nullable.hpp

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// Copyright 2024 Man Group Operations Limited
//
// 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.
 
#pragma once
 
#include <algorithm>
#include <compare>
#include <concepts>
#include <exception>
#if __has_include(<format>)
#    include <format>
#    include <ostream>
#endif
#include <type_traits>
#include <variant>
 
#include "sparrow/config/config.hpp"
#include "sparrow/utils/mp_utils.hpp"
 
 
#if defined(SPARROW_CONSTEXPR)
#    error "SPARROW_CONSTEXPR already defined"
#endif
 
// clang workaround: clang instantiates the constructor in SFINAE context,
// which is incompatible with the implementation of standard libraries which
// are not libc++. This leads to wrong compilation errors. Making the constructor
// not constexpr prevents the compiler from instantiating it.
#if defined(__clang__) && not defined(_LIBCPP_VERSION)
#    define SPARROW_CONSTEXPR
#else
#    define SPARROW_CONSTEXPR constexpr
#endif
 
namespace sparrow
{
    template <class T, mpl::boolean_like B>
    class nullable;
 
    template <class T>
    struct is_nullable : std::false_type
    {
    };
 
    template <class T, mpl::boolean_like B>
    struct is_nullable<nullable<T, B>> : std::true_type
    {
    };
 
    template <class T>
    inline constexpr bool is_nullable_v = is_nullable<T>::value;
 
    template <class N, class T>
    concept nullable_of = is_nullable_v<N> && std::same_as<typename N::stored_value_type, T>;
 
    template <class N, class T>
    concept nullable_of_convertible_to = is_nullable_v<N>
                                         && std::convertible_to<typename N::stored_value_type, T>;
 
    /*
     * Matches a range of nullables objects.
     *
     * A range is considered a range of nullables if it is a range and its value type is a nullable.
     *
     * @tparam RangeOfNullables The range to check.
     */
    template <class RangeOfNullables>
    concept range_of_nullables = std::ranges::range<RangeOfNullables>
                                 && is_nullable<std::ranges::range_value_t<RangeOfNullables>>::value;
 
    // TODO: nullable_get_fn should not be required since nullable implements all the required overloads
    // of get(), but some compilers fail to build without it.
    struct nullable_get_fn
    {
        template <class N>
            requires is_nullable_v<std::remove_cvref_t<N>>
        constexpr decltype(auto) operator()(N&& nullable_value) const
        {
            if constexpr (std::is_lvalue_reference_v<N&&>)
            {
                return std::forward<N>(nullable_value).get();
            }
            else
            {
                using value_type = std::remove_cvref_t<decltype(std::forward<N>(nullable_value).get())>;
                return value_type(std::forward<N>(nullable_value).get());
            }
        }
    };
 
    inline constexpr nullable_get_fn nullable_get{};
 
    /*
     * Default traits for the nullable class. These traits should be specialized
     * for proxy classes whose reference and const_reference types are not
     * defined as usual. For instance:
     *
     * @code{.cpp}
     * struct nullable_traits<string_proxy>
     * {
     *     using value_type = std::string;
     *     using reference = string_proxy;
     *     using const_reference = std::string_view;
     *     using rvalue_reverence = std::string&&;
     *     using const_rvalue_reference = const std::string&&;
     * };
     * @endcode
     */
    template <class T>
    struct nullable_traits
    {
        using value_type = T;
        using reference = std::add_lvalue_reference_t<value_type>;
        using const_reference = std::add_lvalue_reference_t<std::add_const_t<value_type>>;
        using rvalue_reference = value_type&&;
        using const_rvalue_reference = const value_type&&;
    };
 
    template <class T>
    struct nullable_traits<T&>
    {
        using value_type = T;
        using reference = std::add_lvalue_reference_t<value_type>;
        using const_reference = std::add_lvalue_reference_t<std::add_const_t<value_type>>;
        using rvalue_reference = reference;
        using const_rvalue_reference = const_reference;
    };

    class bad_nullable_access : public std::exception
    {
    public:
 
        bad_nullable_access() noexcept = default;
        bad_nullable_access(const bad_nullable_access&) noexcept = default;
        bad_nullable_access& operator=(const bad_nullable_access&) noexcept = default;

