Yeah but I'd file that under C++'s ABI problems category. I don't think anything in std::optional's spec requires that it has additional padding. It could have

Yeah but I'd file that under C++'s ABI problems category. I don't think anything in std::optional's spec requires that it has additional padding. It could have an optimized representation for known types of T & thus specialize T* to use nullptr as the sentinel instead of an extra boolean, similar to how std::vector specializes for bool (although that one is probably a mistake, but whatever). But unfortunately also optional references was never resolved & thus punted ( see https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3527.html#rationale.refs https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n35... ), even though boost::optional supported them