cache.get() not working with fallback

[Gregoirevda]

Bug report

With <SWRConfig fallback={{'/foo': 'bar'}} >
and useSWR('/foo') I can persist the cache with persist(cache.get('/foo'))
but if no single fetch is made eg useSWRImmutable('/foo') as '/foo' is provided in fallback, cache.get('/foo') returns null.

Expected Behavior

fallback should be set in cache, so it can be retrieved by cache.get() no matter if a revalidation occurred or not

Additional Context

SWR version: 2.3.3

[Gregoirevda]

A specific use case is persisting cache.

• Entering app for 1st time: no persisted cache

• useSWRImmutable in app fetches once

• leaving app: persist SWR cache (cache.get(key) works)

• Entering app: put persisted data in SWRConfig fallback

• useSWRImmutable doesn't revalidate on keys added in fallback

• leaving app: cache.get() on keys set in fallback are not available.

• Entering app => no persisted cache

• useSWRImmutable fetches data

• leaving app => cache.get() works

etc

Instead of adding fallback to cache, it would be even better to revalidate keys in useSWRImmutable that are set in fallback, but aren't in cache, so they're at least revalidated once and only the response is added to the cache as it is now.