/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// @author: Andrei Alexandrescu

#pragma once

#include <cstdint>
#include <functional>
#include <limits>
#include <memory>
#include <tuple>
#include <type_traits>

#include <folly/Portability.h>

namespace folly {

template <typename...>
struct tag_t {};

template <typename... T>
FOLLY_INLINE_VARIABLE constexpr tag_t<T...> tag{};

#if __cpp_lib_bool_constant || _MSC_VER

using std::bool_constant;

#else

//  mimic: std::bool_constant, C++17
template <bool B>
using bool_constant = std::integral_constant<bool, B>;

#endif

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

//  always_false
//
//  A variable template that is always false but requires template arguments to
//  be provided (which are then ignored). This is useful in very specific cases
//  where we want type-dependent expressions to defer static_assert's.
//
//  A common use-case is for exhaustive constexpr if branches:
//
//    template <typename T>
//    void foo(T value) {
//      if constexpr (std::is_integral_v<T>) foo_integral(value);
//      else if constexpr (std::is_same_v<T, std::string>) foo_string(value);
//      else static_assert(always_false<T>, "Unsupported type");
//    }
//
//  If we had used static_assert(false), then this would always fail to compile,
//  even if foo is never instantiated!
//
//  Another use case is if a template that is expected to always be specialized
//  is erroneously instantiated with the base template.
//
//    template <typename T>
//    struct Foo {
//      static_assert(always_false<T>, "Unsupported type");
//    };
//    template <>
//    struct Foo<int> {};
//
//    Foo<int> a;         // fine
//    Foo<std::string> b; // fails! And you get a nice (custom) error message
//
//  This is similar to leaving the base template undefined but we get a nicer
//  compiler error message with static_assert.
template <typename...>
FOLLY_INLINE_VARIABLE constexpr bool always_false = false;

namespace detail {

//  is_instantiation_of_v
//  is_instantiation_of
//
//  A trait variable and type to check if a given type is an instantiation of a
//  class template.
//
//  Note that this only works with type template parameters. It does not work
//  with non-type template parameters, template template parameters, or alias
//  templates.
template <template <typename...> class, typename>
FOLLY_INLINE_VARIABLE constexpr bool is_instantiation_of_v = false;
template <template <typename...> class C, typename... T>
FOLLY_INLINE_VARIABLE constexpr bool is_instantiation_of_v<C, C<T...>> = true;
template <template <typename...> class C, typename... T>
struct is_instantiation_of : bool_constant<is_instantiation_of_v<C, T...>> {};

template <typename, typename>
FOLLY_INLINE_VARIABLE constexpr bool is_similar_instantiation_v = false;
template <template <typename...> class C, typename... A, typename... B>
FOLLY_INLINE_VARIABLE constexpr bool
    is_similar_instantiation_v<C<A...>, C<B...>> = true;
template <typename A, typename B>
struct is_similar_instantiation
    : bool_constant<is_similar_instantiation_v<A, B>> {};

} // namespace detail

namespace detail {

struct is_constexpr_default_constructible_ {
  template <typename T>
  static constexpr auto make(tag_t<T>) -> decltype(void(T()), 0) {
    return (void(T()), 0);
  }
  // second param should just be: int = (void(T()), 0)
  // but under clang 10, crash: https://bugs.llvm.org/show_bug.cgi?id=47620
  // and, with assertions disabled, expectation failures showing compiler
  // deviation from the language spec
  // xcode renumbers clang versions so detection is tricky, but, if detection
  // were desired, a combination of __apple_build_version__ and __clang_major__
  // may be used to reduce frontend overhead under correct compilers: clang 12
  // under xcode and clang 10 otherwise
  template <typename T, int = make(tag<T>)>
  static std::true_type sfinae(T*);
  static std::false_type sfinae(void*);
  template <typename T>
  static constexpr bool apply =
      decltype(sfinae(static_cast<T*>(nullptr)))::value;
};