        [[nodiscard]] const char* what() const noexcept override
        {
            return message;
        }
 
    private:
 
        static constexpr const char* message = "Invalid access to nullable underlying value";
    };

    struct nullval_t
    {
        constexpr explicit nullval_t(int)
        {
        }
    };

    inline constexpr nullval_t nullval(0);
 
    namespace impl
    {
        template <class T, class TArgs, class U, class UArgs>
        concept both_constructible_from_cref = std::constructible_from<T, mpl::add_const_lvalue_reference_t<TArgs>>
                                               and std::constructible_from<U, mpl::add_const_lvalue_reference_t<UArgs>>;
 
        template <class To1, class From1, class To2, class From2>
        concept both_convertible_from_cref = std::convertible_to<mpl::add_const_lvalue_reference_t<From1>, To1>
                                             and std::convertible_to<mpl::add_const_lvalue_reference_t<From2>, To2>;
 
        template <class T, class... Args>
        concept constructible_from_one = (std::constructible_from<T, Args> || ...);
 
        template <class T, class... Args>
        concept convertible_from_one = (std::convertible_to<Args, T> || ...);
 
        template <class T, class... Args>
        concept initializable_from_one = constructible_from_one<T, Args...> || convertible_from_one<T, Args...>;
 
        template <class T, class Arg>
        concept initializable_from_refs = initializable_from_one<T, Arg&, const Arg&, Arg&&, const Arg&&>;
 
        // We prefer std::is_assignable_v to std::assignable_from<To, From> because
        // std::assignable_from requires the existence of an implicit conversion
        // from From to To
        template <class To, class... Args>
        concept assignable_from_one = (std::is_assignable_v<std::add_lvalue_reference<To>, Args> && ...);
 
        template <class T, class Arg>
        concept assignable_from_refs = assignable_from_one<T, Arg&, const Arg&, Arg&&, const Arg&&>;
 
        template <class T, class U>
        using conditional_ref_t = std::conditional_t<std::is_reference_v<T>, const std::decay_t<U>&, std::decay_t<U>&&>;
 
        template <class T, class Targs, class U, class UArgs>
        concept both_constructible_from_cond_ref = std::constructible_from<T, conditional_ref_t<T, Targs>>
                                                   and std::constructible_from<U, conditional_ref_t<U, UArgs>>;
 
        template <class To1, class From1, class To2, class From2>
        concept both_convertible_from_cond_ref = std::convertible_to<conditional_ref_t<To1, From1>, To1>
                                                 and std::convertible_to<conditional_ref_t<To2, From2>, To2>;
 
        template <class To1, class From1, class To2, class From2>
        concept both_assignable_from_cref = std::is_assignable_v<
                                                std::add_lvalue_reference_t<To1>,
                                                mpl::add_const_lvalue_reference_t<From1>>
                                            and std::is_assignable_v<
                                                std::add_lvalue_reference_t<To2>,
                                                mpl::add_const_lvalue_reference_t<From2>>;
 
        template <class To1, class From1, class To2, class From2>
        concept both_assignable_from_cond_ref = std::is_assignable_v<
                                                    std::add_lvalue_reference_t<To1>,
                                                    conditional_ref_t<To1, From1>>
                                                and std::is_assignable_v<
                                                    std::add_lvalue_reference_t<To2>,
                                                    conditional_ref_t<To2, From2>>;
    }

    template <class T, mpl::boolean_like B = bool>
    class nullable
    {
    public:
 
        using stored_value_type = T;
        using self_type = nullable<T, B>;
        using value_traits = nullable_traits<T>;
        using value_type = typename value_traits::value_type;
        using reference = typename value_traits::reference;
        using const_reference = typename value_traits::const_reference;
        using rvalue_reference = typename value_traits::rvalue_reference;
        using const_rvalue_reference = typename value_traits::const_rvalue_reference;
        using flag_traits = nullable_traits<B>;
        using flag_type = typename flag_traits::value_type;
        using flag_reference = typename flag_traits::reference;
        using flag_const_reference = typename flag_traits::const_reference;
        using flag_rvalue_reference = typename flag_traits::rvalue_reference;
        using flag_const_rvalue_reference = typename flag_traits::const_rvalue_reference;

