[boost] [iterator][type_traits] A test is failing with gcc in c++03 mode

The zip_iterator_test_std_pair is failing with gcc in c++-03 mode. The problem reduces viqa Boost type traits to the fact that in gcc c++03 mode the expression: __is_abstract(std::pair<int&, std::string&>) where __is_abstract is a compiler intrinsic, gives an error of: error: forming reference to reference type 'std::__cxx11::basic_string<char>&' pair(const _T1& __a, const _T2& __b) ^~~~ Is this a valid error according to the C++ 2003 standard ? The test works in c++11 or c++14 mode.