} // namespace detail

//  is_constexpr_default_constructible_v
//  is_constexpr_default_constructible
//
//  A trait variable and type which determines whether the type parameter is
//  constexpr default-constructible, that is, default-constructible in a
//  constexpr context.
template <typename T>
FOLLY_INLINE_VARIABLE constexpr bool is_constexpr_default_constructible_v =
    detail::is_constexpr_default_constructible_::apply<T>;
template <typename T>
struct is_constexpr_default_constructible
    : bool_constant<is_constexpr_default_constructible_v<T>> {};

/***
 *  _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;

/**
 * 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;

namespace detail {
template <typename Src>
struct like_ {
  template <typename Dst>
  using apply = Dst;
};
template <typename Src>
struct like_<Src const> {
  template <typename Dst>
  using apply = Dst const;
};
template <typename Src>
struct like_<Src volatile> {
  template <typename Dst>
  using apply = Dst volatile;
};
template <typename Src>
struct like_<Src const volatile> {
  template <typename Dst>
  using apply = Dst const volatile;
};
template <typename Src>
struct like_<Src&> {
  template <typename Dst>
  using apply = typename like_<Src>::template apply<Dst>&;
};
template <typename Src>
struct like_<Src&&> {
  template <typename Dst>
  using apply = typename like_<Src>::template apply<Dst>&&;
};
} // namespace detail

//  mimic: like_t, p0847r0
template <typename Src, typename Dst>
using like_t = typename detail::like_<Src>::template apply<remove_cvref_t<Dst>>;

//  mimic: like, p0847r0
template <typename Src, typename Dst>
struct like {
  using type = like_t<Src, Dst>;
};

/**
 *  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...>;

//  nonesuch
//
//  A tag type which traits may use to indicate lack of a result type.
//
//  Similar to void in that no values of this type may be constructed. Different
//  from void in that no functions may be defined with this return type and no
//  complete expressions may evaluate with this expression type.
//
//  mimic: std::experimental::nonesuch, Library Fundamentals TS v2
struct nonesuch {
  ~nonesuch() = delete;
  nonesuch(nonesuch const&) = delete;
  void operator=(nonesuch const&) = delete;
};

namespace detail {

template <typename Void, typename D, template <typename...> class, typename...>
struct detected_ {
  using value_t = std::false_type;
  using type = D;
};
template <typename D, template <typename...> class T, typename... A>
struct detected_<void_t<T<A...>>, D, T, A...> {
  using value_t = std::true_type;
  using type = T<A...>;
};

} // namespace detail

//  detected_or
//
//  If T<A...> substitutes, has member type alias value_t as std::true_type
//  and has member type alias type as T<A...>. Otherwise, has member type
//  alias value_t as std::false_type and has member type alias type as D.
//
//  mimic: std::experimental::detected_or, Library Fundamentals TS v2
template <typename D, template <typename...> class T, typename... A>
using detected_or = detail::detected_<void, D, T, A...>;

//  detected_or_t
//
//  A trait type alias which results in T<A...> if substitution would succeed
//  and in D otherwise.
//
//  Equivalent to detected_or<D, T, A...>::type.
//
//  mimic: std::experimental::detected_or_t, Library Fundamentals TS v2
template <typename D, template <typename...> class T, typename... A>
using detected_or_t = typename detected_or<D, T, A...>::type;

//  detected_t
//
//  A trait type alias which results in T<A...> if substitution would succeed
//  and in nonesuch otherwise.
//
//  Equivalent to detected_or_t<nonesuch, T, A...>.
//
//  mimic: std::experimental::detected_t, Library Fundamentals TS v2
template <template <typename...> class T, typename... A>
using detected_t = detected_or_t<nonesuch, T, A...>;

