I'm not sure that's true. For ZOE, the compiler knows whether it's returning a failure or not, and thus whether to set/clear the carry flag. The callee immedia

I'm not sure that's true. For ZOE, the compiler knows whether it's returning a failure or not, and thus whether to set/clear the carry flag. The callee immediately handles it so there's no need to store the carry flag. For a generic user defined union, neither of those is true; the compiler won't be able to deduce which field is active in all situations and won't be able to preserve the carry flag (even if it wasn't modified by other instructions, how would you handle 2 discriminated union variables?) so the flag needs to be stored in the union. The compiler could automatically add a flag variable and update it appropriately, but then again, that can be done today in c++ ... call it std::variant.