change return type of useQuery to discriminating union

[MrLoh]

To fully take advantage of typescript, it would be nice if the return type of useQuery would be a discriminating union.

const Component = () => {
  const res = useQuery<Data, Error>('key', fetchData);
  return res.status === 'loading' ? (
    <div>Loading ...</div>
  ) : res.status === 'error' ? (
    <div>{res.error.message}</div> // ts understands that error is not undefined
  ) : res.status === 'success' ? (
    <div>{res.data.name}</div> // ts understands that data is not undefined
  ) : null;
};

Has this been discussed, are there any concerns about this? I could potentially look into providing a PR.

This probably applies to the other hooks too.

[TkDodo]

I think this would be awesome 😎 , not sure if there is a reason to not have the status property discriminate the data / error fields. It would give me another argument to use the status field over loading booleans :)

[TkDodo]

@boschni pointed out some time ago that the discriminating unions have been removed:

#922 (comment)

[MrLoh]

I don't understand the argument though. I created a PR to add them back, #1108 . Maybe we can continue the discussion there.