1 // PR c++/78006 2 // { dg-do compile { target c++14 } } 3 4 template<typename T> T&& declval() noexcept; 5 6 template<typename... _Tp> 7 struct common_type; 8 9 template<typename _Tp> 10 struct common_type<_Tp> 11 { typedef _Tp type; }; 12 13 template<typename _Tp, typename _Up> 14 struct common_type<_Tp, _Up> 15 { typedef decltype(true ? declval<_Tp>() : declval<_Up>()) type; }; 16 17 template<typename _Tp, typename _Up, typename... _Vp> 18 struct common_type<_Tp, _Up, _Vp...> 19 { 20 typedef typename 21 common_type<typename common_type<_Tp, _Up>::type, _Vp...>::type type; 22 }; 23 24 template<typename... _Tp> 25 using common_type_t = typename common_type<_Tp...>::type; 26 27 template <typename... TFs> 28 auto x(TFs&&... fs) 29 { 30 using rt = common_type_t<decltype(fs(0))...>; 31 return [](auto) -> rt { }; 32 } 33 34 int main() 35 { 36 x([](int){})(0); 37 } 38