        template <std::default_initializable U = T, std::default_initializable BB = B>
        constexpr nullable() noexcept
            : m_value()
            , m_null_flag(false)
        {
        }

        template <std::default_initializable U = T, std::default_initializable BB = B>
        constexpr nullable(nullval_t) noexcept
            : m_value()
            , m_null_flag(false)
        {
        }

        template <class U>
            requires(not std::same_as<self_type, std::decay_t<U>> and std::constructible_from<T, U &&>)
        explicit(not std::convertible_to<U&&, T>) constexpr nullable(U&& value) noexcept(
            noexcept(T(std::declval<U>()))
        )
            : m_value(std::forward<U>(value))
            , m_null_flag(true)
        {
        }

        constexpr nullable(const self_type& rhs) = default;

        template <class TO, mpl::boolean_like BO>
            requires(impl::both_constructible_from_cref<T, TO, B, BO>
                     and not impl::initializable_from_refs<T, nullable<TO, BO>>)
        explicit(not impl::both_convertible_from_cref<T, TO, B, BO>) SPARROW_CONSTEXPR nullable(
            const nullable<TO, BO>& rhs
        )
            : m_value(rhs.get())
            , m_null_flag(rhs.null_flag())
        {
        }
 
#ifdef __clang__
        template <class TO, mpl::boolean_like BO>
            requires(impl::both_constructible_from_cref<T, TO, B, BO> and std::same_as<std::decay_t<T>, bool>)
        explicit(not impl::both_convertible_from_cref<T, TO, B, BO>) SPARROW_CONSTEXPR nullable(
            const nullable<TO, BO>& rhs
        )
            : m_value(rhs.get())
            , m_null_flag(rhs.null_flag())
        {
        }
#endif

        constexpr nullable(self_type&& rhs) noexcept = default;

        template <class TO, mpl::boolean_like BO>
            requires(impl::both_constructible_from_cond_ref<T, TO, B, BO>
                     and not impl::initializable_from_refs<T, nullable<TO, BO>>)
        explicit(not impl::both_convertible_from_cond_ref<T, TO, B, BO>) SPARROW_CONSTEXPR nullable(
            nullable<TO, BO>&& rhs
        )
            : m_value(std::move(rhs).get())
            , m_null_flag(std::move(rhs).null_flag())
        {
        }
 
#ifdef __clang__
        template <class TO, mpl::boolean_like BO>
            requires(impl::both_constructible_from_cond_ref<T, TO, B, BO>
                     and std::same_as<std::decay_t<T>, bool>)
        explicit(not impl::both_convertible_from_cond_ref<T, TO, B, BO>) SPARROW_CONSTEXPR nullable(
            nullable<TO, BO>&& rhs
        )
            : m_value(std::move(rhs).get())
            , m_null_flag(std::move(rhs).null_flag())
        {
        }
#endif

        constexpr nullable(value_type&& value, flag_type&& null_flag)
            : m_value(std::move(value))
            , m_null_flag(std::move(null_flag))
        {
        }

        constexpr nullable(std::add_lvalue_reference_t<T> value, std::add_lvalue_reference_t<B> null_flag)
            : m_value(value)
            , m_null_flag(null_flag)
        {
        }

        constexpr nullable(value_type&& value, std::add_lvalue_reference_t<B> null_flag)
            : m_value(std::move(value))
            , m_null_flag(null_flag)
        {
        }

        constexpr nullable(std::add_lvalue_reference_t<T> value, flag_type&& null_flag)
            : m_value(value)
            , m_null_flag(std::move(null_flag))
        {
        }

        template <class U, class V>
            requires(std::same_as<std::remove_cvref_t<U>, T> && std::same_as<std::remove_cvref_t<V>, B>
                     && std::is_const_v<std::remove_reference_t<U>>
                     && not std::is_const_v<std::remove_reference_t<V>> && not std::is_reference_v<T>
                     && not std::is_reference_v<B> && std::is_lvalue_reference_v<U &&>
                     && std::is_lvalue_reference_v<V &&>)
        constexpr nullable(U& value, V& null_flag)
            : m_value(value)
            , m_null_flag(null_flag)
        {
        }

