Why wouldn't it work with fundamental types? I explicitly marked them as supporting the protocol. Maybe I did a bad job of explaining myself.
And again some raises std::expected, it's not related. This is for every container that can conditionally carry a value, std::optional is the best example but I encountered the problem while working on a different class (an interval type).
I don't think it's worth the effort if it cannot work for fundamental
types, and the ABI for std::optional is set in stone. Ideally, we
would be able to say that std::optional<X*> where nullptr stores the
empty value, or std::optional<int> where -1 represents an empty value,
etc.
Those things seem feasible with std::expected with a bit of library
support. We could have something like
std::expected<int, std::nonvalue<-1>>
Which would use the value "-1" to represent an unexpected object. This
would also not break any existing code since it would use novel types
and a novel protocol, even in the case of fundamental types.