Greetings, I've stumbled across a problem in mp_math in the sandbox, revision 50541. The code below (a Boost.Test test case) demonstrates that the problem is present only for mp_int<> and not for int. The message I get is wrong-value.cpp(16): error in "Mixed_integers<N5boost7mp_math6mp_intISaIvENS0_13mp_int_traitsIjyEEEE>": check j == 4 failed [-4 != 4] Regards, Jeroen. #define BOOST_TEST_DYN_LINK #define BOOST_TEST_MODULE Wrong_sign #include <boost/test/unit_test.hpp> #include <boost/mp_math/mp_int.hpp> #include <boost/mpl/list.hpp> typedef boost::mpl::list< int, boost::mp_math::mp_int<> > test_types; BOOST_AUTO_TEST_CASE_TEMPLATE( Mixed_integers, Type, test_types ) { Type i(-1); BOOST_CHECK_EQUAL(i, -1); Type j = i + 5; BOOST_CHECK_EQUAL(j, 4); // Line 16. }