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_REFERENCE_H
10 #define _LIBCPP___TYPE_TRAITS_COMMON_REFERENCE_H
11 
12 #include <__config>
13 #include <__type_traits/common_type.h>
14 #include <__type_traits/copy_cv.h>
15 #include <__type_traits/copy_cvref.h>
16 #include <__type_traits/is_convertible.h>
17 #include <__type_traits/is_reference.h>
18 #include <__type_traits/remove_cv.h>
19 #include <__type_traits/remove_cvref.h>
20 #include <__type_traits/remove_reference.h>
21 #include <__utility/declval.h>
22 
23 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
24 #  pragma GCC system_header
25 #endif
26 
27 _LIBCPP_BEGIN_NAMESPACE_STD
28 
29 // common_reference
30 #if _LIBCPP_STD_VER > 17
31 // Let COND_RES(X, Y) be:
32 template <class _Xp, class _Yp>
33 using __cond_res =
34     decltype(false ? std::declval<_Xp(&)()>()() : std::declval<_Yp(&)()>()());
35 
36 // Let `XREF(A)` denote a unary alias template `T` such that `T<U>` denotes the same type as `U`
37 // with the addition of `A`'s cv and reference qualifiers, for a non-reference cv-unqualified type
38 // `U`.
39 // [Note: `XREF(A)` is `__xref<A>::template __apply`]
40 template <class _Tp>
41 struct __xref {
42   template<class _Up>
43   using __apply = __copy_cvref_t<_Tp, _Up>;
44 };
45 
46 // Given types A and B, let X be remove_reference_t<A>, let Y be remove_reference_t<B>,
47 // and let COMMON-REF(A, B) be:
48 template<class _Ap, class _Bp, class _Xp = remove_reference_t<_Ap>, class _Yp = remove_reference_t<_Bp>>
49 struct __common_ref;
50 
51 template<class _Xp, class _Yp>
52 using __common_ref_t = typename __common_ref<_Xp, _Yp>::__type;
53 
54 template<class _Xp, class _Yp>
55 using __cv_cond_res = __cond_res<__copy_cv_t<_Xp, _Yp>&, __copy_cv_t<_Yp, _Xp>&>;
56 
57 
58 //    If A and B are both lvalue reference types, COMMON-REF(A, B) is
59 //    COND-RES(COPYCV(X, Y)&, COPYCV(Y, X)&) if that type exists and is a reference type.
60 template<class _Ap, class _Bp, class _Xp, class _Yp>
61 requires requires { typename __cv_cond_res<_Xp, _Yp>; } && is_reference_v<__cv_cond_res<_Xp, _Yp>>
62 struct __common_ref<_Ap&, _Bp&, _Xp, _Yp>
63 {
64     using __type = __cv_cond_res<_Xp, _Yp>;
65 };
66 
67 //    Otherwise, let C be remove_reference_t<COMMON-REF(X&, Y&)>&&. ...
68 template <class _Xp, class _Yp>
69 using __common_ref_C = remove_reference_t<__common_ref_t<_Xp&, _Yp&>>&&;
70 
71 
72 //    .... If A and B are both rvalue reference types, C is well-formed, and
73 //    is_convertible_v<A, C> && is_convertible_v<B, C> is true, then COMMON-REF(A, B) is C.
74 template<class _Ap, class _Bp, class _Xp, class _Yp>
75 requires
76   requires { typename __common_ref_C<_Xp, _Yp>; } &&
77   is_convertible_v<_Ap&&, __common_ref_C<_Xp, _Yp>> &&
78   is_convertible_v<_Bp&&, __common_ref_C<_Xp, _Yp>>
79 struct __common_ref<_Ap&&, _Bp&&, _Xp, _Yp>
80 {
81     using __type = __common_ref_C<_Xp, _Yp>;
82 };
83 
84 //    Otherwise, let D be COMMON-REF(const X&, Y&). ...
85 template <class _Tp, class _Up>
86 using __common_ref_D = __common_ref_t<const _Tp&, _Up&>;
87 
88 //    ... If A is an rvalue reference and B is an lvalue reference and D is well-formed and
89 //    is_convertible_v<A, D> is true, then COMMON-REF(A, B) is D.
90 template<class _Ap, class _Bp, class _Xp, class _Yp>
91 requires requires { typename __common_ref_D<_Xp, _Yp>; } &&
92          is_convertible_v<_Ap&&, __common_ref_D<_Xp, _Yp>>
93 struct __common_ref<_Ap&&, _Bp&, _Xp, _Yp>
94 {
95     using __type = __common_ref_D<_Xp, _Yp>;
96 };
97 
98 //    Otherwise, if A is an lvalue reference and B is an rvalue reference, then
99 //    COMMON-REF(A, B) is COMMON-REF(B, A).