        template <class U, class V>
            requires(std::same_as<std::remove_cvref_t<U>, T> && std::same_as<std::remove_cvref_t<V>, B>
                     && not std::is_reference_v<T> && not std::is_reference_v<B>
                     && (std::is_const_v<std::remove_reference_t<U>>
                         || std::is_const_v<std::remove_reference_t<V>>) )
        constexpr nullable(U&& value, V&& null_flag)
            : m_value(std::forward<U>(value))
            , m_null_flag(std::forward<V>(null_flag))
        {
        }

        constexpr self_type& operator=(nullval_t) noexcept
        {
            m_null_flag = false;
            return *this;
        }

        template <class TO>
            requires(not std::same_as<self_type, TO> and std::assignable_from<std::add_lvalue_reference_t<T>, TO>)
        constexpr self_type& operator=(TO&& rhs) noexcept
        {
            m_value = std::forward<TO>(rhs);
            m_null_flag = true;
            return *this;
        }

        constexpr self_type& operator=(const self_type& rhs) noexcept
        {
            m_value = rhs.get();
            m_null_flag = rhs.null_flag();
            return *this;
        }

        template <class TO, mpl::boolean_like BO>
            requires(
                impl::both_assignable_from_cref<T, TO, B, BO>
                and not impl::initializable_from_refs<T, nullable<TO, BO>>
                and not impl::assignable_from_refs<T, nullable<TO, BO>>
            )
        constexpr self_type& operator=(const nullable<TO, BO>& rhs) noexcept
        {
            m_value = rhs.get();
            m_null_flag = rhs.null_flag();
            return *this;
        }

        constexpr self_type& operator=(self_type&& rhs) noexcept
        {
            m_value = std::move(rhs).get();
            m_null_flag = std::move(rhs).null_flag();
            return *this;
        }

        template <class TO, mpl::boolean_like BO>
            requires(
                impl::both_assignable_from_cond_ref<T, TO, B, BO>
                and not impl::initializable_from_refs<T, nullable<TO, BO>>
                and not impl::assignable_from_refs<T, nullable<TO, BO>>
            )
        constexpr self_type& operator=(nullable<TO, BO>&& rhs) noexcept
        {
            m_value = std::move(rhs).get();
            m_null_flag = std::move(rhs).null_flag();
            return *this;
        }

        constexpr explicit operator bool() const noexcept;

        [[nodiscard]] constexpr bool has_value() const noexcept;

        [[nodiscard]] constexpr flag_reference null_flag() & noexcept;

        [[nodiscard]] constexpr flag_const_reference null_flag() const& noexcept;

        [[nodiscard]] constexpr flag_rvalue_reference null_flag() && noexcept;

        [[nodiscard]] constexpr flag_const_rvalue_reference null_flag() const&& noexcept;

        [[nodiscard]] constexpr reference get() & noexcept;

        [[nodiscard]] constexpr const_reference get() const& noexcept;

        [[nodiscard]] constexpr rvalue_reference get() && noexcept;

        [[nodiscard]] constexpr const_rvalue_reference get() const&& noexcept;

        [[nodiscard]] constexpr reference value() &;

        [[nodiscard]] constexpr const_reference value() const&;

        [[nodiscard]] constexpr rvalue_reference value() &&;

        [[nodiscard]] constexpr const_rvalue_reference value() const&&;

        template <class U>
        [[nodiscard]] constexpr value_type value_or(U&& default_value) const&;

        template <class U>
        [[nodiscard]] constexpr value_type value_or(U&& default_value) &&;

        void swap(self_type& other) noexcept;

        void reset() noexcept;
 
    private:

        void throw_if_null() const;
 
        T m_value;      
        B m_null_flag;  
 
        template <class TO, mpl::boolean_like BO>
        friend class nullable;
    };

    template <class T, class B>
    constexpr void swap(nullable<T, B>& lhs, nullable<T, B>& rhs) noexcept;

    template <class T, class B>
    constexpr bool operator==(const nullable<T, B>& lhs, nullval_t) noexcept;

    template <class T, mpl::boolean_like B>
    constexpr std::strong_ordering operator<=>(const nullable<T, B>& lhs, nullval_t dummy) noexcept;

