Existing check is lost after type union discrimination

[princed]

TypeScript Version: 2.4.1, 3.5.1, 3.6.0-dev.20190727

Search Terms:
type union discrimination, narrowing, widening

Code

interface Base { common?: string }

interface Sub1 extends Base { type: 'sub1' }

interface Sub2 extends Base { type: 'sub2' }

type Union = Sub1 | Sub2;

function test(u: Union) {
    if (u.common) {
        if (u.type === "sub1") {
			u.common.toString(); // <-- Object is possibly 'undefined'.
        }
    }
}

Expected behavior:
No error message, since u.common is already proved to exist.

Actual behavior:
Error message

Playground Link:
Playground link

Related Issues:
The closest report I could find was #11787 (comment), but apparently, it's related to type guards, which impose some design limitation as far as I understand.

[fatcerberus]

Duplicate of #31495 (among others).

[princed]

Oh, I see, thanks.