I love sum types, and i find it very hard to work without them when i am in C++. However your examples i don't think are particularly convincing. The examples

I love sum types, and i find it very hard to work without them when i am in C++. However your examples i don't think are particularly convincing. The examples you gave can mostly be done with std::variant<> and are simply examples are unions. (Result == std::expected) In term of non nullable smart pointer, it would be easy to enforce the the non nullability in the constructor, and have the same ergonomy as Option<Box<T>>. In fact you can do something like std::expected<std::reference<T>> but why would you instead of just std::unique_ptr and handle the nullable case. What makes sum type powerfull is to have full Algebraic DataType (tags + sum types + product types) + pattern matching + good compiler errors.