    template <class T, class B, class U>
        requires(!is_nullable_v<U> && mpl::weakly_equality_comparable_with<T, U>)
    constexpr bool operator==(const nullable<T, B>& lhs, const U& rhs) noexcept;

    template <class T, class B, class U>
        requires(!is_nullable_v<U> && std::three_way_comparable_with<U, T>)
    constexpr std::compare_three_way_result_t<T, U>
    operator<=>(const nullable<T, B>& lhs, const U& rhs) noexcept;

    template <class T, class B, class U, class UB>
        requires(mpl::weakly_equality_comparable_with<T, U>)
    constexpr bool operator==(const nullable<T, B>& lhs, const nullable<U, UB>& rhs) noexcept;

    template <class T, class B, std::three_way_comparable_with<T> U, class UB>
    constexpr std::compare_three_way_result_t<T, U>
    operator<=>(const nullable<T, B>& lhs, const nullable<U, UB>& rhs) noexcept;

    template <class T, mpl::boolean_like B = bool>
    constexpr nullable<T, B> make_nullable(T&& value, B&& flag = true);

    template <std::ranges::range R, typename T = typename std::ranges::range_value_t<R>::value_type>
        requires(nullable_of<std::ranges::range_value_t<R>, T>)
    constexpr void zero_null_values(R& range, const T& default_value = T{});

    template <class... T>
        requires(sizeof...(T) > 0 && (is_nullable_v<T> && ...))
    class nullable_variant : public std::variant<T...>
    {
    public:
 
        using base_type = std::variant<T...>;
        using base_type::base_type;
 
        template <class U, mpl::boolean_like B>
            requires((std::same_as<nullable<U, B>, T> || ...))
        constexpr nullable_variant(const nullable<U, B>& value)
            : base_type(std::in_place_type<nullable<U, B>>, value)
        {
        }
 
        constexpr nullable_variant(const nullable_variant&) = default;
        constexpr nullable_variant(nullable_variant&&) noexcept = default;

        constexpr nullable_variant& operator=(const nullable_variant&);

        constexpr nullable_variant& operator=(nullable_variant&&) noexcept;

        constexpr explicit operator bool() const;

        constexpr bool has_value() const;
    };
 
    template <class T>
    struct is_nullable_variant : std::false_type
    {
    };
 
    template <class... T>
    struct is_nullable_variant<nullable_variant<T...>> : std::true_type
    {
    };
 
    template <class T>
    inline constexpr bool is_nullable_variant_v = is_nullable_variant<T>::value;
}
 
namespace std
{
    namespace mpl = sparrow::mpl;
 
    // Specialization of basic_common_reference for nullable proxies so
    // we can use ranges algorithm on iterators returning nullable
    template <class T, mpl::boolean_like TB, class U, mpl::boolean_like UB, template <class> class TQual, template <class> class UQual>
        requires std::common_reference_with<T, U> && std::common_reference_with<TB, UB>
    struct basic_common_reference<sparrow::nullable<T, TB>, sparrow::nullable<U, UB>, TQual, UQual>
    {
        using type = sparrow::
            nullable<std::common_reference_t<TQual<T>, UQual<U>>, std::common_reference_t<TQual<TB>, UQual<UB>>>;
    };
}
 
namespace sparrow
{
    /***************************
     * nullable implementation *
     ***************************/
 
    template <class T, mpl::boolean_like B>
    constexpr nullable<T, B>::operator bool() const noexcept
    {
        return m_null_flag;
    }
 
    template <class T, mpl::boolean_like B>
    constexpr bool nullable<T, B>::has_value() const noexcept
    {
        return m_null_flag;
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::null_flag() & noexcept -> flag_reference
    {
        return m_null_flag;
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::null_flag() const& noexcept -> flag_const_reference
    {
        return m_null_flag;
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::null_flag() && noexcept -> flag_rvalue_reference
    {
        if constexpr (std::is_reference_v<B>)
        {
            return m_null_flag;
        }
        else
        {
            return flag_rvalue_reference(m_null_flag);
        }
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::null_flag() const&& noexcept -> flag_const_rvalue_reference
    {
        if constexpr (std::is_reference_v<B>)
        {
            return m_null_flag;
        }
        else
        {
            return flag_const_rvalue_reference(m_null_flag);
        }
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::get() & noexcept -> reference
    {
        return m_value;
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::get() const& noexcept -> const_reference
    {
        return m_value;
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::get() && noexcept -> rvalue_reference
    {
        if constexpr (std::is_reference_v<T>)
        {
            return m_value;
        }
        else
        {
            return rvalue_reference(m_value);
        }
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::get() const&& noexcept -> const_rvalue_reference
    {
        if constexpr (std::is_reference_v<T>)
        {
            return m_value;
        }
        else
        {
            return const_rvalue_reference(m_value);
        }
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::value() & -> reference
    {
        throw_if_null();
        return get();
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::value() const& -> const_reference
    {
        throw_if_null();
        return get();
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::value() && -> rvalue_reference
    {
        throw_if_null();
        return std::move(*this).get();
    }
 
