variant.hpp

#pragma once
 
#include <exception>
#include <type_traits>
#include <utility> // swap
#include <limits> // used for index_type
#include <initializer_list>
#include <cassert>
 
#ifndef PLUGIFY_VARIANT_NO_CONSTEXPR_EMPLACE
#  include <memory>
#else
#  include <new>
#endif
 
#ifndef PLUGIFY_VARIANT_NO_STD_HASH
#  include <functional>
#endif
 
#define PLG_FWD(x) static_cast<decltype(x)&&>(x)
#define PLG_MOV(x) static_cast< std::remove_reference_t<decltype(x)>&& >(x)
 
#include "macro.hpp"
 
// from https://github.com/groundswellaudio/swl-variant
namespace plg {
#if PLUGIFY_EXCEPTIONS
    class bad_variant_access  final : public std::exception {
        const char* message = ""; // llvm test requires a well formed what() on default init
        public :
        explicit bad_variant_access(const char* str) noexcept : message{str} {}
        bad_variant_access() noexcept = default;
        bad_variant_access(const bad_variant_access&) noexcept = default;
        bad_variant_access& operator=(const bad_variant_access&) noexcept = default;
        const char* what() const noexcept override { return message; }
    };
#endif // PLUGIFY_EXCEPTIONS
 
    namespace detail {
        struct variant_tag{};
        struct emplacer_tag{};
    }
    
    template<class T>
    struct in_place_type_t : private detail::emplacer_tag {};
    
    template<std::size_t Index>
    struct in_place_index_t : private detail::emplacer_tag {};
    
    template<std::size_t Index>
    inline static constexpr in_place_index_t<Index> in_place_index;
    
    template<class T>
    inline static constexpr in_place_type_t<T> in_place_type;
    
    namespace detail {
        template<int N>
        constexpr int find_first_true(bool (&&arr)[N])  {
            for (int k = 0; k < N; ++k)
                if (arr[k])
                    return k;
            return -1;
        }
 
        template<class T, class... Ts>
        inline constexpr bool appears_exactly_once = (static_cast<unsigned short>(std::is_same_v<T, Ts>) + ...) == 1;
 
        // ============= type pack element
 
#if __has_builtin(__type_pack_element)
        template<std::size_t K, class... Ts>
        using type_pack_element = __type_pack_element<K, Ts...>;
#else
        template<unsigned char = 1>
        struct find_type_i;
 
        template<>
        struct find_type_i<1> {
            template<std::size_t Idx, class T, class... Ts>
            using f = typename find_type_i<(Idx != 1)>::template f<Idx - 1, Ts...>;
        };
 
        template<>
        struct find_type_i<0> {
            template<std::size_t, class T, class... Ts>
            using f = T;
        };
 
        template<std::size_t K, class... Ts>
        using type_pack_element = typename find_type_i<(K != 0)>::template f<K, Ts...>;
#endif
 
        // ============= overload match detector. to be used for variant generic assignment
 
        template<class T>
        using arr1 = T[1];
 
        template<std::size_t N, class A>
        struct overload_frag {
            using type = A;
            template<class T>
#if __cpp_concepts
                requires requires { arr1<A>{std::declval<T>()}; }
#endif // __cpp_concepts
            auto operator()(A, T&&) -> overload_frag<N, A>;
        };
 
        template<class Seq, class... Args>
        struct make_overload;
 
        template<std::size_t... Idx, class... Args>
        struct make_overload<std::integer_sequence<std::size_t, Idx...>, Args...>
            : overload_frag<Idx, Args>... {
            using overload_frag<Idx, Args>::operator()...;
        };
 
        template<class T, class... Ts>
        using best_overload_match = typename decltype(
                make_overload<std::make_index_sequence<sizeof...(Ts)>, Ts...>{}
                (std::declval<T>(), std::declval<T>())
                        )::type;
 
#if __cpp_concepts
        template<class T, class... Ts>
        concept has_non_ambiguous_match =
                requires { typename best_overload_match<T, Ts...>; };
 
