Distinguish missing and undefined

[sandersn]

Typescript should distinguish missing and undefined at the type level in order to distinguish the following two statements:

// @strictNullChecks: true
type A = { x?: string }
let x: A = {}
let y: A = { x: undefined } // should not be allowed, but currently is

[dgoldstein0]

I believe this is a dupe of #13195

[mhegazy]

you are right. closing.