//  is_detected_v
//  is_detected
//
//  A trait variable and type to test whether some metafunction from types to
//  types would succeed or fail in substitution over a given set of arguments.
//
//  The trait variable is_detected_v<T, A...> is equivalent to
//  detected_or<nonesuch, T, A...>::value_t::value.
//  The trait type is_detected<T, A...> unambiguously inherits bool_constant<V>
//  where V is is_detected_v<T, A...>.
//
//  mimic: std::experimental::is_detected, std::experimental::is_detected_v,
//    Library Fundamentals TS v2
//
//  Note: the trait type is_detected differs here by being deferred.
template <template <typename...> class T, typename... A>
FOLLY_INLINE_VARIABLE constexpr bool is_detected_v =
    detected_or<nonesuch, T, A...>::value_t::value;
template <template <typename...> class T, typename... A>
struct is_detected : detected_or<nonesuch, T, A...>::value_t {};

template <typename T>
using aligned_storage_for_t =
    typename std::aligned_storage<sizeof(T), alignof(T)>::type;

// Older versions of libstdc++ do not provide std::is_trivially_copyable
#if defined(__clang__) && !defined(_LIBCPP_VERSION)
template <class T>
struct is_trivially_copyable : bool_constant<__is_trivially_copyable(T)> {};
#else
template <class T>
using is_trivially_copyable = std::is_trivially_copyable<T>;
#endif

template <class T>
FOLLY_INLINE_VARIABLE constexpr bool is_trivially_copyable_v =
    is_trivially_copyable<T>::value;

/**
 * IsRelocatable<T>::value describes the ability of moving around
 * memory a value of type T by using memcpy (as opposed to the
 * conservative approach of calling the copy constructor and then
 * destroying the old temporary. Essentially for a relocatable type,
 * the following two sequences of code should be semantically
 * equivalent:
 *
 * void move1(T * from, T * to) {
 *   new(to) T(from);
 *   (*from).~T();
 * }
 *
 * void move2(T * from, T * to) {
 *   memcpy(to, from, sizeof(T));
 * }
 *
 * Most C++ types are relocatable; the ones that aren't would include
 * internal pointers or (very rarely) would need to update remote
 * pointers to pointers tracking them. All C++ primitive types and
 * type constructors are relocatable.
 *
 * This property can be used in a variety of optimizations. Currently
 * fbvector uses this property intensively.
 *
 * The default conservatively assumes the type is not
 * relocatable. Several specializations are defined for known
 * types. You may want to add your own specializations. Do so in
 * namespace folly and make sure you keep the specialization of
 * IsRelocatable<SomeStruct> in the same header as SomeStruct.
 *
 * You may also declare a type to be relocatable by including
 *    `typedef std::true_type IsRelocatable;`
 * in the class header.
 *
 * It may be unset in a base class by overriding the typedef to false_type.
 */
/*
 * IsZeroInitializable describes the property that default construction is the
 * same as memset(dst, 0, sizeof(T)).
 */

namespace traits_detail {

#define FOLLY_HAS_TRUE_XXX(name)                                             \
  template <typename T>                                                      \
  using detect_##name = typename T::name;                                    \
  template <class T>                                                         \
  struct name##_is_true : std::is_same<typename T::name, std::true_type> {}; \
  template <class T>                                                         \
  struct has_true_##name : std::conditional<                                 \
                               is_detected_v<detect_##name, T>,              \
                               name##_is_true<T>,                            \
                               std::false_type>::type {}

FOLLY_HAS_TRUE_XXX(IsRelocatable);
FOLLY_HAS_TRUE_XXX(IsZeroInitializable);

#undef FOLLY_HAS_TRUE_XXX

} // namespace traits_detail

struct Ignore {
  Ignore() = default;
  template <class T>
  constexpr /* implicit */ Ignore(const T&) {}
  template <class T>
  const Ignore& operator=(T const&) const {
    return *this;
  }
};

template <class...>
using Ignored = Ignore;

namespace traits_detail_IsEqualityComparable {
Ignore operator==(Ignore, Ignore);

template <class T, class U = T>
struct IsEqualityComparable
    : std::is_convertible<
          decltype(std::declval<T>() == std::declval<U>()),
          bool> {};
} // namespace traits_detail_IsEqualityComparable

