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 ? std::declval<_Tp>() : std::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, __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 57 template <class, class = void> 58 struct __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__ 63 template <class... _Tp> 64 struct __common_types; 65 template <class... _Tp> 66 struct _LIBCPP_TEMPLATE_VIS common_type; 67 #else 68 # define _LIBCPP_OPTIONAL_PACK(...) 69 struct __no_arg; 70 template <class _Tp, class _Up, class = __no_arg> 71 struct __common_types; 72 template <class _Tp = __no_arg, class _Up = __no_arg, class _Vp = __no_arg, 73 class _Unused = __no_arg> 74 struct 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 80 template <class _Tp, class _Up> 81 struct __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 87 template <class _Tp, class _Up, class _Vp _LIBCPP_OPTIONAL_PACK(class... _Rest)> 88 struct __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 97 template <> 98 struct _LIBCPP_TEMPLATE_VIS common_type<> {}; 99 100 // bullet 2 - sizeof...(Tp) == 1 101 102 template <class _Tp> 103 struct _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 ..." 109 template <class _Tp, class _Up> 110 struct _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) > 2 119 120 template <class _Tp, class _Up, class _Vp _LIBCPP_OPTIONAL_PACK(class... _Rest)> 121 struct _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 > 11 129 template <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