Eigen Serialization Fix

[varunagrawal]

Eigen has backported some fixes which enable correct serialization when using GCC 8 and above.

The issue is detailed here:
https://gitlab.com/libeigen/eigen/-/issues/1676

Basically, boost serialization with Eigen throws an error because of incorrect template deduction handling by GCC. This issue is not seen in Clang, but the Eigen team went ahead and made a simple fix.

The only issue left to resolve is what happens when a user uses the system Eigen rather than the packaged version under 3rdparty. One potential solution is to specify the required Eigen version in CMake (cmake/HandleEigen.cmake). However, I wanted more discussion on this before pushing that change.

An example of the error thrown is when we try to serialize the Link object in GTDynamics, which has a Vector3 for the inertial matrix diagonal:

In file included from /usr/local/include/gtsam/3rdparty/Eigen/Eigen/Core:365,
                 from /usr/local/include/gtsam/inference/FactorGraph.h:29,
                 from /usr/local/include/gtsam/inference/MetisIndex.h:21,
                 from /usr/local/include/gtsam/inference/Ordering.h:25,
                 from /usr/local/include/gtsam/linear/VectorValues.h:21,
                 from /home/varun/borglab/GTDynamics/tests/testLink.cpp:18:
/usr/local/include/gtsam/3rdparty/Eigen/Eigen/src/Core/util/ForwardDeclarations.h: In instantiation of ‘struct Eigen::internal::accessors_level<boost::serialization::U>’:
/usr/local/include/boost/serialization/split_free.hpp:58:13:   required from ‘static void boost::serialization::free_loader<Archive, T>::invoke(Archive&, T&, unsigned int) [with Archive = boost::archive::xml_iarchive; T = Eigen::Matrix<double, 3, 3>]’
/usr/local/include/boost/serialization/split_free.hpp:74:18:   required from ‘void boost::serialization::split_free(Archive&, T&, unsigned int) [with Archive = boost::archive::xml_iarchive; T = Eigen::Matrix<double, 3, 3>]’
/usr/local/include/gtsam/base/Matrix.h:594:17:   required from ‘void boost::serialization::serialize(Archive&, Eigen::Matrix<Scalar_, Rows_, Cols_, Ops_, MaxRows_, MaxCols_>&, unsigned int) [with Archive = boost::archive::xml_iarchive; Scalar_ = double; int Rows_ = 3; int Cols_ = 3; int Ops_ = 0; int MaxRows_ = 3; int MaxCols_ = 3]’
/usr/local/include/boost/serialization/serialization.hpp:126:14:   required from ‘void boost::serialization::serialize_adl(Archive&, T&, unsigned int) [with Archive = boost::archive::xml_iarchive; T = Eigen::Matrix<double, 3, 3>]’
/usr/local/include/boost/archive/detail/iserializer.hpp:187:40:   [ skipping 22 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/usr/local/include/boost/archive/detail/interface_iarchive.hpp:68:9:   required from ‘Archive& boost::archive::detail::interface_iarchive<Archive>::operator>>(T&) [with T = const boost::serialization::nvp<gtdynamics::Link>; Archive = boost::archive::xml_iarchive]’
/usr/local/include/gtsam/base/serialization.h:128:14:   required from ‘void gtsam::deserializeFromXMLStream(std::istream&, T&, const string&) [with T = gtdynamics::Link; std::istream = std::basic_istream<char>; std::string = std::__cxx11::basic_string<char>]’
/usr/local/include/gtsam/base/serialization.h:145:27:   required from ‘void gtsam::deserializeFromXMLString(const string&, T&, const string&) [with T = gtdynamics::Link; std::string = std::__cxx11::basic_string<char>]’
/usr/local/include/gtsam/base/serialization.h:181:27:   required from ‘void gtsam::deserializeXML(const string&, T&, const string&) [with T = gtdynamics::Link; std::string = std::__cxx11::basic_string<char>]’
/usr/local/include/gtsam/base/serializationTestHelpers.h:98:17:   required from ‘void gtsam::serializationTestHelpers::roundtripXML(const T&, T&) [with T = gtdynamics::Link]’
/usr/local/include/gtsam/base/serializationTestHelpers.h:122:18:   required from ‘bool gtsam::serializationTestHelpers::equalsXML(const T&) [with T = gtdynamics::Link]’
/home/varun/borglab/GTDynamics/tests/testLink.cpp:88:3:   required from here
/usr/local/include/gtsam/3rdparty/Eigen/Eigen/src/Core/util/ForwardDeclarations.h:32:54: error: incomplete type ‘Eigen::internal::traits<boost::serialization::U>’ used in nested name specifier
   32 |   enum { has_direct_access = (traits<Derived>::Flags & DirectAccessBit) ? 1 : 0,
      |                              ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
/usr/local/include/gtsam/3rdparty/Eigen/Eigen/src/Core/util/ForwardDeclarations.h:33:53: error: incomplete type ‘Eigen::internal::traits<boost::serialization::U>’ used in nested name specifier
   33 |          has_write_access = (traits<Derived>::Flags & LvalueBit) ? 1 : 0,
      |                             ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~

[dellaert]

Is there another way to side-step the issue in Link?

[ProfFan]

@dellaert #940 (comment)

[varunagrawal]

Updated!

[dellaert]

I’m not following. I guess this PR is no longer an Eigen upgrade, but a workaround for serialization? Note this was not what I advised in the associated issue.

[varunagrawal]

My understanding was that you preferred the workaround, so I went with that approach. Eigen is no longer touched (we're still at good ol' 3.3.7) but it resolves the serialization issue.

[dellaert]

Approved after Ci succeeds