/* using override */ using traits_detail_IsEqualityComparable::
    IsEqualityComparable;

namespace traits_detail_IsLessThanComparable {
Ignore operator<(Ignore, Ignore);

template <class T, class U = T>
struct IsLessThanComparable
    : std::is_convertible<
          decltype(std::declval<T>() < std::declval<U>()),
          bool> {};
} // namespace traits_detail_IsLessThanComparable

/* using override */ using traits_detail_IsLessThanComparable::
    IsLessThanComparable;

namespace traits_detail_IsNothrowSwappable {
#if defined(__cpp_lib_is_swappable) || (_CPPLIB_VER && _HAS_CXX17)
// MSVC already implements the C++17 P0185R1 proposal which adds
// std::is_nothrow_swappable, so use it instead if C++17 mode is
// enabled.
template <typename T>
using IsNothrowSwappable = std::is_nothrow_swappable<T>;
#elif _CPPLIB_VER
// MSVC defines the base even if C++17 is disabled, and MSVC has
// issues with our fallback implementation due to over-eager
// evaluation of noexcept.
template <typename T>
using IsNothrowSwappable = std::_Is_nothrow_swappable<T>;
#else
/* using override */ using std::swap;

template <class T>
struct IsNothrowSwappable
    : bool_constant<std::is_nothrow_move_constructible<T>::value&& noexcept(
          swap(std::declval<T&>(), std::declval<T&>()))> {};
#endif
} // namespace traits_detail_IsNothrowSwappable

/* using override */ using traits_detail_IsNothrowSwappable::IsNothrowSwappable;

template <class T>
struct IsRelocatable
    : std::conditional<
          is_detected_v<traits_detail::detect_IsRelocatable, T>,
          traits_detail::has_true_IsRelocatable<T>,
          // TODO add this line (and some tests for it) when we
          // upgrade to gcc 4.7
          // std::is_trivially_move_constructible<T>::value ||
          is_trivially_copyable<T>>::type {};

template <class T>
struct IsZeroInitializable
    : std::conditional<
          is_detected_v<traits_detail::detect_IsZeroInitializable, T>,
          traits_detail::has_true_IsZeroInitializable<T>,
          bool_constant<!std::is_class<T>::value>>::type {};

namespace detail {
template <bool>
struct conditional_;
template <>
struct conditional_<false> {
  template <typename, typename T>
  using apply = T;
};
template <>
struct conditional_<true> {
  template <typename T, typename>
  using apply = T;
};
} // namespace detail

//  conditional_t
//
//  Like std::conditional_t but with only two total class template instances,
//  rather than as many class template instances as there are uses.
//
//  As one effect, the result can be used in deducible contexts, allowing
//  deduction of conditional_t<V, T, F> to work when T or F is a template param.
template <bool V, typename T, typename F>
using conditional_t = typename detail::conditional_<V>::template apply<T, F>;

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

template <typename T>
struct Negation : bool_constant<!T::value> {};

template <bool... Bs>
struct Bools {
  using valid_type = bool;
  static constexpr std::size_t size() { return sizeof...(Bs); }
};

// Lighter-weight than Conjunction, but evaluates all sub-conditions eagerly.
template <class... Ts>
struct StrictConjunction
    : std::is_same<Bools<Ts::value...>, Bools<(Ts::value || true)...>> {};

template <class... Ts>
struct StrictDisjunction
    : Negation<
          std::is_same<Bools<Ts::value...>, Bools<(Ts::value && false)...>>> {};

namespace detail {
template <typename T>
using is_transparent_ = typename T::is_transparent;
} // namespace detail

//  is_transparent_v
//  is_transparent
//
//  A trait variable and type to test whether a less, equal-to, or hash type
//  follows the is-transparent protocol used by containers with optional
//  heterogeneous access.
template <typename T>
FOLLY_INLINE_VARIABLE constexpr bool is_transparent_v =
    is_detected_v<detail::is_transparent_, T>;
template <typename T>
struct is_transparent : bool_constant<is_transparent_v<T>> {};

namespace detail {
template <typename T>
using sizeof_t = decltype(sizeof(T));
} // namespace detail

