The union avoids requiring an allocation for, say, returning a Result.

I am not sure what using an object vs using a struct with an enum + a union has to do with allocation. My C++ is rusty (no pun intended).

The equivalent of a function returning Result<T, E> in Rust would be returning something like unique_ptr<ResultBaseClass<T,E>> because the two subclasses have different sizes and thus can't be returned directly (or else one will be object slicing). The struct version stores everything inline from the start (it has a single static size) and so can be returned without needing a pointer.

My C++ skills have atrophied worse than i thought - I completely forgot you can't return an abstract class by value.

I find it easier to reason about.

Interesting. I tend to view ADT as syntax sugar for a shallow class hierarchy of small immutable objects with no encapsulation or behaviour.