It might have something to do with operations on temporaries in parentheses? Whatever heres more context to my other example example . Note if you want to try this out in pqs _3_0_4 you need to add boost::pqs:: in front of t1_quantity at line 143 In <boost/pqs/t1_quantity/operations/add_subtract.hpp>. Thats an error in pqs_3_0_4 but only affects VC8 Then in the same file disable the conditonal defines for vc8 around namespace open and close parentheses for operator + and operator - . Maybe this will work on simpler udts in the same sort of situation. Will try this next. #include <boost/pqs/t1_quantity/types/out/velocity.hpp> #include <boost/pqs/t1_quantity/types/out/acceleration.hpp> #include <boost/pqs/t1_quantity/types/out/time.hpp> // Note all operators except addition/subtraction // are defined in global namespace for this test // #define FUNC1 only --> compiles OK // #define FUNC1 and FUNC2 --> compiles OK // #define FUNC2 --> error no '-' operator found #define FUNC1 #define FUNC2 #ifdef FUNC1 boost::pqs::acceleration::m_div_s2 Func1( const boost::pqs::velocity::mm_div_s& a, const boost::pqs::velocity::m_div_s& b, const boost::pqs::time::s & t ) { // succeeds not affected by parentheses; boost::pqs::velocity::m_div_s v = (a - b); // succeeds //boost::pqs::velocity::m_div_s v = a - b; boost::pqs::acceleration::m_div_s2 result = v / t; return result; } #endif #ifdef FUNC2 boost::pqs::acceleration::m_div_s2 Func( const boost::pqs::velocity::mm_div_s & a, const boost::pqs::velocity::m_div_s& b, const boost::pqs::time::s & t ) { // fails boost::pqs::acceleration::m_div_s2 result = (a-b) / t; // fails // boost::pqs::acceleration::m_div_s2 result = 1 / (t / (a-b)); // succeeds //boost::pqs::acceleration::m_div_s2 result = (boost::pqs::operator-(a, b)) / t; // succeeds //boost::pqs::acceleration::m_div_s2 result = a / t - b/ t; return result; } #endif