    template <class T, mpl::boolean_like B>
    constexpr auto nullable<T, B>::value() const&& -> const_rvalue_reference
    {
        throw_if_null();
        return std::move(*this).get();
    }
 
    template <class T, mpl::boolean_like B>
    template <class U>
    constexpr auto nullable<T, B>::value_or(U&& default_value) const& -> value_type
    {
        return *this ? get() : value_type(std::forward<U>(default_value));
    }
 
    template <class T, mpl::boolean_like B>
    template <class U>
    constexpr auto nullable<T, B>::value_or(U&& default_value) && -> value_type
    {
        return *this ? get() : value_type(std::forward<U>(default_value));
    }
 
    template <class T, mpl::boolean_like B>
    void nullable<T, B>::swap(self_type& other) noexcept
    {
        using std::swap;
        swap(m_value, other.m_value);
        swap(m_null_flag, other.m_null_flag);
    }
 
    template <class T, mpl::boolean_like B>
    void nullable<T, B>::reset() noexcept
    {
        m_null_flag = false;
    }
 
    template <class T, mpl::boolean_like B>
    void nullable<T, B>::throw_if_null() const
    {
        if (!m_null_flag)
        {
            throw bad_nullable_access{};
        }
    }
 
    template <class T, class B>
    constexpr void swap(nullable<T, B>& lhs, nullable<T, B>& rhs) noexcept
    {
        lhs.swap(rhs);
    }
 
    template <class T, class B>
    constexpr bool operator==(const nullable<T, B>& lhs, nullval_t) noexcept
    {
        return !lhs;
    }
 
    template <class T, class B>
    constexpr std::strong_ordering operator<=>(const nullable<T, B>& lhs, nullval_t) noexcept
    {
        return lhs <=> false;
    }
 
    template <class T, class B, class U>
        requires(!is_nullable_v<U> && mpl::weakly_equality_comparable_with<T, U>)
    constexpr bool operator==(const nullable<T, B>& lhs, const U& rhs) noexcept
    {
        return lhs && (lhs.get() == rhs);
    }
 
    template <class T, class B, class U>
        requires(!is_nullable_v<U> && std::three_way_comparable_with<U, T>)
    constexpr std::compare_three_way_result_t<T, U> operator<=>(const nullable<T, B>& lhs, const U& rhs) noexcept
    {
        return lhs ? lhs.get() <=> rhs : std::strong_ordering::less;
    }
 
    template <class T, class B, class U, class UB>
        requires(mpl::weakly_equality_comparable_with<T, U>)
    constexpr bool operator==(const nullable<T, B>& lhs, const nullable<U, UB>& rhs) noexcept
    {
        return rhs ? lhs == rhs.get() : !lhs;
    }
 
    template <class T, class B, std::three_way_comparable_with<T> U, class UB>
    constexpr std::compare_three_way_result_t<T, U>
    operator<=>(const nullable<T, B>& lhs, const nullable<U, UB>& rhs) noexcept
    {
        return (lhs && rhs) ? lhs.get() <=> rhs.get() : bool(lhs) <=> bool(rhs);
    }
 
    template <class T, mpl::boolean_like B>
    constexpr nullable<T, B> make_nullable(T&& value, B&& flag)
    {
        return nullable<T, B>(std::forward<T>(value), std::forward<B>(flag));
    }
 
    template <std::ranges::range R, typename T>
        requires(nullable_of<std::ranges::range_value_t<R>, T>)
    constexpr void zero_null_values(R& range, const T& default_value)
    {
        for (auto nullable_value : range)
        {
            if (!nullable_value.has_value())
            {
                nullable_value.get() = default_value;
            }
        }
    }
 
