Hi All Ok, I should probably have started this thread with this self contained code example. What's wrong with this? #include <vector> #include <boost/concept/requires.hpp> #include <boost/range/concepts.hpp> #include <boost/utility/enable_if.hpp> #include <boost/type_traits/is_same.hpp> template <typename T> BOOST_CONCEPT_REQUIRES( (( boost::SinglePassRangeConcept<T> )), // ( void )) ( typename boost::enable_if<boost::is_same<typename boost::range_value<T>::type, int>, void>::type )) f( const T & ) { } int main( ) { std::vector<int> v; f( v ); } g++ -c -o enable_if.o enable_if.cpp enable_if.cpp: In function ‘int main()’: enable_if.cpp:17: error: no matching function for call to ‘f(std::vector<int, std::allocator<int> >&)’ If I comment out the enable_if line and comment in the void line it's fine. TIA, Rob.