//  is_complete_v
//  is_complete
//
//  A trait to test if the provided type T is a complete type or not.
//  See https://en.cppreference.com/w/cpp/language/type.
//
//  If is_complete_v<T> is used (possibly cv-ref qualified), then the type T
//  must always be incomplete. That is, it's not safe to instantiate
//  is_complete_v<T> on a forward-declared class type T and then again after the
//  class T is defined. Doing so would be an ODR violation for is_complete_v.
template <typename T>
FOLLY_INLINE_VARIABLE constexpr bool is_complete_v =
    Disjunction<is_detected<detail::sizeof_t, T>, std::is_function<T>>::value;
template <typename T>
struct is_complete : bool_constant<is_complete_v<T>> {};

} // namespace folly

/**
 * Use this macro ONLY inside namespace folly. When using it with a
 * regular type, use it like this:
 *
 * // Make sure you're at namespace ::folly scope
 * template <> FOLLY_ASSUME_RELOCATABLE(MyType)
 *
 * When using it with a template type, use it like this:
 *
 * // Make sure you're at namespace ::folly scope
 * template <class T1, class T2>
 * FOLLY_ASSUME_RELOCATABLE(MyType<T1, T2>)
 */
#define FOLLY_ASSUME_RELOCATABLE(...) \
  struct IsRelocatable<__VA_ARGS__> : std::true_type {}

/**
 * The FOLLY_ASSUME_FBVECTOR_COMPATIBLE* macros below encode the
 * assumption that the type is relocatable per IsRelocatable
 * above. Many types can be assumed to satisfy this condition, but
 * it is the responsibility of the user to state that assumption.
 * User-defined classes will not be optimized for use with
 * fbvector (see FBVector.h) unless they state that assumption.
 *
 * Use FOLLY_ASSUME_FBVECTOR_COMPATIBLE with regular types like this:
 *
 * FOLLY_ASSUME_FBVECTOR_COMPATIBLE(MyType)
 *
 * The versions FOLLY_ASSUME_FBVECTOR_COMPATIBLE_1, _2, _3, and _4
 * allow using the macro for describing templatized classes with 1, 2,
 * 3, and 4 template parameters respectively. For template classes
 * just use the macro with the appropriate number and pass the name of
 * the template to it. Example:
 *
 * template <class T1, class T2> class MyType { ... };
 * ...
 * // Make sure you're at global scope
 * FOLLY_ASSUME_FBVECTOR_COMPATIBLE_2(MyType)
 */

// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE(...) \
  namespace folly {                           \
  template <>                                 \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__);      \
  }
// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE_1(...) \
  namespace folly {                             \
  template <class T1>                           \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__<T1>);    \
  }
// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE_2(...)  \
  namespace folly {                              \
  template <class T1, class T2>                  \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__<T1, T2>); \
  }
// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE_3(...)      \
  namespace folly {                                  \
  template <class T1, class T2, class T3>            \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__<T1, T2, T3>); \
  }
// Use this macro ONLY at global level (no namespace)
#define FOLLY_ASSUME_FBVECTOR_COMPATIBLE_4(...)          \
  namespace folly {                                      \
  template <class T1, class T2, class T3, class T4>      \
  FOLLY_ASSUME_RELOCATABLE(__VA_ARGS__<T1, T2, T3, T4>); \
  }

namespace folly {

// STL commonly-used types
template <class T, class U>
struct IsRelocatable<std::pair<T, U>>
    : bool_constant<IsRelocatable<T>::value && IsRelocatable<U>::value> {};

// Is T one of T1, T2, ..., Tn?
template <typename T, typename... Ts>
using IsOneOf = StrictDisjunction<std::is_same<T, Ts>...>;

/*
 * Complementary type traits for integral comparisons.
 *
 * For instance, `if(x < 0)` yields an error in clang for unsigned types
 *  when -Werror is used due to -Wtautological-compare
 *
 *
 * @author: Marcelo Juchem <marcelo@fb.com>
 */