        // ================================== rel ops
 
        template<class From, class To>
        concept convertible = std::is_convertible_v<From, To>;
 
        template<class T>
        concept has_eq_comp = requires (T a, T b) {
            { a == b } -> convertible<bool>;
        };
 
        template<class T>
        concept has_lesser_comp = requires (T a, T b) {
            { a < b } -> convertible<bool>;
        };
 
        template<class T>
        concept has_less_or_eq_comp = requires (T a, T b) {
            { a <= b } -> convertible<bool>;
        };
#endif // __cpp_concepts
 
        template<class A>
        struct emplace_no_dtor_from_elem {
            template<class T, class NT, NT N>
            constexpr void operator()(T&& elem, std::integral_constant<NT, N> index_) const {
                a.template emplace_no_dtor<index_>(static_cast<T&&>(elem));
            }
            A& a;
        };
 
        template<class E, class T>
        constexpr void destruct(T& obj)  {
            if constexpr (not std::is_trivially_destructible_v<E>)
                obj.~E();
        }
 
        // =============================== variant union types
 
        // =================== base variant storage type
        // this type is used to build a N-ary tree of union.
 
        struct dummy_type{ static constexpr int elem_size = 0; }; // used to fill the back of union nodes
 
        using union_index_t = unsigned;
 
#define TRAIT(trait) (std::is_##trait##_v<A> && std::is_##trait##_v<B>)
 
#ifdef __cpp_concepts
#  define SFM_DEF_REQ(trait) \
    requires (TRAIT(trait) and not TRAIT(trivially_##trait))
#else // !__cpp_concepts
#  define SFM_DEF_REQ(trait)
#endif // __cpp_concepts
 
#define SFM(signature, impl) \
    signature \
    impl \
 
#define SFM2(signature, req, impl) \
    signature = default; \
    signature \
        req \
    impl
 
#if __cpp_constexpr >= 201907L
#  define SFM_DTOR(X) constexpr ~X()
#else // __cpp_constexpr < 201907L
#  define SFM_DTOR(X) ~X()
#endif
 
// given the two members of type A and B of an union X
// this create the proper conditionally trivial special members functions
#ifdef __cpp_concepts
#  define INJECT_UNION_SFM(X) \
    SFM2(constexpr X (const X &), SFM_DEF_REQ(copy_constructible), {}) \
    SFM2(constexpr X (X&&) noexcept, SFM_DEF_REQ(move_constructible), {}) \
    SFM2(constexpr X& operator=(const X&), SFM_DEF_REQ(copy_assignable), { return *this; }) \
    SFM2(constexpr X& operator=(X&&) noexcept, SFM_DEF_REQ(move_assignable), { return *this; }) \
    SFM2(SFM_DTOR(X), SFM_DEF_REQ(destructible), {})
#  else // !__cpp_concepts
#  define INJECT_UNION_SFM(X) \
    SFM(constexpr X (const X &), {}) \
    SFM(constexpr X (X&&) noexcept, {}) \
    SFM(constexpr X& operator=(const X&), { return *this; }) \
    SFM(constexpr X& operator=(X&&) noexcept, { return *this; }) \
    SFM(SFM_DTOR(X), {})
#endif // __cpp_concepts
 
        template<bool IsLeaf>
        struct node_trait;
 
        template<>
        struct node_trait<true> {
 
            template<class A, class B>
            static constexpr auto elem_size = not(std::is_same_v<B, dummy_type>) ? 2 : 1;
 
            template<std::size_t, class>
            static constexpr char ctor_branch = 0;
        };
 
        template<>
        struct node_trait<false> {
            template<class A, class B>
            static constexpr auto elem_size = A::elem_size + B::elem_size;
 
            template<std::size_t Index, class A>
            static constexpr char ctor_branch = (Index < A::elem_size) ? 1 : 2;
        };
 
        template<bool IsLeaf, class A, class B>
        struct union_node {
            union {
                A a;
                B b;
            };
 
            static constexpr auto elem_size = node_trait<IsLeaf>::template elem_size<A, B>;
 
            constexpr union_node() = default;
 
            template<std::size_t Index, class... Args>
#ifdef __cpp_concepts
                requires (node_trait<IsLeaf>::template ctor_branch<Index, A> == 1)
#endif // __cpp_concepts
            constexpr explicit union_node(in_place_index_t<Index>, Args&&... args)
                : a{in_place_index<Index>, static_cast<Args&&>(args)...}
            {}
 
