1 // PR c++/59204 2 // { dg-do compile { target c++11 } } 3 4 template< class T > 5 using void_t = void; 6 7 template< class T, class = void > 8 struct has_type 9 { constexpr static bool value = false; }; 10 11 template< class T > 12 struct has_type<T, void_t<typename T::type>> 13 { constexpr static bool value = true; }; 14 15 struct yes { using type = int; }; 16 struct no { }; 17 18 int 19 main( ) 20 { 21 static_assert( has_type<yes>::value, "false negative!" ); 22 static_assert( not has_type<no >::value, "false positive!" ); 23 } 24