Wrong union behaviour

[dfilatov]

TypeScript Version: 2.3.2

Code

interface Action<Type> {
    type: Type;
}

interface ActionWithPayload<Type, Payload> extends Action<Type> {
    payload: Payload;
}

type MyAction = Action<'type1'> | ActionWithPayload<'type2', number>;

const action: MyAction = {
    type: 'type1',
    payload: 3
};

Expected behavior:
I'd expect to see an error about unexpected payload property. And if I use only one type from union I'll get it:

const action: Action<'type1'> = {
    type: 'type1',
    payload: 3
};

Object literal may only specify known properties, and 'payload' does not exist in type 'Action<"type1">'.

Actual behavior:
There is no any errors

[mhegazy]

duplicate of #10575

looks like you are looking for #14094

A workaround would be to define your MyAction as:

type MyAction = ActionWithPayload<'type1', undefined> | ActionWithPayload<'type2', number>;

[dfilatov]

@mhegazy Yes, exclusive unions would solve this. Thank you.

[dfilatov]

@mhegazy If I use suggested workaround, I can't do anymore:

const action: MyAction = {
    type: 'type1'    
};

I have to do:

const action: MyAction = {
    type: 'type1',
    payload: undefined
};

[mhegazy]

that is correct. consider setting payload?: undefined instead:

type MyAction = { type: "type1", payload?: undefined } | { type: "type2", payload: number };

[dfilatov]

Cool! This works.

[AJamesPhillips]

This works like a charm. Just use it as discussed here: DefinitelyTyped/DefinitelyTyped#16416 (comment) Thanks @mhegazy !

[mhegazy]

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