Flow: Error when destructing union

[iddan]

When destructing a union object flow fails

type Student = {
   type: 'student'
};

type Teacher = {
    type: 'teacher'
};

const getType1 = (someone : Student | Teacher) =>  someone.type; // works

const getType2 = ({ type } : Student | Teacher) => type;
//                ^ string literal `teacher` [1] is incompatible with string literal `student` [2].

https://flow.org/try/#0C4TwDgpgBAysCuATCA7YUC8UDeAoAkKJAFxQDkAzgsmmbgL665HQAqEAhgMYAWEATphwEWpMsE68BdRri4B7FFSgBzCMFbhoWABTYoLKPSik4SVOgA+Udtz78AlJgB8BrUA

[apsavin]

Use generics like this

[iddan]

It has nothing to do with generics and your example fails

[apsavin]

It does not "fails". Read the error message. I added the error intentionally to show use that it works.
Here's the version that typechecks.

[iddan]

This was just an example. This error occurs allot when destructing React component's props. Here's another example:

type Message = { type: 'image', data: { url: string } } | { type: 'text', data: { text: string } };

// Doesn't work
const MessageBubble = ({ type, data } : Message) => {
    switch (type) {
      case 'image': {
        return <img src={data.url} />;
      }
      case 'text': {
        return <div>{data.text}</div>;
      }
      default: {
        throw new Error('Unknown message type');
      }
    }
}

// Works
const MessageBubble2 = (props : Message) => {
    switch (props.type) {
      case 'image': {
        return <img src={props.data.url} />;
      }
      case 'text': {
        return <div>{props.data.text}</div>;
      }
      default: {
        throw new Error('Unknown message type');
      }
    }
}

https://flow.org/try/#0C4TwDgpgBAshDO8CGBzaBeKBvKpIC4oByASwFtUIiAaKAEyWCUJwFcAnAG0PmHZIB2KKAF9RUAD7Zc4CISLAIAD2A16jZtMUqefQcLEiA3ACgTAYwD2A3lABKEJOeBRMWEWYD0nqABFLCAIKUADuluwA1hbWtnCIlABCrABGyZwYUAAUOHgQtAxM4oRxyGgAlK4AfNgmUFAAkPAhJMDmABZZuRVYtXV95kjw0KQUaEQsvX0N7BDAHAJQADzkwvDs5uhYBUgAdBycYp6VplN1HqdQA0PE2qoTF-Uzc+wLi3QkAG6VWxo7tyKLTzvL4nU7nU50CAAMyQrE4wHup3qwDa7EsISgAggGIAouw0exMkQAKoCCICdELMgIUrQXJEMqgqbgs4mDwmbxQADq4Qi8GiNhcJUSKTSEAATK4smA0WB4FBijTKBV0NUenVGs1Wh1MjLLHK-rJupN+oNhuRKOMag8nvMlisoGsNlg9QbtnsuIdjibWRcrsNblb1UjbS8lsDvq74Dt3f9ARGmX0WX1ITC4QjrUiUWiMVjcfjwkTSeTKVBqfE0DJIAzE76k2ygA

[apsavin]

Oh, sorry, it seems like a duplicate of #3918
Am I wrong?

[iddan]

No, it says unions doesn't work well with destructions

[apsavin]

I personally can't see any difference between your latest example and an example from #3918
Well, the title of issue is different, but it does not matter.
Could you please clarify the difference?

[iddan]

Because this has nothing to do with refinements

[apsavin]

Well, what about #3932 ?

[goodmind]

Duplicate of #3932