Ignore cache for nil on non-nil types?

Say you have some logic that may return nil, but you don't want to cache nil... Is this something where we could do this?

cache.fetch(key, as: String) do
  [nil, "string"].sample
end

With this, I specify that the value is String, but if it returns nil, then we don't cache it. Running it again would only cache if the value returns "string". However, doing this...

cache.fetch(key, as: String?) do
  [nil, "string"].sample
end

would say that I allow a nil value, so cache whatever I get back, even if it's nil.

I think we can catch that at compile-time and alter the logic. But the question is, would that be nice? Or would that cause issues?

Start by reviewing the cache.fetch API and the two Crystal examples to understand how the as: String and as: String? forms currently behave. Define the intended nil-caching semantics and the compile-time conditions before identifying the tests needed to verify both cases.