使用clang的pahole工具，结果如下

class optional<int> : _Optional_base<int, true, true> {
public:
};

struct _Optional_base<int, true, true> : _Optional_base_impl<int, std::_Optional_base<int, true, true> > {
    struct _Optional_payload<int, true, true, true> _M_payload; /*     0     8 */
};

class _Optional_base_impl<int, std::_Optional_base<int, true, true> > {
protected:
};

struct _Optional_payload<int, true, true, true> : _Optional_payload_base<int> {
};

struct _Optional_payload_base<int> {
    union _Storage<int, true>  _M_payload;           /*     0     4 */
    bool                       _M_engaged;           /*     4     1 */
};

union _Storage<int, true> {
    struct _Empty_byte         _M_empty;           /*     0     0 */
    int                        _M_value;           /*     0     4 */
};

struct _Empty_byte {
};

Godbolt：https://godbolt.org/z/r4KTfYKoW
最终这相当于

class optional<int> { 
    int value;
    bool used;
};