// same as `x < 0`
template <typename T>
constexpr bool is_negative(T x) {
  return std::is_signed<T>::value && x < T(0);
}

// same as `x <= 0`
template <typename T>
constexpr bool is_non_positive(T x) {
  return !x || folly::is_negative(x);
}

// same as `x > 0`
template <typename T>
constexpr bool is_positive(T x) {
  return !is_non_positive(x);
}

// same as `x >= 0`
template <typename T>
constexpr bool is_non_negative(T x) {
  return !x || is_positive(x);
}

namespace detail {

// folly::to integral specializations can end up generating code
// inside what are really static ifs (not executed because of the templated
// types) that violate -Wsign-compare and/or -Wbool-compare so suppress them
// in order to not prevent all calling code from using it.
FOLLY_PUSH_WARNING
FOLLY_GNU_DISABLE_WARNING("-Wsign-compare")
FOLLY_GCC_DISABLE_WARNING("-Wbool-compare")
FOLLY_MSVC_DISABLE_WARNING(4287) // unsigned/negative constant mismatch
FOLLY_MSVC_DISABLE_WARNING(4388) // sign-compare
FOLLY_MSVC_DISABLE_WARNING(4804) // bool-compare

template <typename RHS, RHS rhs, typename LHS>
bool less_than_impl(LHS const lhs) {
  // clang-format off
  return
      // Ensure signed and unsigned values won't be compared directly.
      (!std::is_signed<RHS>::value && is_negative(lhs)) ? true :
      (!std::is_signed<LHS>::value && is_negative(rhs)) ? false :
      rhs > std::numeric_limits<LHS>::max() ? true :
      rhs <= std::numeric_limits<LHS>::lowest() ? false :
      lhs < rhs;
  // clang-format on
}

template <typename RHS, RHS rhs, typename LHS>
bool greater_than_impl(LHS const lhs) {
  // clang-format off
  return
      // Ensure signed and unsigned values won't be compared directly.
      (!std::is_signed<RHS>::value && is_negative(lhs)) ? false :
      (!std::is_signed<LHS>::value && is_negative(rhs)) ? true :
      rhs > std::numeric_limits<LHS>::max() ? false :
      rhs < std::numeric_limits<LHS>::lowest() ? true :
      lhs > rhs;
  // clang-format on
}

FOLLY_POP_WARNING

} // namespace detail

template <typename RHS, RHS rhs, typename LHS>
bool less_than(LHS const lhs) {
  return detail::
      less_than_impl<RHS, rhs, typename std::remove_reference<LHS>::type>(lhs);
}

template <typename RHS, RHS rhs, typename LHS>
bool greater_than(LHS const lhs) {
  return detail::
      greater_than_impl<RHS, rhs, typename std::remove_reference<LHS>::type>(
          lhs);
}
} // namespace folly

// Assume nothing when compiling with MSVC.
#ifndef _MSC_VER
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_2(std::unique_ptr)
FOLLY_ASSUME_FBVECTOR_COMPATIBLE_1(std::shared_ptr)
#endif

namespace folly {

//  Some compilers have signed __int128 and unsigned __int128 types, and some
//  libraries with some compilers have traits for those types. It's a mess.
//  Import things into folly and then fill in whatever is missing.
//
//  The aliases:
//    int128_t
//    uint128_t
//
//  The traits:
//    is_arithmetic
//    is_arithmetic_v
//    is_integral
//    is_integral_v
//    is_signed
//    is_signed_v
//    is_unsigned
//    is_unsigned_v
//    make_signed
//    make_signed_t
//    make_unsigned
//    make_unsigned_t

template <typename T>
struct is_arithmetic : std::is_arithmetic<T> {};
template <typename T>
FOLLY_INLINE_VARIABLE constexpr bool is_arithmetic_v = is_arithmetic<T>::value;

template <typename T>
struct is_integral : std::is_integral<T> {};
template <typename T>
FOLLY_INLINE_VARIABLE constexpr bool is_integral_v = is_integral<T>::value;

