Narrowing sources' falsyness

[XiNiHa]

When writing dependent queries, it's common to expect the query function to depend on the value passed to the enabled option and its truthyness. If useQuery can ensure the behavior with some type narrowing, it would make all falsyness checks in the query function redundant. For example:

const { data: user }  = useQuery({
  ...opts,
  queryFn: fetchUser,
})
const { data: organization } = useQuery({
  // the name of the option wouldn't make sense anymore and would need a new one if adopted, like `source`
  enabled: user,
  // ctx.enabled is typed as `Exclude<typeof user, Falsy>`, ensuring that the value is truthy and need no null check
  queryFn: ({ enabled: user }) => fetchOrganization(user.organizationId)
})

This design is already adopted in SolidJS's createResource() (the overload that uses a source signal) and works pretty conveniently.

[TkDodo]

this looks interesting