#ifdef __cpp_concepts
            template<std::size_t Index, class... Args>
                requires (node_trait<IsLeaf>::template ctor_branch<Index, A> == 2)
            constexpr explicit union_node(in_place_index_t<Index>, Args&&... args)
                : b{in_place_index<Index - A::elem_size>, static_cast<Args&&>(args)...}
            {}
#endif // __cpp_concepts
 
            template<class... Args>
#ifdef __cpp_concepts
                requires (IsLeaf)
#endif // __cpp_concepts
            constexpr explicit union_node(in_place_index_t<0>, Args&&... args)
                : a{static_cast<Args&&>(args)...}
            {}
 
            template<class... Args>
#ifdef __cpp_concepts
                requires (IsLeaf)
#endif // __cpp_concepts
            constexpr explicit union_node(in_place_index_t<1>, Args&&... args)
                : b{static_cast<Args&&>(args)...}
            {}
 
            constexpr explicit union_node(dummy_type)
#ifdef __cpp_concepts
                requires (std::is_same_v<dummy_type, B>)
#endif // __cpp_concepts
                : b{}
            {}
 
            template<union_index_t Index>
            constexpr auto& get() {
                if constexpr (IsLeaf) {
                    if constexpr (Index == 0)
                        return a;
                    else
                        return b;
                } else {
                    if constexpr (Index < A::elem_size)
                        return a.template get<Index>();
                    else
                        return b.template get<Index - A::elem_size>();
                }
            }
 
            template<union_index_t Index>
            constexpr const auto& get() const {
                return const_cast<union_node&>(*this).get<Index>();
            }
 
            INJECT_UNION_SFM(union_node)
        };
 
#undef INJECT_UNION_SFM
#undef SFM
#undef TRAIT
 
        // =================== algorithm to build the tree of unions
        // take a sequence of types and perform an order preserving fold until only one type is left
        // the first parameter is the numbers of types remaining for the current pass
 
        constexpr unsigned char pick_next(unsigned remaining)  {
            return static_cast<unsigned char>(remaining >= 2 ? 2 : remaining);
        }
 
        template<unsigned char Pick, unsigned char GoOn, bool FirstPass>
        struct make_tree;
 
        template<bool IsFirstPass>
        struct make_tree<2, 1, IsFirstPass> {
            template<unsigned Remaining, class A, class B, class... Ts>
            using f = typename make_tree
                    <
                            pick_next(Remaining - 2),
                            sizeof...(Ts) != 0,
                            IsFirstPass
                            >
                    ::template f
                    <
                            Remaining - 2,
                            Ts...,
                            union_node<IsFirstPass, A, B>
                            >;
        };
 
        // only one type left, stop
        template<bool F>
        struct make_tree<0, 0, F> {
            template<unsigned, class A>
            using f = A;
        };
 
        // end of one pass, restart
        template<bool IsFirstPass>
        struct make_tree<0, 1, IsFirstPass> {
            template<unsigned Remaining, class... Ts>
            using f = typename make_tree
                    <
                            pick_next(sizeof...(Ts)),
                            (sizeof...(Ts) != 1),
                            false  // <- both first pass and tail call recurse into a tail call
                            >
                    ::template f<sizeof...(Ts), Ts...>;
        };
 
        // one odd type left in the pass, put it at the back to preserve the order
        template<>
        struct make_tree<1, 1, false> {
            template<unsigned Remaining, class A, class... Ts>
            using f = typename make_tree<0, sizeof...(Ts) != 0, false>
                    ::template f<0, Ts..., A>;
        };
 
        // one odd type left in the first pass, wrap it in an union
        template<>
        struct make_tree<1, 1, true> {
            template<unsigned, class A, class... Ts>
            using f = typename make_tree<0, sizeof...(Ts) != 0, false>
                    ::template f<0, Ts..., union_node<true, A, dummy_type>>;
        };
 
        template<class... Ts>
        using make_tree_union = typename
                make_tree<pick_next(sizeof...(Ts)), 1, true>::template f<sizeof...(Ts), Ts...>;
 
