Type narrowing on a property with disjoint set of values

[colltoaction]

TypeScript Version: 2.3.4 / Current playground version

Code

export interface AuthenticatedProfile {
    readonly userId: string;
    readonly name: string;
}
export interface AnonymousProfile {
    readonly userId: undefined;
    readonly otherProp: string;
}
export type Profile = AnonymousProfile | AuthenticatedProfile;

function handleProfile(prof: Profile) {
    if (prof.userId !== undefined) {
        console.log(prof.name);
    }
}

Expected behavior:

Code would compile, since by checking that the userId property is not undefined we definitely know we are handling an AuthenticatedProfile instance.

Actual behavior:

Type checks fail and you need workarounds such as the ones on this question: https://stackoverflow.com/q/44583342/625581.

What makes me think this is a bug (or at least can be added as a feature) is that this other similar code works. I've read the handbook and it seems that casting is an option, but the userId property is already part of both instances and I wouldn't want to add the boilerplate of casting when there's information right there.

[ahejlsberg]

Duplicate of #12600.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.