g2opy compilation error on mac

The problem occurs when I compiled g2opy on Mac OS X Mojave. The solution is to edit as follows:

    // this is code is one before edit
        .def("x", (double (Eigen::Quaterniond::*) () const) &Eigen::Quaterniond::x)
        .def("y", (double (Eigen::Quaterniond::*) () const) &Eigen::Quaterniond::y)
        .def("z", (double (Eigen::Quaterniond::*) () const) &Eigen::Quaterniond::z)
        .def("w", (double (Eigen::Quaterniond::*) () const) &Eigen::Quaterniond::w)
    // this is code is one after edit
        .def("x", (const double & (Eigen::Quaterniond::*) () const) &Eigen::Quaterniond::x)
        .def("y", (const double & (Eigen::Quaterniond::*) () const) &Eigen::Quaterniond::y)
        .def("z", (const double & (Eigen::Quaterniond::*) () const) &Eigen::Quaterniond::z)
        .def("w", (const double & (Eigen::Quaterniond::*) () const) &Eigen::Quaterniond::w)

The problem is related to the return value of Eigen::Quaterniond::x(), y(), z(), and w(). As the following error message says, the return value is different.

In file included from /Users/oyamada/Documents/code/python/extern/g2opy/python/g2o.cpp:12:
/Users/oyamada/Documents/code/python/extern/g2opy/python/core/eigen_types.h:185:62: error: 
      address of overloaded function 'x' does not match required type 'double () const'
        .def("x", (double (Eigen::Quaterniond::*) () const) &Eigen::Quaterniond::x)
                                                             ^~~~~~~~~~~~~~~~~~~~~
/usr/local/include/eigen3/Eigen/src/Geometry/Quaternion.h:66:44: note: candidate function has
      different return type ('double' expected but has
      'Eigen::QuaternionBase<Eigen::Quaternion<double, 0> >::CoeffReturnType'
      (aka 'const double &'))
  EIGEN_DEVICE_FUNC inline CoeffReturnType x() const { return this->derived().coeffs()....
                                           ^
/usr/local/include/eigen3/Eigen/src/Geometry/Quaternion.h:75:52: note: candidate function has
      different return type ('double' expected but has
      'Eigen::QuaternionBase<Eigen::Quaternion<double, 0> >::NonConstCoeffReturnType'
      (aka 'double &'))
  EIGEN_DEVICE_FUNC inline NonConstCoeffReturnType x() { return this->derived().coeffs().x(); }