You've managed to miss the entire point of using a union: the value is either a success payload or an error value, never both.
You can't encode that mutual exclusivity if you return a std::pair or std::tuple. That's exactly why std::expected, std::variant, or Rust enums exist, to make that constraint explicit in the type system.
You can't encode that mutual exclusivity if you return a std::pair or std::tuple. That's exactly why std::expected, std::variant, or Rust enums exist, to make that constraint explicit in the type system.