1 // RUN: %clang_cc1 -std=c++11 %s -fsyntax-only -fcxx-exceptions
2 
3 template<typename T> struct remove_reference { typedef T type; };
4 template<typename T> struct remove_reference<T&> { typedef T type; };
5 template<typename T> struct remove_reference<T&&> { typedef T type; };
6 
7 template<typename T> constexpr T &&forward(typename remove_reference<T>::type &t) noexcept { return static_cast<T&&>(t); }
8 template<typename T> constexpr T &&forward(typename remove_reference<T>::type &&t) noexcept { return static_cast<T&&>(t); }
9 template<typename T> constexpr typename remove_reference<T>::type &&move(T &&t) noexcept { return static_cast<typename remove_reference<T>::type&&>(t); }
10 
11 template<typename T> T declval() noexcept;
12 
13 namespace detail {
14   template<unsigned N> struct select {}; // : integral_constant<unsigned, N> {};
15   template<typename T> struct type {};
16 
17   template<typename...T> union either_impl;
18 
19   template<> union either_impl<> {
20     void get(...);
21     void destroy(...) { throw "logic_error"; }
22   };
23 
24   template<typename T, typename...Ts> union either_impl<T, Ts...> {
25   private:
26     T val;
27     either_impl<Ts...> rest;
28     typedef either_impl<Ts...> rest_t;
29 
30   public:
31     constexpr either_impl(select<0>, T &&t) : val(move(t)) {}
32 
33     template<unsigned N, typename U>
34     constexpr either_impl(select<N>, U &&u) : rest(select<N-1>(), move(u)) {}
35 
36     constexpr static unsigned index(type<T>) { return 0; }
37     template<typename U>
38     constexpr static unsigned index(type<U> t) {
39       return decltype(rest)::index(t) + 1;
40     }
41 
42     void destroy(unsigned elem) {
43       if (elem)
44         rest.destroy(elem - 1);
45       else
46         val.~T();
47     }
48 
49     constexpr const T &get(select<0>) { return val; }
50     template<unsigned N> constexpr const decltype(static_cast<const rest_t&>(rest).get(select<N-1>{})) get(select<N>) {
51       return rest.get(select<N-1>{});
52     }
53   };
54 }
55 
56 template<typename T>
57 struct a {
58   T value;
59   template<typename...U>
60   constexpr a(U &&...u) : value{forward<U>(u)...} {}
61 };
62 template<typename T> using an = a<T>;
63 
64 template<typename T, typename U> T throw_(const U &u) { throw u; }
65 
66 template<typename...T>
67 class either {
68   unsigned elem;
69   detail::either_impl<T...> impl;
70   typedef decltype(impl) impl_t;
71 
72 public:
73   template<typename U>
74   constexpr either(a<U> &&t) :
75     elem(impl_t::index(detail::type<U>())),
76     impl(detail::select<impl_t::index(detail::type<U>())>(), move(t.value)) {}
77 
78   // Destruction disabled to allow use in a constant expression.
79   // FIXME: declare a destructor iff any element has a nontrivial destructor
80   //~either() { impl.destroy(elem); }
81 
82   constexpr unsigned index() noexcept { return elem; }
83 
84   template<unsigned N> using const_get_result =
85     decltype(static_cast<const impl_t&>(impl).get(detail::select<N>{}));
86 
87   template<unsigned N>
88   constexpr const_get_result<N> get() {
89     // Can't just use throw here, since that makes the conditional a prvalue,
90     // which means we return a reference to a temporary.
91     return (elem != N ? throw_<const_get_result<N>>("bad_either_get")
92                       : impl.get(detail::select<N>{}));
93   }
94 
95   template<typename U>
96   constexpr const U &get() {
97     return get<impl_t::index(detail::type<U>())>();
98   }
99 };
100 
101 typedef either<int, char, double> icd;
102 constexpr icd icd1 = an<int>(4);
103 constexpr icd icd2 = a<char>('x');
104 constexpr icd icd3 = a<double>(6.5);
105 
106 static_assert(icd1.get<int>() == 4, "");
107 static_assert(icd2.get<char>() == 'x', "");
108 static_assert(icd3.get<double>() == 6.5, "");
109 
110 struct non_triv {
111   constexpr non_triv() : n(5) {}
112   int n;
113 };
114 constexpr either<const icd*, non_triv> icd4 = a<const icd*>(&icd2);
115 constexpr either<const icd*, non_triv> icd5 = a<non_triv>();
116 
117 static_assert(icd4.get<const icd*>()->get<char>() == 'x', "");
118 static_assert(icd5.get<non_triv>().n == 5, "");
119