        // ============================================================
 
        // Ts... must be sorted in ascending size
        template<std::size_t Num, class... Ts>
        using smallest_suitable_integer_type =
                type_pack_element<(static_cast<unsigned char>(Num > std::numeric_limits<Ts>::max()) + ...),
                                  Ts...
                                  >;
 
        // why do we need this again? i think something to do with GCC?
        namespace swap_trait {
            using std::swap;
 
#ifdef __cpp_concepts
            template<class A>
            concept able = requires (A a, A b) { swap(a, b); };
#endif // __cpp_concepts
 
            template<class A>
            inline constexpr bool nothrow = noexcept(swap(std::declval<A&>(), std::declval<A&>()));
        }
 
#ifndef PLUGIFY_VARIANT_NO_STD_HASH
#  ifdef __cpp_concepts
        template<class T>
        inline constexpr bool has_std_hash =
            requires (T t)
        {
            std::size_t(::std::hash<std::remove_cvref_t<T>>{}(t));
        };
#  endif // __cpp_concepts
#endif // PLUGIFY_VARIANT_NO_STD_HASH
 
        template<class T>
        inline constexpr T* addressof(T& obj) noexcept {
#if defined(__GNUC__) || defined(__clang__)
            return __builtin_addressof(obj);
#elif defined(PLUGIFY_VARIANT_NO_CONSTEXPR_EMPLACE)
            // if & is overloaded, use the ugly version
            if constexpr (requires { obj.operator&(); })
                return reinterpret_cast<T*>
                        (&const_cast<char&>(reinterpret_cast<const volatile char&>(obj)));
            else
                return &obj;
#else
            return std::addressof(obj);
#endif
        }
 
        // ========================= visit dispatcher
 
        template<class Fn, class... Vars>
        using rtype_visit = decltype((std::declval<Fn>()(std::declval<Vars>().template unsafe_get<0>()...)));
 
        template<class Fn, class Var>
        using rtype_index_visit = decltype((std::declval<Fn>()(std::declval<Var>().template unsafe_get<0>(),
                                                               std::integral_constant<std::size_t, 0>{}))
        );
 
        inline namespace v1 {
 
#define DEC(N) X((N)) X((N) + 1) X((N) + 2) X((N) + 3) X((N) + 4) X((N) + 5) X((N) + 6) X((N) + 7) X((N) + 8) X((N) + 9)
 
#define SEQ30(N) DEC((N) + 0) DEC((N) + 10) DEC((N) + 20)
#define SEQ100(N) SEQ30((N) + 0) SEQ30((N) + 30) SEQ30((N) + 60) DEC((N) + 90)
#define SEQ200(N) SEQ100((N) + 0) SEQ100((N) + 100)
#define SEQ400(N) SEQ200((N) + 0) SEQ200((N) + 200)
#define CAT(M, N) M##N
#define CAT2(M, N) CAT(M, N)
#define INJECTSEQ(N) CAT2(SEQ, N)(0)
 
            // single-visitation
 
            template<unsigned Offset, class Rtype, class Fn, class V>
            constexpr Rtype single_visit_tail(Fn&& fn, V&& v) {
 
                constexpr auto RemainingIndex = std::decay_t<V>::size - Offset;
 
#define X(N) case (N + Offset) : \
        if constexpr (N < RemainingIndex) { \
            return static_cast<Fn&&>(fn)(static_cast<V&&>(v).template unsafe_get<N+Offset>()); \
            break; \
        } else PLUGIFY_UNREACHABLE();
 
#define SEQSIZE 100
 
                switch (v.index()) {
 
                    INJECTSEQ(SEQSIZE)
 
                    default:
                        if constexpr (SEQSIZE < RemainingIndex)
                            return detail::single_visit_tail<Offset + SEQSIZE, Rtype>(static_cast<Fn&&>(fn), static_cast<V&&>(v));
                        else
                            PLUGIFY_UNREACHABLE();
                }
 
#undef X
#undef SEQSIZE
            }
 
            template<unsigned Offset, class Rtype, class Fn, class V>
            constexpr Rtype single_visit_w_index_tail(Fn&& fn, V&& v) {
 
                constexpr auto RemainingIndex = std::decay_t<V>::size - Offset;
 
