Near-future TS changes for possible swr API changes

[devjmetivier]

TypeScript is soon to release some changes that would allow for better type inference from destructured discriminated union types: microsoft/TypeScript#46266

It would be great if swr could update its public types to include discriminated union types for the SWRResponse interface with the addition of a property that could leverage these new type inference improvements.

Example of possible new swr API/usage:

const Component = () => {
  const { data, error, status } = useSWR('');

  if (status === 'validating' || !data) return <Loading />;

  if (status === 'error') return <Error error={error} />;

  if (status === 'success') {
    return <>{data.name}</>;
  }

  return null;
};

The improvements to TS type inference would be that you'd no longer need to check if data is nullish before using it in the above example.

And below would be a contrived example of how the SWRResponse type might change with the addition of a status property:

export type SWRResponse<Data, Error> =
  | {
      data?: Data;
      error?: Error;
      status: 'validating';
      mutate: KeyedMutator<Data>;
      isValidating: true;
    }
  | {
      data: Data;
      error?: Error;
      status: 'success';
      mutate: KeyedMutator<Data>;
      isValidating: false;
    }
  | {
      data?: Data;
      error: Error;
      status: 'error';
      mutate: KeyedMutator<Data>;
      isValidating: false;
    };

I imagine that this would reduce the burden of manual type checking for the data and/or error properties when using those bits of data from the hook. It's also worth noting that at the time of writing this I don't believe these TS improvements have reached any stable release.

Let me know what you all think and if I'm missing something :)