1 // Copyright Louis Dionne 2013-2017 2 // Distributed under the Boost Software License, Version 1.0. 3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) 4 5 #include <boost/hana/core/when.hpp> 6 namespace hana = boost::hana; 7 8 9 template <typename T, typename = hana::when<true>> 10 struct base_template; 11 12 template <typename T> 13 struct base_template<T, hana::when_valid<typename T::first_type>> { }; 14 15 template <typename T> 16 struct base_template<T, hana::when_valid<typename T::second_type>> { }; 17 18 struct First { struct first_type; }; 19 struct Second { struct second_type; }; 20 21 template struct base_template<First>; 22 template struct base_template<Second>; 23 main()24int main() { } 25