Regarding a comment below the video I posted 2 days ago. My case

[zarichniuk]

Didn't have time to dive deep into the question of why it doesn't work. But I promise I'll try despite lack of that deep knowledge :)

Unless you solve it first ;)

[kommade]

For OneOf to work, there cannot be conflicting types in the array.

This is because of the type OnlyFirst<F, S> = F & {[Key in keyof Omit<S, keyof F>]?: never};

Your example:
interface AuthenticatedUser {
user: User;
isAuthenticated: true;
isLoading: false;
}

interface UnauthenticatedUser {
user: null;
isAuthenticated: false;
isLoading: false;
}

Then F = AuthenticatedUser and S = UnauthenticatedUser. Onlyfirst returns the keys in F only.

Because user cannot be both null and User, it becomes never.

Something like this would work:

type User = {
someValue: unknown;
};

type BaseUser = {
user: User | null,
};

type AuthenticatedUser = BaseUser & {
user: User;
};

type UnauthenticatedUser = BaseUser & {
isAuthenticated: false;
};

type PendingUser = BaseUser & {
isPending: true
};

type AuthenticatedUserState = OneOf<[AuthenticatedUser, UnauthenticatedUser, PendingUser]>;

const user: AuthenticatedUserState = {
user: null,
isAuthenticated: false,
}