I have defined a simple ID type, and a type that should represent all suitable properties from a given object type that might hold this ID:

export type MyId = string | number;

export type MyIdProp<T extends object> = {
    [K in keyof T]: T[K] extends (MyId | undefined) ? K : never
}[keyof T];

I think this should be correct:
MyIdProp assumes, for a given object type T, an object type that maps properties of T to themselves, but only those properties whose value is assignable to MyId or undefined.
And then, only the properties listed in this assumed object are taken into account to form the resulting union.

Now, I’m trying to retrieve the ID value from a given object in a generic function:

function doSomething<T extends object>(obj: T, idAttr: MyIdProp<T>): MyId {
    const objId = obj[idAttr];
    if (objId === undefined || objId === null) {
        throw new Error('No ID supplied!');
    }

    return objId;
}

However, this fails with error 2322 on the line return objId;, saying:

Type ‘T[MyIdProp] & {}’ is not assignable to type ‘MyId’.

Why? Where does that & {} come from and how do I prevent this?