AMDG This is a bit more complex than it needs to be. has_conversion_helper uses SFINAE to get a bool and then you pass the bool to enable_if to do more SFINAE. It's simpler to use SFINAE once: template<class Q1, class Q2> using require_conversion_factor = void_t<decltype(conversion_factor(Q1{}, Q2{}))>; template<class Q1, class Q2> using require_conversion_helper = void_t<decltype(conversion_helper<Q1, Q2>::convert(Q1{}))> This of course, only works in C++11, but I think you need to make it conditional anyway, to avoid the possibility of errors from over-eager evaluation of the SFINAE test. In Christ, Steven Watanabe

AMDG

To make it work in C++03, you need to avoid SFINAE on expressions. // limit the scope of the extra overload namespace has_conversion_factor_impl { // a fallback overload that will be found only // if no other overload of conversion_factor exists. struct no_conversion_factor {}; no_conversion_factor conversion_factor(...); // identify whether the fallback was used. // conversion_factor should never return void, // so we don't need `operator,` magic. no& test(no_conversion_factor); yes& test(...); template<class Q1, class Q2> struct has_conversion_factor { enum { value = sizeof((test)(conversion_factor( declval<Q1>(), declval<Q2>()))) == sizeof(yes) }; }; } using has_conversion_factor_impl::has_conversion_factor; (Bleh.) has_conversion_helper can be handled by modifying the primary template of conversion_helper to make it possible to identify: template<class Q1, class Q2> struct conversion_helper { typedef void _is_primary_template; }; template<class Q1, class Q2, class = void> struct has_conversion_helper : true_type {}; template<class Q1, class Q2> struct has_conversion_helper<Q1, Q1, typename conversion_helper<Q1, Q2>::_is_primary_template> : false_type {};

and BOOST_NO_SFINAE_EXPR and BOOST_NO_CXX11_TEMPLATE_ALIASES. In Christ, Steven Watanabe

AMDG

You need either C++17 void_t: template<class...> using void_t = void; (depends on CWG 1558) or extra indirection (make conversion_helper inherit from another template and put the SFINAE there.) In Christ, Steven Watanabe