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/type_identity.h> 18 #include <__type_traits/void_t.h> 19 #include <__utility/declval.h> 20 #include <__utility/empty.h> 21 22 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 23 # pragma GCC system_header 24 #endif 25 26 _LIBCPP_BEGIN_NAMESPACE_STD 27 28 #if __has_builtin(__builtin_common_type) 29 30 template <class... _Args> 31 struct common_type; 32 33 template <class... _Args> 34 using __common_type_t _LIBCPP_NODEBUG = typename common_type<_Args...>::type; 35 36 template <class... _Args> 37 struct common_type : __builtin_common_type<__common_type_t, __type_identity, __empty, _Args...> {}; 38 39 #else 40 # if _LIBCPP_STD_VER >= 20 41 // Let COND_RES(X, Y) be: 42 template <class _Tp, class _Up> 43 using __cond_type _LIBCPP_NODEBUG = decltype(false ? std::declval<_Tp>() : std::declval<_Up>()); 44 45 template <class _Tp, class _Up, class = void> 46 struct __common_type3 {}; 47 48 // sub-bullet 4 - "if COND_RES(CREF(D1), CREF(D2)) denotes a type..." 49 template <class _Tp, class _Up> 50 struct __common_type3<_Tp, _Up, void_t<__cond_type<const _Tp&, const _Up&>>> { 51 using type = remove_cvref_t<__cond_type<const _Tp&, const _Up&>>; 52 }; 53 54 template <class _Tp, class _Up, class = void> 55 struct __common_type2_imp : __common_type3<_Tp, _Up> {}; 56 # else 57 template <class _Tp, class _Up, class = void> 58 struct __common_type2_imp {}; 59 # endif 60 61 // sub-bullet 3 - "if decay_t<decltype(false ? declval<D1>() : declval<D2>())> ..." 62 template <class _Tp, class _Up> 63 struct __common_type2_imp<_Tp, _Up, __void_t<decltype(true ? std::declval<_Tp>() : std::declval<_Up>())> > { 64 using type _LIBCPP_NODEBUG = __decay_t<decltype(true ? std::declval<_Tp>() : std::declval<_Up>())>; 65 }; 66 67 template <class, class = void> 68 struct __common_type_impl {}; 69 70 template <class... _Tp> 71 struct __common_types; 72 template <class... _Tp> 73 struct _LIBCPP_TEMPLATE_VIS common_type; 74 75 template <class _Tp, class _Up> 76 struct __common_type_impl< __common_types<_Tp, _Up>, __void_t<typename common_type<_Tp, _Up>::type> > { 77 typedef typename common_type<_Tp, _Up>::type type; 78 }; 79 80 template <class _Tp, class _Up, class _Vp, class... _Rest> 81 struct __common_type_impl<__common_types<_Tp, _Up, _Vp, _Rest...>, __void_t<typename common_type<_Tp, _Up>::type> > 82 : __common_type_impl<__common_types<typename common_type<_Tp, _Up>::type, _Vp, _Rest...> > {}; 83 84 // bullet 1 - sizeof...(Tp) == 0 85 86 template <> 87 struct _LIBCPP_TEMPLATE_VIS common_type<> {}; 88 89 // bullet 2 - sizeof...(Tp) == 1 90 91 template <class _Tp> 92 struct _LIBCPP_TEMPLATE_VIS common_type<_Tp> : public common_type<_Tp, _Tp> {}; 93 94 // bullet 3 - sizeof...(Tp) == 2 95 96 // sub-bullet 1 - "If is_same_v<T1, D1> is false or ..." 97 template <class _Tp, class _Up> 98 struct _LIBCPP_TEMPLATE_VIS common_type<_Tp, _Up> 99 : __conditional_t<_IsSame<_Tp, __decay_t<_Tp> >::value && _IsSame<_Up, __decay_t<_Up> >::value, 100 __common_type2_imp<_Tp, _Up>, 101 common_type<__decay_t<_Tp>, __decay_t<_Up> > > {}; 102 103 // bullet 4 - sizeof...(Tp) > 2 104 105 template <class _Tp, class _Up, class _Vp, class... _Rest> 106 struct _LIBCPP_TEMPLATE_VIS common_type<_Tp, _Up, _Vp, _Rest...> 107 : __common_type_impl<__common_types<_Tp, _Up, _Vp, _Rest...> > {}; 108 109 #endif 110 111 #if _LIBCPP_STD_VER >= 14 112 template <class... _Tp> 113 using common_type_t = typename common_type<_Tp...>::type; 114 #endif 115 116 _LIBCPP_END_NAMESPACE_STD 117 118 #endif // _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H 119