#define X(N) case (N + Offset) : \
        if constexpr (N < RemainingIndex) { \
            return static_cast<Fn&&>(fn)(static_cast<V&&>(v).template unsafe_get<N+Offset>(), std::integral_constant<unsigned, N+Offset>{}); \
            break; \
        } else PLUGIFY_UNREACHABLE();
 
#define SEQSIZE 100
 
                switch (v.index()) {
 
                    INJECTSEQ(SEQSIZE)
 
                    default:
                        if constexpr (SEQSIZE < RemainingIndex)
                            return detail::single_visit_w_index_tail<Offset + SEQSIZE, Rtype>(static_cast<Fn&&>(fn), static_cast<V&&>(v));
                        else
                            PLUGIFY_UNREACHABLE();
                }
 
#undef X
#undef SEQSIZE
            }
 
            template<class Fn, class V>
            constexpr decltype(auto) visit(Fn&& fn, V&& v) {
                return detail::single_visit_tail<0, rtype_visit<Fn&&, V&&>>(PLG_FWD(fn), PLG_FWD(v));
            }
 
            // unlike other visit functions, this takes the variant first!
            // this is confusing, but make the client code easier to read
            template<class Fn, class V>
            constexpr decltype(auto) visit_with_index(V&& v, Fn&& fn) {
                return detail::single_visit_w_index_tail<0, rtype_index_visit<Fn&&, V&&>>(PLG_FWD(fn), PLG_FWD(v));
            }
 
            template<class Fn, class Head, class... Tail>
            constexpr decltype(auto) multi_visit(Fn&& fn, Head&& head, Tail&&... tail) {
 
                // visit them one by one, starting with the last
                auto vis = [&fn, &head] (auto&&... args) -> decltype(auto) {
                    return detail::visit([&fn, &args...] (auto&& elem) -> decltype(auto) {
                        return PLG_FWD(fn)(PLG_FWD(elem), PLG_FWD(args)...);
                    }, PLG_FWD(head));
                };
 
                if constexpr (sizeof...(tail) == 0)
                    return PLG_FWD(vis)();
                else if constexpr (sizeof...(tail) == 1)
                    return detail::visit(PLG_FWD(vis), PLG_FWD(tail)...);
                else
                    return detail::multi_visit(PLG_FWD(vis), PLG_FWD(tail)...);
            }
 
#undef DEC
#undef SEQ30
#undef SEQ100
#undef SEQ200
#undef SEQ400
#undef CAT
#undef CAT2
#undef INJECTSEQ
 
        } // inline namespace v1
 
        struct variant_npos_t {
            template<class T>
            constexpr bool operator==(T idx) const noexcept { return idx == std::numeric_limits<T>::max(); }
        };
    }
 
    inline static constexpr detail::variant_npos_t variant_npos;
 
    template<class... Ts>
    class variant;
 
    template<typename T>
    struct is_variant_impl : std::false_type {};
 
    template<typename... Ts>
    struct is_variant_impl<variant<Ts...>> : std::true_type {};
 
    template<typename T>
    struct is_variant : is_variant_impl<std::decay_t<T>> {};
 
    template<typename T>
    inline constexpr bool is_variant_v = is_variant<T>::value;
 
    // ill-formed variant, an empty specialization prevents some really bad errors messages on gcc
#ifdef __cpp_concepts
    template<class... Ts>
        requires (
            (std::is_array_v<Ts> || ...)
            || (std::is_reference_v<Ts> || ...)
            || (std::is_void_v<Ts> || ...)
            || sizeof...(Ts) == 0
        )
    class variant<Ts...> {
        static_assert(sizeof...(Ts) > 0, "A variant cannot be empty.");
        static_assert(not(std::is_reference_v<Ts> || ...), "A variant cannot contain references, consider using reference wrappers instead.");
        static_assert(not(std::is_void_v<Ts> || ...), "A variant cannot contains void.");
        static_assert(not(std::is_array_v<Ts> || ...), "A variant cannot contain a raw array type, consider using std::array instead.");
    };
#endif // __cpp_concepts
 
