Type simultaneously does and does not have property

[jwalton]

TypeScript Version: 3.4.1 and 3.2.2

Code

interface TextChannel {
    id: string;
    type: 'text';
    phoneNumber: string;
}

interface EmailChannel {
    id: string;
    type: 'email';
    addres: string;
}

type Channel = TextChannel | EmailChannel;

export type ChannelType = Channel extends { type: infer R } ? R : never; // 'text' | 'email'

// Stolen from lodash.
type Omit<T, K extends keyof T> = Pick<
    T,
    ({ [P in keyof T]: P } & { [P in K]: never } & { [x: string]: never })[keyof T]
>;

type ChannelOfType<T extends ChannelType, A = Channel> = A extends { type: T }
    ? A
    : never;

/**
 * A NewChannel is a Channel with a 'type', a 'localChannelId', with no id,
 * and every other field is optional.
 */
export type NewChannel<T extends Channel> = Pick<T, 'type'> &
    Partial<Omit<T, 'type' | 'id'>> & { localChannelId: string };

/**
 * Create a NewChannel of the specified type, ready to hand off to, for example,
 * a react form which can fill it in with values and turn it into a real Channel.
 */
export function makeNewChannel<T extends ChannelType>(type: T): NewChannel<ChannelOfType<T>> {
    const localChannelId = `blahblahblah`;
    return { type, localChannelId };
}

// Here's the exciting bit:

const newTextChannel = makeNewChannel('text');
// Property 'phoneNumber' does not exist on type 'NewChannel<TextChannel>'. ts(2339)
newTextChannel.phoneNumber = '613-555-1234';

const newTextChannel2 : NewChannel<TextChannel> = makeNewChannel('text');
// But this works!
newTextChannel2.phoneNumber = '613-555-1234';

Expected behavior:

Since newTextChannel and newTextChannel2 are both of type NewChannel<TextChannel>, I'd expect them both to have a phoneNumber property (or both to not have a phoneNumber property).

Actual behavior:

newTextChannel does not have this property, and newTextChannel2 does, even though these two seem to be of the same type!

Playground Link: link

Related Issues: No

[jwalton]

It even works if you do:

const newTextChannel3 : NewChannel<ChannelOfType<'text'>> = makeNewChannel('text');
newTextChannel3.phoneNumber = '613-555-1234';

[ahejlsberg]

There's definitely something odd going on here. It appears related to the ChannelOfType type. For example, it works if you change ChannelOfType to:

type ChannelOfType<T extends ChannelType> =
  T extends 'text' ? TextChannel :
  T extends 'email' ? EmailChannel :
  never;

[weswigham]

OK, so this is only strange because we have two completely different types which instantiate to the same alias symbol and alias type arguments; under the hood we actually are talking about two completely separate types. NewChannel<ChannelOfType<T>> instantiates to NewChannel<TextChannel> alias-wise, however NewChannel<ChannelOfType<T>> is really NewChannel<ChannelOfType<T, TextChannel> | ChannelOfType<T, EmailChannel>> if you stop printing ChannelOfType's alias, which is in turn

type NewChannel<ChannelOfType<T, TextChannel> | ChannelOfType<T, EmailChannel>> = 
  & Pick<ChannelOfType<T, TextChannel> | ChannelOfType<T, EmailChannel>, "type">
  & Partial<Pick<ChannelOfType<T, TextChannel> | ChannelOfType<T, EmailChannel>, (
    | ({ [P in keyof ChannelOfType<T, TextChannel>]: P; } & { type: never; id: never; } & { [x: string]: never; })
    | ({ [P in keyof ChannelOfType<T, EmailChannel>]: P; } & { type: never; id: never; } & { [x: string]: never; })
  )[keyof ChannelOfType<T, TextChannel> & keyof ChannelOfType<T, EmailChannel>]>>
  & { localChannelId: string; }

if you stop printing NewChannel's alias. When T="text", a bunch of stuff reduces like so:

type NewChannel<TextChannel> = 
  & Pick<TextChannel, "type">
  & Partial<Pick<TextChannel, (
    | ({ [P in keyof TextChannel]: P; } & { type: never; id: never; } & { [x: string]: never; })
    | ({ [P in keyof never]: P; } & { type: never; id: never; } & { [x: string]: never; })
  )[keyof TextChannel & never]>>
  & { localChannelId: string; }

OK, now what's gone wrong should be obvious - keyof ChannelOfType<T, TextChannel> & keyof ChannelOfType<T, EmailChannel> instantiates to never, causing us to index that Pick by never and thus pull out no keys. The root cause is keyof never reducing to never - it should be string | number | symbol (the keyof-specific unknown direction bound). This makes sense; since never is the subtype of all types, it must have all keys.