Required return type when mutating

[gt-codes]

Bug report

Description / Observed Behavior

When using mutate from useSWR<User>, the async function I have to pass must return User as well. This makes me have to do a bit more in my api. For example: my useUpdateUser hook didn't return anything because it didn't need to. I had to update it to return the user. I do this in different parts of my application where it takes a bit more to return the required type.

Expected Behavior

If you have populateCache: false then it's not needed to return the User type since I don't intend on updating the cache.

Repro Steps / Code Example

mutate(useUpdateUser(data), {
  optimisticData: data as User,
  populateCache: true,
  revalidate: true,
  rollbackOnError: true,
});

Additional Context

SWR version: ^1.2.0-beta.0