libcxx/include/__iterator/readable_traits.h

// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// 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___ITERATOR_READABLE_TRAITS_H
#define _LIBCPP___ITERATOR_READABLE_TRAITS_H

#include <__config>
#include <concepts>
#include <type_traits>

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

_LIBCPP_BEGIN_NAMESPACE_STD

#if !defined(_LIBCPP_HAS_NO_RANGES)

// [readable.traits]
template<class> struct __cond_value_type {};

template<class _Tp>
requires is_object_v<_Tp>
struct __cond_value_type<_Tp> { using value_type = remove_cv_t<_Tp>; };

template<class _Tp>
concept __has_member_value_type = requires { typename _Tp::value_type; };

template<class _Tp>
concept __has_member_element_type = requires { typename _Tp::element_type; };

template<class> struct indirectly_readable_traits {};

template<class _Ip>
requires is_array_v<_Ip>
struct indirectly_readable_traits<_Ip> {
  using value_type = remove_cv_t<remove_extent_t<_Ip>>;
};

template<class _Ip>
struct indirectly_readable_traits<const _Ip> : indirectly_readable_traits<_Ip> {};

template<class _Tp>
struct indirectly_readable_traits<_Tp*> : __cond_value_type<_Tp> {};

template<__has_member_value_type _Tp>
struct indirectly_readable_traits<_Tp>
  : __cond_value_type<typename _Tp::value_type> {};

template<__has_member_element_type _Tp>
struct indirectly_readable_traits<_Tp>
  : __cond_value_type<typename _Tp::element_type> {};

// Pre-emptively applies LWG3541
template<__has_member_value_type _Tp>
requires __has_member_element_type<_Tp>
struct indirectly_readable_traits<_Tp> {};
template<__has_member_value_type _Tp>
requires __has_member_element_type<_Tp> &&
         same_as<remove_cv_t<typename _Tp::element_type>,
                 remove_cv_t<typename _Tp::value_type>>
struct indirectly_readable_traits<_Tp>
  : __cond_value_type<typename _Tp::value_type> {};

template <class>
struct iterator_traits;

// Let `RI` be `remove_cvref_t<I>`. The type `iter_value_t<I>` denotes
// `indirectly_readable_traits<RI>::value_type` if `iterator_traits<RI>` names a specialization
// generated from the primary template, and `iterator_traits<RI>::value_type` otherwise.
template <class _Ip>
using iter_value_t = typename conditional_t<__is_primary_template<iterator_traits<remove_cvref_t<_Ip> > >::value,
                                            indirectly_readable_traits<remove_cvref_t<_Ip> >,
                                            iterator_traits<remove_cvref_t<_Ip> > >::value_type;

#endif // !defined(_LIBCPP_HAS_NO_RANGES)

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___ITERATOR_READABLE_TRAITS_H