Property not checked in union of objects

[OliverJAsh]

TypeScript Version: 3.3.3333

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Code

type MyPhotoA = { id: number };
type MyPhotoB = MyPhotoA & { userId: number };

type MyPhoto = MyPhotoA | MyPhotoB;

declare const myPhoto: MyPhoto;

const myPhoto2: MyPhoto = {
    ...myPhoto,
    // Expected type error but got none. `userId` should be string
    userId: {},
};

if ('userId' in myPhoto2) {
    // No type error, but runtime exception!! Very bad…
    myPhoto2.userId.toFixed();
}

IIUC, this is because TS considers myPhoto2 assignable to MyPhotoA with an excess property userId.

Shouldn't TS check that the excess property is not used in other members of the union?

[jack-williams]

Think this is a duplicate of #20863.

The real design issue here is that excess property checking runs exclusively before further type-checking, delivering a yes/no answer. To correctly check this case you need excess property checking to effectively prune off MyPhotoA from the union MyPhoto when comparing the types of the properties. For that to work you either need excess property checking to work in tandem with type-checking, or you need excess property checking to return more than just yes/no.

[OliverJAsh]

Closing due to duplicate