Eric Niebler wrote:
I made Meta SFINAE-friendly on a lark and found it useful in practice. (See this thread[*] where Louis and I compared common_type implementations with Meta and Hana.) ... [*] http://lists.boost.org/Archives/boost/2015/03/220446.php
Having a SFINAE-friendly fold helps, but here's my implementation of this challenge. // common_type template<class...> struct common_type { using type_not_present = void***[]; }; template<class T> struct common_type<T>: std::decay<T> { }; template<class T1, class T2, class... T> using common_type_impl = common_type<decltype(declval<bool>()? declval<T1>(): declval<T2>()), T...>; template<class T1, class T2, class...T> struct common_type<T1, T2, T...>: eval_or_default<common_type<>, common_type_impl, T1, T2, T...> { }; #include <iostream> #include <typeinfo> int main() { std::cout << typeid( common_type<char volatile[], void*, int const[], void*>::type ).name() << std::endl; std::cout << typeid( common_type<char, float, int*, double>::type_not_present ).name() << std::endl; } where eval_or_default<Def, F, T...> checks is_evaluable<F, T...> (from Bruno Dutra's earlier message) and returns F<T...> if evaluable, Def if not. Getting it right was a bit tricky, but I find the final result quite readable. (This implements the specification in the latest C++17 draft.)