awaiting a Promise recasts top variable to initial type

[bondz]

TypeScript Version: 2.6.1, 2.7.0-dev.20171110

function test(a: boolean): false | Date {
    if (a) {
        return new Date();
    }

    return false;
}


async function test2() {
    let b = test(true);

    if (!b) {
        b = new Date();
    }
    
    await Promise.all([]).then(() => {
        b.getDate(); //Should compile.
            
    })
}

Expected behavior:
Should compile
Actual behavior:
Throws with Property 'getDate' does not exist on type 'true'.

---

This can be reproduced with any return type and even when type inference is used.

[aluanhaddad]

Likely duplicate of #13180. Also, see #9998 for detailed commentary and discussion.

Note: this is not related to promises.

Simple fix:

async function test2() {
  const b = test(true) || new Date();
  await Promise.all([]).then(() => {
      b.getDate(); // compiles.
  });
}

[bondz]

Thanks @aluanhaddad. The simple fix works and looks like #9998 should cover this. Closing this ticket.