rocksdb/third-party/folly/folly/Traits.h

//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).

#pragma once

#include <type_traits>
#include <utility>

namespace folly {

#if !defined(_MSC_VER)
template <class T>
struct is_trivially_copyable
    : std::integral_constant<bool, __has_trivial_copy(T)> {};
#else
template <class T>
using is_trivially_copyable = std::is_trivially_copyable<T>;
#endif

/***
 *  _t
 *
 *  Instead of:
 *
 *    using decayed = typename std::decay<T>::type;
 *
 *  With the C++14 standard trait aliases, we could use:
 *
 *    using decayed = std::decay_t<T>;
 *
 *  Without them, we could use:
 *
 *    using decayed = _t<std::decay<T>>;
 *
 *  Also useful for any other library with template types having dependent
 *  member types named `type`, like the standard trait types.
 */
template <typename T>
using _t = typename T::type;

/**
 *  type_t
 *
 *  A type alias for the first template type argument. `type_t` is useful for
 *  controlling class-template and function-template partial specialization.
 *
 *  Example:
 *
 *    template <typename Value>
 *    class Container {
 *     public:
 *      template <typename... Args>
 *      Container(
 *          type_t<in_place_t, decltype(Value(std::declval<Args>()...))>,
 *          Args&&...);
 *    };
 *
 *  void_t
 *
 *  A type alias for `void`. `void_t` is useful for controling class-template
 *  and function-template partial specialization.
 *
 *  Example:
 *
 *    // has_value_type<T>::value is true if T has a nested type `value_type`
 *    template <class T, class = void>
 *    struct has_value_type
 *        : std::false_type {};
 *
 *    template <class T>
 *    struct has_value_type<T, folly::void_t<typename T::value_type>>
 *        : std::true_type {};
 */

/**
 * There is a bug in libstdc++, libc++, and MSVC's STL that causes it to
 * ignore unused template parameter arguments in template aliases and does not
 * cause substitution failures. This defect has been recorded here:
 * http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558.
 *
 * This causes the implementation of std::void_t to be buggy, as it is likely
 * defined as something like the following:
 *
 *  template <typename...>
 *  using void_t = void;
 *
 * This causes the compiler to ignore all the template arguments and does not
 * help when one wants to cause substitution failures.  Rather declarations
 * which have void_t in orthogonal specializations are treated as the same.
 * For example, assuming the possible `T` types are only allowed to have
 * either the alias `one` or `two` and never both or none:
 *
 *  template <typename T,
 *            typename std::void_t<std::decay_t<T>::one>* = nullptr>
 *  void foo(T&&) {}
 *  template <typename T,
 *            typename std::void_t<std::decay_t<T>::two>* = nullptr>
 *  void foo(T&&) {}
 *
 * The second foo() will be a redefinition because it conflicts with the first
 * one; void_t does not cause substitution failures - the template types are
 * just ignored.
 */

namespace traits_detail {
template <class T, class...>
struct type_t_ {
  using type = T;
};
} // namespace traits_detail

template <class T, class... Ts>
using type_t = typename traits_detail::type_t_<T, Ts...>::type;
template <class... Ts>
using void_t = type_t<void, Ts...>;

/**
 * A type trait to remove all const volatile and reference qualifiers on a
 * type T
 */
template <typename T>
struct remove_cvref {
  using type =
      typename std::remove_cv<typename std::remove_reference<T>::type>::type;
};
template <typename T>
using remove_cvref_t = typename remove_cvref<T>::type;

template <class T>
struct IsNothrowSwappable
    : std::integral_constant<
          bool,
          std::is_nothrow_move_constructible<T>::value&& noexcept(
              std::swap(std::declval<T&>(), std::declval<T&>()))> {};

template <typename...>
struct Conjunction : std::true_type {};
template <typename T>
struct Conjunction<T> : T {};
template <typename T, typename... TList>
struct Conjunction<T, TList...>
    : std::conditional<T::value, Conjunction<TList...>, T>::type {};

template <typename T>
struct Negation : std::integral_constant<bool, !T::value> {};

template <std::size_t I>
using index_constant = std::integral_constant<std::size_t, I>;

} // namespace folly