    template<class... Ts>
    class variant {
        using storage = detail::union_node<false, detail::make_tree_union<Ts...>, detail::dummy_type>;
 
        static constexpr bool is_trivial           = std::is_trivial_v<storage>;
        static constexpr bool has_copy_ctor        = std::is_copy_constructible_v<storage>;
        static constexpr bool trivial_copy_ctor    = is_trivial || std::is_trivially_copy_constructible_v<storage>;
        static constexpr bool has_copy_assign      = std::is_copy_constructible_v<storage>;
        static constexpr bool trivial_copy_assign  = is_trivial || std::is_trivially_copy_assignable_v<storage>;
        static constexpr bool has_move_ctor        = std::is_move_constructible_v<storage>;
        static constexpr bool trivial_move_ctor    = is_trivial || std::is_trivially_move_constructible_v<storage>;
        static constexpr bool has_move_assign      = std::is_move_assignable_v<storage>;
        static constexpr bool trivial_move_assign  = is_trivial || std::is_trivially_move_assignable_v<storage>;
        static constexpr bool trivial_dtor         = std::is_trivially_destructible_v<storage>;
 
    public:
        template<std::size_t Idx>
        using alternative = std::remove_reference_t<decltype(std::declval<storage&>().template get<Idx>())>;
 
        static constexpr bool can_be_valueless = not is_trivial;
 
        static constexpr unsigned size = sizeof...(Ts);
 
        using index_type = detail::smallest_suitable_integer_type<sizeof...(Ts) + can_be_valueless, unsigned char, unsigned short, unsigned>;
 
        static constexpr index_type npos = static_cast<index_type>(-1);
 
        template<class T>
        static constexpr int index_of = detail::find_first_true({std::is_same_v<T, Ts>...});
 
        // ============================================= constructors (20.7.3.2)
 
        // default constructor
        constexpr variant()
            noexcept(std::is_nothrow_default_constructible_v<alternative<0>>)
#ifdef __cpp_concepts
            requires std::is_default_constructible_v<alternative<0>>
#endif // __cpp_concepts
        : _storage{in_place_index<0>}, _current{0}
        {}
    
        // copy constructor (trivial)
#ifdef __cpp_concepts
        constexpr variant(const variant&)
            requires trivial_copy_ctor
        = default;
#endif // __cpp_concepts
    
        // note : both the copy and move constructor cannot be meaningfully constexpr without std::construct_at
        // copy constructor
        constexpr variant(const variant& o)
#ifdef __cpp_concepts
            requires (has_copy_ctor and not trivial_copy_ctor)
#endif // __cpp_concepts
        : _storage{detail::dummy_type{}} {
            construct_from(o);
        }
    
        // move constructor (trivial)
#ifdef __cpp_concepts
        constexpr variant(variant&&)
            requires trivial_move_ctor
        = default;
#endif // __cpp_concepts
 
        // move constructor
        constexpr variant(variant&& o)
            noexcept((std::is_nothrow_move_constructible_v<Ts> && ...))
#ifdef __cpp_concepts
            requires (has_move_ctor and not trivial_move_ctor)
#endif // __cpp_concepts
        : _storage{detail::dummy_type{}} {
            construct_from(static_cast<variant&&>(o));
        }
 
        // generic constructor
        template<class T, class M = detail::best_overload_match<T&&, Ts...>, class D = std::decay_t<T>>
        constexpr variant(T&& t)
            noexcept(std::is_nothrow_constructible_v<M, T&&>)
#ifdef __cpp_concepts
            requires (not std::is_same_v<D, variant> and not std::is_base_of_v<detail::emplacer_tag, D>)
#endif // __cpp_concepts
        : variant{in_place_index<index_of<M>>, static_cast<T&&>(t)}
        {}
 
        // construct at index
        template<std::size_t Index, class... Args>
#ifdef __cpp_concepts
            requires (Index < size && std::is_constructible_v<alternative<Index>, Args&&...>)
#endif // __cpp_concepts
        explicit constexpr variant(in_place_index_t<Index> tag, Args&&... args)
        : _storage{tag, static_cast<Args&&>(args)...}, _current(Index)
        {}
 
