Flow does not refines type of class union as it does with types
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Here is an [example of failure](https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBjGBDAzrsAeQGswAebAPjAG9UwwBLXEgLjABcAnAVwFN6YAG7YY-VtlQBfdFjwEAoly5wu5AB7U6DZmzBRRuAQz7LVrddPQcAngAc+YAEp9cPGBw0AaMFTABeImIKagAfMCUVLjJNQVQ+dTtVTww4ADtcFIALbEY0gO9fHwAjSgAKQSh2MpExPnZsAEoA6hc3DxiSyi9BLld3DnY2gc7fSlRGof6O9S6W2kFGKDAyvvaOADpdYmbtBjA+jh4ufKhV6c3a-kbBKTA+GCMF-YO+I5PX9durIA) that I'd expect to work:
```js
/* @flow */
class Ok {
isOk: true
value:a
}
class Error {
isOk: false
error:x
}
It fails to refine Result to `Ok` and `Error` by `isOk` sentinel field as following erros seem to indicate:
```
20: return f(result.value)
^ property `value`. Property not found in
20: return f(result.value)
^ Error
22: return result
^ Ok. This type is incompatible with
18: ): Result => {
^ union: type application of polymorphic type: class type: Ok | type application of polymorphic type: class type: Error
22: return result
^ Ok. This type is incompatible with
18: ): Result => {
^ union: type application of polymorphic type: class type: Ok | type application of polymorphic type: class type: Error
25:1
```
What's interesting that if I [change classes to type definitions](https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVAXAngBwKZgDyA1mADwCGAfGALxgDeqYYAlgM4kBcYGATgFc8AGmZgAbhRhCuFVAF902fGACifPnD7kAHjXpMWHbmChT2IsXg1auOhUtwEASnnYCYGXcLDU6RYkoaAB81Gz4yPTFUPB0cLS8AYzgAO3YkgAsKVhT-SJ8KHwAjKgAKMSgeUslpPB4KAEo6Gld3T3ywEtEWPjcPDB5W-o7qVAbBvvadYv0aQzYoMFLetowAOmNiJvmevAwBPlyoZcn1mqEGsXkwPBgLRjFd-cOwFf6rhyA) everything works as expected, but [same error is present if interfaces are used](https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBLAdgFwKYBOUAhgMZ5gDyA1mADzEB8YA3qmGBgM40BcYOAgFc8AGnZgAbsRgjexVAF902fETIUAogQJwC9AB7M2HbnzAkYXMRMK6CvA0vQ4AngAcKAJTxchMHENRMCYwAF4qagZmAB8wbXs6IwlUPAN3PUDSOCwuLIALYmxwoJDggCNGAAoJKH4q6Vk8fmIASnDmHz8ApIrGcQ4CX38cfi6R3pDGVFax4Z6DPo7WCQwoMCqh7pwAOjNqdpMOMCGcIQIsC0353caRVolFMDwrCiPBvDOLk5vH5yA)
For whatever reason [type refinements via switch](https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBLAdgFwKYBOUAhgMZ5gDyA1mADzEB8YA3qmGBgM40BcYOAgFc8AGnZgAbsRgjexVAF902fETIUAogQJwC9AB7M2HbnzAkYXMRMK6CvA0vQ4AngAcKAJTxchMHENRMCYwAF4qagZmAB8wbXs6IwlUPAN3PUDSOCwuLIALYmxwoJDggCNGAAoJKH4q6Vk8fmIASnDmHz8ApIrGcQ4CX38cfi6R3pDGVFax4Z6DPo7WCS4EDBxSfKqh7pwAOjNqdpMOMFJiawFhZtQASDOwIZwhAiwLHfmDxpFWiQ4ACZ4EgjXj3R7PV7vXYjCTKZRAA) instead of `if` seems to work in all cases:
```js
/* @flow */
interface Ok {
isOk: true,
value:a
}
interface Error {
isOk: false,
error:x
}
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