std_variant serialization does not instantiate variant alternatives via boost::serialization::access

[KJTsanaktsidis]

I have a variant that has some alternatives which do not have public default constructors. They do have private default constructors though, and are friended to boost::serialization::access. However, the variant serializer in boost/serialization/std_variant.hpp can't construct these types.

I wonder if the variant serializaer should, instead of this:

serialization/include/boost/serialization/std_variant.hpp

Line 125 in 8a8c628

type value;

Do something along the lines of

alignas(type) unsigned char value_storage[sizeof(type)];
type *value_ptr = reinterpret_cast<type *>(value_storage);
access::construct(value_ptr);
std::experimental::scope_exit cleanup([&]() { access::destroy(value_ptr) };

ar >> BOOST_SERIALIZATION_NVP(*value_ptr);
v = std::move(*value_ptr);

If this sounds right I can send a PR. thoughts?