tsc optional chaining with functions that may return void or an object

[dannypike]

TypeScript Version: 3.8.0-dev.20191221
(it also behaves this way with 3.7.3 and 3.7.4)

Code

class A {
  prop: number = 1
}

declare function elsewhere() : void | A

let a = elsewhere()
let b = a?.prop

Expected behavior:

I was expecting the optional chaining to handle the two possibilities:

(a) elsewhere() returns an object that has a property called prop; and
(b) elsewhere() returns undefined.

Actual behavior:

The compiler rejects the code:

index.ts:8:12 - error TS2339: Property 'prop' does not exist on type 'void | A'.
Property 'prop' does not exist on type 'void'.

let b = a?.prop
             ~~~~


Found 1 error.

Have I misunderstood the way that optional chaining is supposed to work?

[MartinJohns]

Looks like a duplicate of #35236.