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 >= 20 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 __decay_t<decltype( 53 true ? std::declval<_Tp>() : std::declval<_Up>() 54 )> type; 55 }; 56 57 template <class, class = void> 58 struct __common_type_impl {}; 59 60 template <class... _Tp> 61 struct __common_types; 62 template <class... _Tp> 63 struct _LIBCPP_TEMPLATE_VIS common_type; 64 65 template <class _Tp, class _Up> 66 struct __common_type_impl< 67 __common_types<_Tp, _Up>, __void_t<typename common_type<_Tp, _Up>::type> > 68 { 69 typedef typename common_type<_Tp, _Up>::type type; 70 }; 71 72 template <class _Tp, class _Up, class _Vp, class... _Rest> 73 struct __common_type_impl< 74 __common_types<_Tp, _Up, _Vp, _Rest...>, 75 __void_t<typename common_type<_Tp, _Up>::type> > 76 : __common_type_impl<__common_types<typename common_type<_Tp, _Up>::type, 77 _Vp, _Rest...> > { 78 }; 79 80 // bullet 1 - sizeof...(Tp) == 0 81 82 template <> 83 struct _LIBCPP_TEMPLATE_VIS common_type<> {}; 84 85 // bullet 2 - sizeof...(Tp) == 1 86 87 template <class _Tp> 88 struct _LIBCPP_TEMPLATE_VIS common_type<_Tp> 89 : public common_type<_Tp, _Tp> {}; 90 91 // bullet 3 - sizeof...(Tp) == 2 92 93 // sub-bullet 1 - "If is_same_v<T1, D1> is false or ..." 94 template <class _Tp, class _Up> 95 struct _LIBCPP_TEMPLATE_VIS common_type<_Tp, _Up> 96 : conditional< 97 _IsSame<_Tp, __decay_t<_Tp> >::value && _IsSame<_Up, __decay_t<_Up> >::value, 98 __common_type2_imp<_Tp, _Up>, 99 common_type<__decay_t<_Tp>, __decay_t<_Up> > 100 >::type 101 {}; 102 103 // bullet 4 - sizeof...(Tp) > 2 104 105 template <class _Tp, class _Up, class _Vp, class... _Rest> 106 struct _LIBCPP_TEMPLATE_VIS 107 common_type<_Tp, _Up, _Vp, _Rest...> 108 : __common_type_impl< 109 __common_types<_Tp, _Up, _Vp, _Rest...> > {}; 110 111 #if _LIBCPP_STD_VER >= 14 112 template <class ..._Tp> using common_type_t = typename common_type<_Tp...>::type; 113 #endif 114 115 _LIBCPP_END_NAMESPACE_STD 116 117 #endif // _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H 118