Non-null assertion type operator `!`

[YosubShin]

TypeScript Version: 2.2.1

Code

interface Foo {
 bar: {
 baz: number;
 } | null;
}

type Baz = Foo["bar"]["baz"]; // This doesn't compile with the error: "Property 'baz' does not exist on type '{ baz: number; } | null'."
let x: Baz = 0;

Expected behavior:
I would expect there to be a way to specify bar is non-null, such as Foo["bar!"].

Actual behavior:
The code doesn't compile with the following error message:
"Property 'baz' does not exist on type '{ baz: number; } | null'."

[mhegazy]

This requires a null assertion type operator, as you noted. something of the form type Baz = (Foo["bar"])!["baz"];.

Also referenced in #13253

[mhegazy]

Related #12636

[KiaraGrouwstra]

Another dupe: #17370