1 // PR c++/63723
2 // { dg-do compile { target c++11 } }
3 
4 template<typename Tp> Tp declval();
5 
6 template<typename Tp, Tp v>
7 struct integral_constant
8 {
9   static constexpr Tp value = v;
10   typedef Tp value_type;
11   typedef integral_constant<Tp, v> type;
value_typeintegral_constant12   constexpr operator value_type() const { return value; }
13 };
14 
15 typedef integral_constant<bool, true>   true_type;
16 typedef integral_constant<bool, false> false_type;
17 
18 template <typename From, typename To>
19 class is_list_convertible_helper
20 {
21   template <typename To2>
22   static void requires_conversion(To2 t);
23 
24   template <typename From2, typename To2,
25       typename = decltype(requires_conversion<To2>({declval<From2>()}))>
26   static true_type helper(int);
27 
28   template <typename From2, typename To2>
29   static false_type helper(...);
30 
31 public:
32   using type = decltype(helper<From, To>(0));
33 };
34 
35 template <typename From, typename To>
36 class is_list_convertible
37   : public is_list_convertible_helper<From, To>::type
38 { };
39 
40 static_assert(!is_list_convertible<double, int>::value,
41 	      "double -> int is narrowing!");
42