type_traits.h

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//===- llvm/Support/type_traits.h - Simplfied type traits -------*- 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
//
//===----------------------------------------------------------------------===//
//
// This file provides useful additions to the standard type_traits library.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_SUPPORT_TYPE_TRAITS_H
#define LLVM_SUPPORT_TYPE_TRAITS_H

#include "llvm/Support/Compiler.h"
#include <type_traits>
#include <utility>

namespace llvm {


/// Metafunction that determines whether the given type is either an
/// integral type or an enumeration type, including enum classes.
///
/// Note that this accepts potentially more integral types than is_integral
/// because it is based on being implicitly convertible to an integral type.
/// Also note that enum classes aren't implicitly convertible to integral types,
/// the value may therefore need to be explicitly converted before being used.
template <typename T> class is_integral_or_enum {
  using UnderlyingT = typename std::remove_reference<T>::type;

public:
  static const bool value =
      !std::is_class<UnderlyingT>::value && // Filter conversion operators.
      !std::is_pointer<UnderlyingT>::value &&
      !std::is_floating_point<UnderlyingT>::value &&
      (std::is_enum<UnderlyingT>::value ||
       std::is_convertible<UnderlyingT, unsigned long long>::value);
};

/// If T is a pointer, just return it. If it is not, return T&.
template<typename T, typename Enable = void>
struct add_lvalue_reference_if_not_pointer { using type = T &; };

template <typename T>
struct add_lvalue_reference_if_not_pointer<
    T, typename std::enable_if<std::is_pointer<T>::value>::type> {
  using type = T;
};

/// If T is a pointer to X, return a pointer to const X. If it is not,
/// return const T.
template<typename T, typename Enable = void>
struct add_const_past_pointer { using type = const T; };

template <typename T>
struct add_const_past_pointer<
    T, typename std::enable_if<std::is_pointer<T>::value>::type> {
  using type = const typename std::remove_pointer<T>::type *;
};

template <typename T, typename Enable = void>
struct const_pointer_or_const_ref {
  using type = const T &;
};
template <typename T>
struct const_pointer_or_const_ref<
    T, typename std::enable_if<std::is_pointer<T>::value>::type> {
  using type = typename add_const_past_pointer<T>::type;
};

namespace detail {
/// Internal utility to detect trivial copy construction.
template<typename T> union copy_construction_triviality_helper {
    T t;
    copy_construction_triviality_helper() = default;
    copy_construction_triviality_helper(const copy_construction_triviality_helper&) = default;
    ~copy_construction_triviality_helper() = default;
};
/// Internal utility to detect trivial move construction.
template<typename T> union move_construction_triviality_helper {
    T t;
    move_construction_triviality_helper() = default;
    move_construction_triviality_helper(move_construction_triviality_helper&&) = default;
    ~move_construction_triviality_helper() = default;
};

template<class T>
union trivial_helper {
    T t;
};

} // end namespace detail

/// An implementation of `std::is_trivially_copy_constructible` since we have
/// users with STLs that don't yet include it.
template <typename T>
struct is_trivially_copy_constructible
    : std::is_copy_constructible<
          ::llvm::detail::copy_construction_triviality_helper<T>> {};
template <typename T>
struct is_trivially_copy_constructible<T &> : std::true_type {};
template <typename T>
struct is_trivially_copy_constructible<T &&> : std::false_type {};

/// An implementation of `std::is_trivially_move_constructible` since we have
/// users with STLs that don't yet include it.
template <typename T>
struct is_trivially_move_constructible
    : std::is_move_constructible<
          ::llvm::detail::move_construction_triviality_helper<T>> {};
template <typename T>
struct is_trivially_move_constructible<T &> : std::true_type {};
template <typename T>
struct is_trivially_move_constructible<T &&> : std::true_type {};


template <typename T>
struct is_copy_assignable {
  template<class F>
    static auto get(F*) -> decltype(std::declval<F &>() = std::declval<const F &>(), std::true_type{});
    static std::false_type get(...);
    static constexpr bool value = decltype(get((T*)nullptr))::value;
};

template <typename T>
struct is_move_assignable {
  template<class F>
    static auto get(F*) -> decltype(std::declval<F &>() = std::declval<F &&>(), std::true_type{});
    static std::false_type get(...);
    static constexpr bool value = decltype(get((T*)nullptr))::value;
};


// An implementation of `std::is_trivially_copyable` since STL version
// is not equally supported by all compilers, especially GCC 4.9.
// Uniform implementation of this trait is important for ABI compatibility
// as it has an impact on SmallVector's ABI (among others).
template <typename T>
class is_trivially_copyable {

  // copy constructors
  static constexpr bool has_trivial_copy_constructor =
      std::is_copy_constructible<detail::trivial_helper<T>>::value;
  static constexpr bool has_deleted_copy_constructor =
      !std::is_copy_constructible<T>::value;

  // move constructors
  static constexpr bool has_trivial_move_constructor =
      std::is_move_constructible<detail::trivial_helper<T>>::value;
  static constexpr bool has_deleted_move_constructor =
      !std::is_move_constructible<T>::value;

  // copy assign
  static constexpr bool has_trivial_copy_assign =
      is_copy_assignable<detail::trivial_helper<T>>::value;
  static constexpr bool has_deleted_copy_assign =
      !is_copy_assignable<T>::value;

  // move assign
  static constexpr bool has_trivial_move_assign =
      is_move_assignable<detail::trivial_helper<T>>::value;
  static constexpr bool has_deleted_move_assign =
      !is_move_assignable<T>::value;

  // destructor
  static constexpr bool has_trivial_destructor =
      std::is_destructible<detail::trivial_helper<T>>::value;

  public:

  static constexpr bool value =
      has_trivial_destructor &&
      (has_deleted_move_assign || has_trivial_move_assign) &&
      (has_deleted_move_constructor || has_trivial_move_constructor) &&
      (has_deleted_copy_assign || has_trivial_copy_assign) &&
      (has_deleted_copy_constructor || has_trivial_copy_constructor);

#ifdef HAVE_STD_IS_TRIVIALLY_COPYABLE
  static_assert(value == std::is_trivially_copyable<T>::value,
                "inconsistent behavior between llvm:: and std:: implementation of is_trivially_copyable");
#endif
};
template <typename T>
class is_trivially_copyable<T*> : public std::true_type {
};


} // end namespace llvm

#endif // LLVM_SUPPORT_TYPE_TRAITS_H