Type of data should be a union type with undefined when a config of type SWRConfiguration is passed to useSWR

[connorch]

Bug report

Description / Observed Behavior

When passing an SWR config of type SWRConfiguration | undefined as an argument to useSWR, data in the swr response is no longer a union type with undefined (regardless of whether the swr config defined suspense: true).

Expected Behavior

I expect data in the below example to be a union type User | undefined because the hook is not running in suspense mode.

Repro Steps / Code Example

Here is a suggested test showcasing the currently failing case:

export function testUndefinedData() {
  const fetcher = (k: string) => Promise.resolve({ value: k })
  const { data } = useSWR('/api', fetcher, {} as SWRConfiguration)
  expectType<Equal<typeof data, { value: string } | undefined>>(true)
}

Additional Context

This issue was newly introduced in 2.1.0.

I have confirmed in a local test environment that it was introduced in this PR: https://github.com/vercel/swr/pull/2452/files

[nstepien]

When doing {} as SWRConfiguration, you're asserting that the value has a different type, I wouldn't expect it to give you the correct type for {} if you assert it to be something else.
Have you tried using {} satisfies SWRConfiguration instead?

[connorch]

Yeah this is a good point. Not a big deal here since we do want to trick typescript into thinking that the {} is of type SWRConfiguration, but satisfies is the safer option 👍.

This PR is merged already though so I'm unfortunately that fix would need to come in a separate PR - please feel free to submit one.

[connorch]

Fixed in #2506