        // construct a given type
        template<class T, class... Args>
#ifdef __cpp_concepts
            requires (detail::appears_exactly_once<T, Ts...> && std::is_constructible_v<T, Args&&...>)
#endif // __cpp_concepts
        explicit constexpr variant(in_place_type_t<T>, Args&&... args)
        : variant{in_place_index<index_of<T>>, static_cast<Args&&>(args)...}
        {}
 
        // initializer-list constructors
        template<std::size_t Index, class U, class... Args>
#ifdef __cpp_concepts
            requires (
                (Index < size) and
                std::is_constructible_v<alternative<Index>, std::initializer_list<U>&, Args&&...>
            )
#endif // __cpp_concepts
        explicit constexpr variant(in_place_index_t<Index> tag, std::initializer_list<U> list, Args&&... args)
        : _storage{tag, list, PLG_FWD(args)...}, _current{Index}
        {}
    
        template<class T, class U, class... Args>
#ifdef __cpp_concepts
            requires (
                detail::appears_exactly_once<T, Ts...>
                && std::is_constructible_v<T, std::initializer_list<U>&, Args&&...>
            )
#endif // __cpp_concepts
        explicit constexpr variant(in_place_type_t<T>, std::initializer_list<U> list, Args&&... args)
        : _storage{in_place_index<index_of<T>>, list, PLG_FWD(args)...}, _current{index_of<T> }
        {}
 
        // ================================ destructors (20.7.3.3)
 
#ifdef __cpp_concepts
#  if __cpp_constexpr >= 201907L
        constexpr ~variant() requires trivial_dtor = default;
        constexpr ~variant() requires (not trivial_dtor) {
            reset();
        }
#  else // __cpp_constexpr < 201907L
        ~variant() requires trivial_dtor = default;
        ~variant() requires (not trivial_dtor) {
            reset();
        }
#  endif // __cpp_constexpr >= 201907L
#else // !__cpp_concepts
        ~variant() {
            reset();
        }
#endif // __cpp_concepts
 
        // ================================ assignment(20.7.3.4)
 
        // copy assignment(trivial)
#ifdef __cpp_concepts
        constexpr variant& operator=(const variant& o)
            requires trivial_copy_assign && trivial_copy_ctor
        = default;
#endif // __cpp_concepts
 
        // copy assignment
        constexpr variant& operator=(const variant& rhs)
#ifdef __cpp_concepts
            requires (has_copy_assign and not(trivial_copy_assign && trivial_copy_ctor))
#endif // __cpp_concepts
        {
            if (this == &rhs) [[unlikely]]
                return *this;
 
            assign_from(rhs, [&](const auto& elem, auto index_cst) {
                if (index() == index_cst)
                    unsafe_get<index_cst>() = elem;
                else {
                    using type = alternative<index_cst>;
                    constexpr bool do_simple_copy =
                            std::is_nothrow_copy_constructible_v<type>
                            or not std::is_nothrow_move_constructible_v<type>;
                    if constexpr (do_simple_copy)
                        emplace<index_cst>(elem);
                    else {
                        alternative<index_cst> tmp = elem;
                        emplace<index_cst>(PLG_MOV(tmp));
                    }
                }
            });
            return *this;
        }
    
        // move assignment(trivial)
#ifdef __cpp_concepts
        constexpr variant& operator=(variant&& o)
            requires (trivial_move_assign and trivial_move_ctor and trivial_dtor)
        = default;
#endif // __cpp_concepts
    
        // move assignment
        constexpr variant& operator=(variant&& o)
            noexcept((std::is_nothrow_move_constructible_v<Ts> && ...) && (std::is_nothrow_move_assignable_v<Ts> && ...))
#ifdef __cpp_concepts
            requires (has_move_assign && has_move_ctor and not(trivial_move_assign and trivial_move_ctor and trivial_dtor))
#endif // __cpp_concepts
            {
            if (this == &o) [[unlikely]]
                return *this;
 
            assign_from(PLG_FWD(o), [&](auto&& elem, auto index_cst) {
                if (index() == index_cst)
                    unsafe_get<index_cst>() = PLG_MOV(elem);
                else
                    emplace<index_cst>(PLG_MOV(elem));
            });
            return *this;
        }
    
