`is` で絞込むやつ

index.tsx

type Modify<T, K extends keyof T, Value> = Pick<T, Exclude<keyof T, K>> &
  {
    [P in K]: Value
  }

function isError<TData, TVariables>(
  result: QueryResult<TData, TVariables>,
): result is Modify<typeof result, 'error', ApolloError> {
  return !!result.error
}