template <typename T>
struct is_signed : std::is_signed<T> {};
template <typename T>
FOLLY_INLINE_VARIABLE constexpr bool is_signed_v = is_signed<T>::value;

template <typename T>
struct is_unsigned : std::is_unsigned<T> {};
template <typename T>
FOLLY_INLINE_VARIABLE constexpr bool is_unsigned_v = is_unsigned<T>::value;

template <typename T>
struct make_signed : std::make_signed<T> {};
template <typename T>
using make_signed_t = typename make_signed<T>::type;

template <typename T>
struct make_unsigned : std::make_unsigned<T> {};
template <typename T>
using make_unsigned_t = typename make_unsigned<T>::type;

#if FOLLY_HAVE_INT128_T

using int128_t = signed __int128;
using uint128_t = unsigned __int128;

template <>
struct is_arithmetic<int128_t> : std::true_type {};
template <>
struct is_arithmetic<uint128_t> : std::true_type {};

template <>
struct is_integral<int128_t> : std::true_type {};
template <>
struct is_integral<uint128_t> : std::true_type {};

template <>
struct is_signed<int128_t> : std::true_type {};
template <>
struct is_signed<uint128_t> : std::false_type {};
template <>
struct is_unsigned<int128_t> : std::false_type {};
template <>
struct is_unsigned<uint128_t> : std::true_type {};

template <>
struct make_signed<int128_t> {
  using type = int128_t;
};
template <>
struct make_signed<uint128_t> {
  using type = int128_t;
};

template <>
struct make_unsigned<int128_t> {
  using type = uint128_t;
};
template <>
struct make_unsigned<uint128_t> {
  using type = uint128_t;
};
#endif // FOLLY_HAVE_INT128_T

namespace traits_detail {
template <std::size_t>
struct uint_bits_t_ {};
template <>
struct uint_bits_t_<8> : type_t_<std::uint8_t> {};
template <>
struct uint_bits_t_<16> : type_t_<std::uint16_t> {};
template <>
struct uint_bits_t_<32> : type_t_<std::uint32_t> {};
template <>
struct uint_bits_t_<64> : type_t_<std::uint64_t> {};
#if FOLLY_HAVE_INT128_T
template <>
struct uint_bits_t_<128> : type_t_<uint128_t> {};
#endif // FOLLY_HAVE_INT128_T
} // namespace traits_detail

template <std::size_t bits>
using uint_bits_t = _t<traits_detail::uint_bits_t_<bits>>;

template <std::size_t lg_bits>
using uint_bits_lg_t = uint_bits_t<(1u << lg_bits)>;

template <std::size_t bits>
using int_bits_t = make_signed_t<uint_bits_t<bits>>;

template <std::size_t lg_bits>
using int_bits_lg_t = make_signed_t<uint_bits_lg_t<lg_bits>>;

#if FOLLY_HAS_BUILTIN(__type_pack_element)
template <std::size_t I, typename... Ts>
using type_pack_element_t = __type_pack_element<I, Ts...>;
#else
namespace traits_detail {

template <std::size_t I, typename T>
struct indexed_type : std::integral_constant<std::size_t, I> {
  using type = T;
};

template <std::size_t I, typename... Ts>
struct type_pack_element_impl {
 private:
  template <typename>
  struct set;

  template <std::size_t... Is>
  struct set<std::index_sequence<Is...>> : indexed_type<Is, Ts>... {};

  template <typename T>
  inline static std::enable_if<true, T> impl(indexed_type<I, T>);

  inline static std::enable_if<false> impl(...);

 public:
  using type = decltype(impl(set<std::index_sequence_for<Ts...>>{}));
};

template <std::size_t I, typename... Ts>
using type_pack_element = typename type_pack_element_impl<I, Ts...>::type;

} // namespace traits_detail

template <std::size_t I, typename... Ts>
using type_pack_element_t =
    typename traits_detail::type_pack_element<I, Ts...>::type;
#endif

} // namespace folly