100 template<class _Ap, class _Bp, class _Xp, class _Yp>
101 struct __common_ref<_Ap&, _Bp&&, _Xp, _Yp> : __common_ref<_Bp&&, _Ap&> {};
102 
103 //    Otherwise, COMMON-REF(A, B) is ill-formed.
104 template<class _Ap, class _Bp, class _Xp, class _Yp>
105 struct __common_ref {};
106 
107 // Note C: For the common_reference trait applied to a parameter pack [...]
108 
109 template <class...>
110 struct common_reference;
111 
112 template <class... _Types>
113 using common_reference_t = typename common_reference<_Types...>::type;
114 
115 // bullet 1 - sizeof...(T) == 0
116 template<>
117 struct common_reference<> {};
118 
119 // bullet 2 - sizeof...(T) == 1
120 template <class _Tp>
121 struct common_reference<_Tp>
122 {
123     using type = _Tp;
124 };
125 
126 // bullet 3 - sizeof...(T) == 2
127 template <class _Tp, class _Up> struct __common_reference_sub_bullet3;
128 template <class _Tp, class _Up> struct __common_reference_sub_bullet2 : __common_reference_sub_bullet3<_Tp, _Up> {};
129 template <class _Tp, class _Up> struct __common_reference_sub_bullet1 : __common_reference_sub_bullet2<_Tp, _Up> {};
130 
131 // sub-bullet 1 - If T1 and T2 are reference types and COMMON-REF(T1, T2) is well-formed, then
132 // the member typedef `type` denotes that type.
133 template <class _Tp, class _Up> struct common_reference<_Tp, _Up> : __common_reference_sub_bullet1<_Tp, _Up> {};
134 
135 template <class _Tp, class _Up>
136 requires is_reference_v<_Tp> && is_reference_v<_Up> && requires { typename __common_ref_t<_Tp, _Up>; }
137 struct __common_reference_sub_bullet1<_Tp, _Up>
138 {
139     using type = __common_ref_t<_Tp, _Up>;
140 };
141 
142 // sub-bullet 2 - Otherwise, if basic_common_reference<remove_cvref_t<T1>, remove_cvref_t<T2>, XREF(T1), XREF(T2)>::type
143 // is well-formed, then the member typedef `type` denotes that type.
144 template <class, class, template <class> class, template <class> class> struct basic_common_reference {};
145 
146 template <class _Tp, class _Up>
147 using __basic_common_reference_t = typename basic_common_reference<
148     remove_cvref_t<_Tp>, remove_cvref_t<_Up>,
149     __xref<_Tp>::template __apply, __xref<_Up>::template __apply>::type;
150 
151 template <class _Tp, class _Up>
152 requires requires { typename __basic_common_reference_t<_Tp, _Up>; }
153 struct __common_reference_sub_bullet2<_Tp, _Up>
154 {
155     using type = __basic_common_reference_t<_Tp, _Up>;
156 };
157 
158 // sub-bullet 3 - Otherwise, if COND-RES(T1, T2) is well-formed,
159 // then the member typedef `type` denotes that type.
160 template <class _Tp, class _Up>
161 requires requires { typename __cond_res<_Tp, _Up>; }
162 struct __common_reference_sub_bullet3<_Tp, _Up>
163 {
164     using type = __cond_res<_Tp, _Up>;
165 };
166 
167 
168 // sub-bullet 4 & 5 - Otherwise, if common_type_t<T1, T2> is well-formed,
169 //                    then the member typedef `type` denotes that type.
170 //                  - Otherwise, there shall be no member `type`.
171 template <class _Tp, class _Up> struct __common_reference_sub_bullet3 : common_type<_Tp, _Up> {};
172 
173 // bullet 4 - If there is such a type `C`, the member typedef type shall denote the same type, if
174 //            any, as `common_reference_t<C, Rest...>`.
175 template <class _Tp, class _Up, class _Vp, class... _Rest>
176 requires requires { typename common_reference_t<_Tp, _Up>; }
177 struct common_reference<_Tp, _Up, _Vp, _Rest...>
178     : common_reference<common_reference_t<_Tp, _Up>, _Vp, _Rest...>
179 {};
180 
181 // bullet 5 - Otherwise, there shall be no member `type`.
182 template <class...> struct common_reference {};
183 
184 #endif // _LIBCPP_STD_VER > 17
185 
186 _LIBCPP_END_NAMESPACE_STD
187 
188 #endif // _LIBCPP___TYPE_TRAITS_COMMON_REFERENCE_H
189