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: 28 template <class _Tp, class _Up> 29 using __cond_type = decltype(false ? declval<_Tp>() : declval<_Up>()); 30 31 template <class _Tp, class _Up, class = void> 32 struct __common_type3 {}; 33 34 // sub-bullet 4 - "if COND_RES(CREF(D1), CREF(D2)) denotes a type..." 35 template <class _Tp, class _Up> 36 struct __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 41 template <class _Tp, class _Up, class = void> 42 struct __common_type2_imp : __common_type3<_Tp, _Up> {}; 43 #else 44 template <class _Tp, class _Up, class = void> 45 struct __common_type2_imp {}; 46 #endif 47 48 // sub-bullet 3 - "if decay_t<decltype(false ? declval<D1>() : declval<D2>())> ..." 49 template <class _Tp, class _Up> 50 struct __common_type2_imp<_Tp, _Up, 51 typename __void_t<decltype( 52 true ? declval<_Tp>() : declval<_Up>() 53 )>::type> 54 { 55 typedef _LIBCPP_NODEBUG typename decay<decltype( 56 true ? declval<_Tp>() : declval<_Up>() 57 )>::type type; 58 }; 59 60 template <class, class = void> 61 struct __common_type_impl {}; 62 63 // Clang provides variadic templates in C++03 as an extension. 64 #if !defined(_LIBCPP_CXX03_LANG) || defined(__clang__) 65 # define _LIBCPP_OPTIONAL_PACK(...) , __VA_ARGS__ 66 template <class... _Tp> 67 struct __common_types; 68 template <class... _Tp> 69 struct _LIBCPP_TEMPLATE_VIS common_type; 70 #else 71 # define _LIBCPP_OPTIONAL_PACK(...) 72 struct __no_arg; 73 template <class _Tp, class _Up, class = __no_arg> 74 struct __common_types; 75 template <class _Tp = __no_arg, class _Up = __no_arg, class _Vp = __no_arg, 76 class _Unused = __no_arg> 77 struct common_type { 78 static_assert(sizeof(_Unused) == 0, 79 "common_type accepts at most 3 arguments in C++03"); 80 }; 81 #endif // _LIBCPP_CXX03_LANG 82 83 template <class _Tp, class _Up> 84 struct __common_type_impl< 85 __common_types<_Tp, _Up>, 86 typename __void_t<typename common_type<_Tp, _Up>::type>::type> 87 { 88 typedef typename common_type<_Tp, _Up>::type type; 89 }; 90 91 template <class _Tp, class _Up, class _Vp _LIBCPP_OPTIONAL_PACK(class... _Rest)> 92 struct __common_type_impl< 93 __common_types<_Tp, _Up, _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)>, 94 typename __void_t<typename common_type<_Tp, _Up>::type>::type> 95 : __common_type_impl<__common_types<typename common_type<_Tp, _Up>::type, 96 _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)> > { 97 }; 98 99 // bullet 1 - sizeof...(Tp) == 0 100 101 template <> 102 struct _LIBCPP_TEMPLATE_VIS common_type<> {}; 103 104 // bullet 2 - sizeof...(Tp) == 1 105 106 template <class _Tp> 107 struct _LIBCPP_TEMPLATE_VIS common_type<_Tp> 108 : public common_type<_Tp, _Tp> {}; 109 110 // bullet 3 - sizeof...(Tp) == 2 111 112 // sub-bullet 1 - "If is_same_v<T1, D1> is false or ..." 113 template <class _Tp, class _Up> 114 struct _LIBCPP_TEMPLATE_VIS common_type<_Tp, _Up> 115 : conditional< 116 _IsSame<_Tp, typename decay<_Tp>::type>::value && _IsSame<_Up, typename decay<_Up>::type>::value, 117 __common_type2_imp<_Tp, _Up>, 118 common_type<typename decay<_Tp>::type, typename decay<_Up>::type> 119 >::type 120 {}; 121 122 // bullet 4 - sizeof...(Tp) > 2 123 124 template <class _Tp, class _Up, class _Vp _LIBCPP_OPTIONAL_PACK(class... _Rest)> 125 struct _LIBCPP_TEMPLATE_VIS 126 common_type<_Tp, _Up, _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)> 127 : __common_type_impl< 128 __common_types<_Tp, _Up, _Vp _LIBCPP_OPTIONAL_PACK(_Rest...)> > {}; 129 130 #undef _LIBCPP_OPTIONAL_PACK 131 132 #if _LIBCPP_STD_VER > 11 133 template <class ..._Tp> using common_type_t = typename common_type<_Tp...>::type; 134 #endif 135 136 _LIBCPP_END_NAMESPACE_STD 137 138 #endif // _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H 139