#include <Eigen/Sparse>

int main()
{
  Eigen::Matrix3d m3d;
  int i_int = 0, j_int = 0;
  std::size_t i_size_t = 0, j_size_t = 0;
  std::ptrdiff_t i_ptrdiff_t = 0, j_ptrdiff_t = 0;

  double d;

  d = m3d(0, 0);                     // ok
  d = m3d(i_int, j_int);             // ok
  d = m3d(i_ptrdiff_t, j_ptrdiff_t); // ok

  d = m3d.coeff(i_int, j_size_t);    // works, but slower


  // d = m3d(i_int, j_size_t);       // fails
  // d = m3d(i_size_t, j_size_t);    // fails
  return 0;
}