by https://github.com/Microsoft/TypeScript/pull/29384