    /***********************************
     * nullable_variant implementation *
     ***********************************/
 
    template <class... T>
        requires(sizeof...(T) > 0 && (is_nullable_v<T> && ...))
    constexpr nullable_variant<T...>& nullable_variant<T...>::operator=(const nullable_variant& rhs)
    {
        base_type::operator=(rhs);
        return *this;
    }
 
    template <class... T>
        requires(sizeof...(T) > 0 && (is_nullable_v<T> && ...))
    constexpr nullable_variant<T...>& nullable_variant<T...>::operator=(nullable_variant&& rhs) noexcept
    {
        base_type::operator=(std::move(rhs));
        return *this;
    }
 
    template <class... T>
        requires(sizeof...(T) > 0 && (is_nullable_v<T> && ...))
    constexpr nullable_variant<T...>::operator bool() const
    {
        return has_value();
    }
 
    template <class... T>
        requires(sizeof...(T) > 0 && (is_nullable_v<T> && ...))
    constexpr bool nullable_variant<T...>::has_value() const
    {
        return std::visit(
            [](const auto& v)
            {
                return v.has_value();
            },
#if SPARROW_GCC_11_2_WORKAROUND
            static_cast<const base_type&>(*this)
#else
            *this
#endif
        );
    }
}
 
#if defined(__cpp_lib_format)
 
template <typename T, sparrow::mpl::boolean_like B>
struct std::formatter<sparrow::nullable<T, B>>
{
    constexpr auto parse(format_parse_context& ctx)
    {
        auto pos = ctx.begin();
        while (pos != ctx.end() && *pos != '}')
        {
            m_format_string.push_back(*pos);
            ++pos;
        }
        m_format_string.push_back('}');
        return pos;
    }
 
    auto format(const sparrow::nullable<T, B>& n, std::format_context& ctx) const
    {
        if (n.has_value())
        {
            return std::vformat_to(ctx.out(), m_format_string, std::make_format_args(n.get()));
        }
        else
        {
            return std::format_to(ctx.out(), "{}", "null");
        }
    }
 
    std::string m_format_string = "{:";
};
 
namespace sparrow
{
    template <typename T, mpl::boolean_like B>
    std::ostream& operator<<(std::ostream& os, const nullable<T, B>& value)
    {
        os << std::format("{}", value);
        return os;
    }
}
 
template <class... T>
struct std::formatter<sparrow::nullable_variant<T...>>
{
    constexpr auto parse(format_parse_context& ctx)
    {
        auto pos = ctx.begin();
        while (pos != ctx.end() && *pos != '}')
        {
            m_format_string.push_back(*pos);
            ++pos;
        }
        m_format_string.push_back('}');
        return pos;
    }
 
    auto format(const sparrow::nullable_variant<T...>& variant, std::format_context& ctx) const
    {
        if (variant.has_value())
        {
            return std::visit(
                [&](const auto& value)
                {
                    return std::vformat_to(ctx.out(), m_format_string, std::make_format_args(value));
                },
                variant
            );
        }
        else
        {
            return std::vformat_to(ctx.out(), m_format_string, std::make_format_args("null"));
        }
    }
 
    std::string m_format_string = "{:";
};
 
namespace sparrow
{
    template <class... T>
    std::ostream& operator<<(std::ostream& os, const nullable_variant<T...>& value)
    {
        os << std::format("{}", value);
        return os;
    }
}
 
template <>
struct std::formatter<sparrow::nullval_t>
{
    constexpr auto parse(format_parse_context& ctx)
    {
        return ctx.begin();  // Simple implementation
    }
 
    auto format(const sparrow::nullval_t&, std::format_context& ctx) const
    {
        return std::format_to(ctx.out(), "nullval");
    }
};
 
#endif
 
namespace sparrow
{
    inline std::ostream& operator<<(std::ostream& os, const nullval_t&)
    {
        constexpr std::string_view nullval_str = "nullval";
        os << nullval_str;
        return os;
    }
}
 
#undef SPARROW_CONSTEXPR