Hello all, One a separate note, why the following SFINAE impl of EqualityComparable returns 0 on std::common_type<int, int>::type? EqualityComparable<int>::value // 1 -- ok EqualityComparable<std::common_type<int>::type>::value // 1 -- ok EqualityComparable<std::common_type<int, int>::type>::value // 0 -- why?? Code: namespace std { class type_info; } #include <boost/type_traits/detail/yes_no_type.hpp> #include <iostream> #include <type_traits> template< typename _1 > struct EqualityComparable { struct requires0 { template< typename T > static boost::type_traits::yes_type check ( T a, T b, char (*) [sizeof(decltype( bool{a == b} ))] ); template< typename T > static boost::type_traits::no_type check ( ... ); template< typename T > struct sizeof_check { T a; T b; static size_t const value = sizeof check<_1>(a , b , 0); }; static bool const value = sizeof(boost::type_traits::yes_type) == sizeof_check< _1 >::value; }; static bool const value = requires0::value; }; int main ( void ) { std::cout << EqualityComparable<int>::value << std::endl; // 1 -- ok std::cout << EqualityComparable<std::common_type<int>::type>::value << std::endl; // 1 -- ok std::cout << EqualityComparable<std::common_type<int, int>::type>::value << std::endl; // 0 -- why?? return 0; } I'm using Clang++ 3.2. Thanks, --Lorenzo