chromium/third_party/libc++/src/include/__type_traits/common_type.h

//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H
#define _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H

#include <__config>
#include <__type_traits/conditional.h>
#include <__type_traits/decay.h>
#include <__type_traits/is_same.h>
#include <__type_traits/remove_cvref.h>
#include <__type_traits/void_t.h>
#include <__utility/declval.h>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#  pragma GCC system_header
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER >= 20
// Let COND_RES(X, Y) be:
__cond_type;

template <class _Tp, class _Up, class = void>
struct __common_type3 { … };

// sub-bullet 4 - "if COND_RES(CREF(D1), CREF(D2)) denotes a type..."
__common_type3<_Tp, _Up, void_t<__cond_type<const _Tp &, const _Up &>>>;

template <class _Tp, class _Up, class = void>
struct __common_type2_imp : __common_type3<_Tp, _Up> { … };
#else
template <class _Tp, class _Up, class = void>
struct __common_type2_imp {};
#endif

// sub-bullet 3 - "if decay_t<decltype(false ? declval<D1>() : declval<D2>())> ..."
__common_type2_imp<_Tp, _Up, __void_t<decltype(true ? std::declval<_Tp>() : std::declval<_Up>())>>;

template <class, class = void>
struct __common_type_impl { … };

template <class... _Tp>
struct __common_types;
template <class... _Tp>
struct _LIBCPP_TEMPLATE_VIS common_type;

__common_type_impl<__common_types<_Tp, _Up>, __void_t<typename common_type<_Tp, _Up>::type>>;

__common_type_impl<__common_types<_Tp, _Up, _Vp, _Rest...>, __void_t<typename common_type<_Tp, _Up>::type>>;

// bullet 1 - sizeof...(Tp) == 0

template <>
struct _LIBCPP_TEMPLATE_VIS common_type<> { … };

// bullet 2 - sizeof...(Tp) == 1

common_type<_Tp>;

// bullet 3 - sizeof...(Tp) == 2

// sub-bullet 1 - "If is_same_v<T1, D1> is false or ..."
common_type<_Tp, _Up>;

// bullet 4 - sizeof...(Tp) > 2

common_type<_Tp, _Up, _Vp, _Rest...>;

#if _LIBCPP_STD_VER >= 14
common_type_t;
#endif

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___TYPE_TRAITS_COMMON_TYPE_H