1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#ifndef _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H
10#define _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H
11
12#include <__config>
13#include <__type_traits/conditional.h>
14#include <__type_traits/decay.h>
15#include <__type_traits/is_same.h>
16#include <__type_traits/remove_cvref.h>
17#include <__type_traits/void_t.h>
18#include <__utility/declval.h>
19
20#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
21#  pragma GCC system_header
22#endif
23
24_LIBCPP_BEGIN_NAMESPACE_STD
25
26#if _LIBCPP_STD_VER > 17
27// Let COND_RES(X, Y) be:
28template <class _Tp, class _Up>
29using __cond_type = decltype(false ? std::declval<_Tp>() : std::declval<_Up>());
30
31template <class _Tp, class _Up, class = void>
32struct __common_type3 {};
33
34// sub-bullet 4 - "if COND_RES(CREF(D1), CREF(D2)) denotes a type..."
35template <class _Tp, class _Up>
36struct __common_type3<_Tp, _Up, void_t<__cond_type<const _Tp&, const _Up&>>>
37{
38    using type = remove_cvref_t<__cond_type<const _Tp&, const _Up&>>;
39};
40
41template <class _Tp, class _Up, class = void>
42struct __common_type2_imp : __common_type3<_Tp, _Up> {};
43#else
44template <class _Tp, class _Up, class = void>
45struct __common_type2_imp {};
46#endif
47
48// sub-bullet 3 - "if decay_t<decltype(false ? declval<D1>() : declval<D2>())> ..."
49template <class _Tp, class _Up>
50struct __common_type2_imp<_Tp, _Up, __void_t<decltype(true ? std::declval<_Tp>() : std::declval<_Up>())> >
51{
52  typedef _LIBCPP_NODEBUG typename decay<decltype(
53                         true ? std::declval<_Tp>() : std::declval<_Up>()
54                         )>::type type;
55};
56
57template <class, class = void>
58struct __common_type_impl {};
59
60// Clang provides variadic templates in C++03 as an extension.
61#if !defined(_LIBCPP_CXX03_LANG) || defined(__clang__)
62# define _LIBCPP_OPTIONAL_PACK(...) , __VA_ARGS__
63template <class... _Tp>
64struct __common_types;
65template <class... _Tp>
66struct _LIBCPP_TEMPLATE_VIS common_type;
67#else
68# define _LIBCPP_OPTIONAL_PACK(...)
69struct __no_arg;
70template <class _Tp, class _Up, class = __no_arg>
71struct __common_types;
72template <class _Tp = __no_arg, class _Up = __no_arg, class _Vp = __no_arg,
73          class _Unused = __no_arg>
74struct common_type {
75  static_assert(sizeof(_Unused) == 0,
76                "common_type accepts at most 3 arguments in C++03");
77};
78#endif // _LIBCPP_CXX03_LANG
79
80template <class _Tp, class _Up>
81struct __common_type_impl<
82    __common_types<_Tp, _Up>, __void_t<typename common_type<_Tp, _Up>::type> >
83{
84  typedef typename common_type<_Tp, _Up>::type type;
85};
86
87template <class _Tp, class _Up, class _Vp _LIBCPP_OPTIONAL_PACK(class... _Rest)>
88struct __common_type_impl<
89    __common_types<_Tp, _Up, _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)>,
90    __void_t<typename common_type<_Tp, _Up>::type> >
91    : __common_type_impl<__common_types<typename common_type<_Tp, _Up>::type,
92                                        _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)> > {
93};
94
95// bullet 1 - sizeof...(Tp) == 0
96
97template <>
98struct _LIBCPP_TEMPLATE_VIS common_type<> {};
99
100// bullet 2 - sizeof...(Tp) == 1
101
102template <class _Tp>
103struct _LIBCPP_TEMPLATE_VIS common_type<_Tp>
104    : public common_type<_Tp, _Tp> {};
105
106// bullet 3 - sizeof...(Tp) == 2
107
108// sub-bullet 1 - "If is_same_v<T1, D1> is false or ..."
109template <class _Tp, class _Up>
110struct _LIBCPP_TEMPLATE_VIS common_type<_Tp, _Up>
111    : conditional<
112        _IsSame<_Tp, typename decay<_Tp>::type>::value && _IsSame<_Up, typename decay<_Up>::type>::value,
113        __common_type2_imp<_Tp, _Up>,
114        common_type<typename decay<_Tp>::type, typename decay<_Up>::type>
115    >::type
116{};
117
118// bullet 4 - sizeof...(Tp) > 2
119
120template <class _Tp, class _Up, class _Vp _LIBCPP_OPTIONAL_PACK(class... _Rest)>
121struct _LIBCPP_TEMPLATE_VIS
122    common_type<_Tp, _Up, _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)>
123    : __common_type_impl<
124          __common_types<_Tp, _Up, _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)> > {};
125
126#undef _LIBCPP_OPTIONAL_PACK
127
128#if _LIBCPP_STD_VER > 11
129template <class ..._Tp> using common_type_t = typename common_type<_Tp...>::type;
130#endif
131
132_LIBCPP_END_NAMESPACE_STD
133
134#endif // _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H
135