undefined type from optional chaining doesn't produce correct generic inference candidate from union

[RyanCavanaugh]

TypeScript Version: 10/18/2019 nightly

Search Terms: optional chaining generic inference undefined

Code

declare function unbox<T>(box: { value: T | undefined }): T;
declare const su: string | undefined;

const b1 = { value: su?.length };
// Incorrect error - T inferred to be number | undefined
const v1: number = unbox(b1);

// No error
const b2 = { value: su?.length as number | undefined };
const v2: number = unbox(b2);

// No error
const b3: { value: number | undefined } = { value: su?.length  };
const v3: number = unbox(b3);

Expected behavior: In all cases, T should be inferred to be number

Actual behavior: When the undefined part of the union of the type of value originates in an optional chaining, it doesn't infer correctly

Playground Link: http://www.typescriptlang.org/play/?ts=Nightly&ssl=1&ssc=1&pln=15&pc=1#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXzQCMcAPAHgBUA+ACmJIC54BveANygmRCYvgB8CqUIiyoQweAF8AlLwDcAWABQoSLARg8AZwzxtyJrphiA5gKEixEpcpVbUu+IQCM8ALwt2nbkeQB+ADoIEFRTDAALaVsAehj4AEl0HBg4THgQVJT4AFp4PjFETLhJDBxnBFRkAFtCTIs0K3Fgex09NhcmKtr6zyJSOhcZWxU4+AA5cuKU1sc9QgAmDy8OLh59AODQ8KiobXhuupgG4RBRZujZpzYFrpqj5f6SOgXhlVH4yYysmCv5gGYmKxVr4Dvd6oJGmdrJIpMtgT51gYgiEwpF4JdlA5roCwT1jn1UPQ6P83sogA (nightly link)

Related Issues: Nope

[RyanCavanaugh]

cc @rbuckton who was looking at this with us

[JannesMeyer]

I think I've come across the same issue. This is my code:

let values = { a: 1 } as { a: number } | undefined;

let a = values?.a;
assertDefined(a); // This does not narrow the type to number
console.log(a.toString()); // Error: Object is possibly undefined

function assertDefined<T>(x: T | undefined | null): asserts x is T {
    if (x == null) {
        throw new Error('Expected a value');
    }
}

Playground link

[rosslovas]

This is in the 3.7.2 milestone but the fix didn't actually make it in (playground link). Would be good to get this in 3.7.3.