        // generic assignment
        template<class T>
#ifdef __cpp_concepts
            requires detail::has_non_ambiguous_match<T, Ts...>
#endif // __cpp_concepts
        constexpr variant& operator=(T&& t)
            noexcept(std::is_nothrow_assignable_v<detail::best_overload_match<T&&, Ts...>, T&&>
                      && std::is_nothrow_constructible_v<detail::best_overload_match<T&&, Ts...>, T&&>) {
            using related_type = detail::best_overload_match<T&&, Ts...>;
            constexpr auto new_index = index_of<related_type>;
    
            if (_current == new_index)
                unsafe_get<new_index>() = PLG_FWD(t);
            else {
                constexpr bool do_simple_emplace =
                    std::is_nothrow_constructible_v<related_type, T>
                    or not std::is_nothrow_move_constructible_v<related_type>;
    
                if constexpr (do_simple_emplace)
                    emplace<new_index>(PLG_FWD(t));
                else {
                    related_type tmp = t;
                    emplace<new_index>(PLG_MOV(tmp));
                }
            }
    
            return *this;
        }
    
        // ================================== modifiers (20.7.3.5)
    
        template<class T, class... Args>
#ifdef __cpp_concepts
            requires (std::is_constructible_v<T, Args&&...> && detail::appears_exactly_once<T, Ts...>)
#endif // __cpp_concepts
        constexpr T& emplace(Args&&... args) {
            return emplace<index_of<T>>(static_cast<Args&&>(args)...);
        }
 
        template<std::size_t Idx, class... Args>
#ifdef __cpp_concepts
            requires (Idx < size and std::is_constructible_v<alternative<Idx>, Args&&...> )
#endif // __cpp_concepts
        constexpr auto& emplace(Args&&... args) {
            return emplace_impl<Idx>(PLG_FWD(args)...);
        }
 
        // emplace with initializer-lists
        template<std::size_t Idx, class U, class... Args>
#ifdef __cpp_concepts
            requires (Idx < size
                && std::is_constructible_v<alternative<Idx>, std::initializer_list<U>&, Args&&...>)
#endif // __cpp_concepts
        constexpr auto& emplace(std::initializer_list<U> list, Args&&... args) {
            return emplace_impl<Idx>(list, PLG_FWD(args)...);
        }
 
        template<class T, class U, class... Args>
#ifdef __cpp_concepts
            requires (std::is_constructible_v<T, std::initializer_list<U>&, Args&&...>
                       && detail::appears_exactly_once<T, Ts...>)
#endif // __cpp_concepts
        constexpr T& emplace(std::initializer_list<U> list, Args&&... args) {
            return emplace_impl<index_of<T>>(list, PLG_FWD(args)...);
        }
 
        // ==================================== value status (20.7.3.6)
 
        [[nodiscard]] constexpr bool valueless_by_exception() const noexcept {
            if constexpr (can_be_valueless)
                return _current == npos;
            else return false;
        }
 
        [[nodiscard]] constexpr index_type index() const noexcept {
            return _current;
        }
 
        // =================================== swap (20.7.3.7)
 
        constexpr void swap(variant& o)
            noexcept((std::is_nothrow_move_constructible_v<Ts> && ...)
                       && (detail::swap_trait::template nothrow<Ts> && ...))
#ifdef __cpp_concepts
            requires (has_move_ctor && (detail::swap_trait::template able<Ts> && ...))
#endif // __cpp_concepts
        {
            if constexpr (can_be_valueless) {
                // if one is valueless, move the element form the non-empty variant,
                // reset it, and set it to valueless
                constexpr auto impl_one_valueless = [](auto&& full, auto& empty) {
                    detail::visit_with_index(PLG_FWD(full), detail::emplace_no_dtor_from_elem<variant&>{empty});
                    full.reset_no_check();
                    full._current = npos;
                };
 
                switch(static_cast<int>(index() == npos) + static_cast<int>(o.index() == npos) * 2) {
                    case 0 :
                        break;
                    case 1 :
                        // "this" is valueless
                        impl_one_valueless(PLG_MOV(o), *this);
                        return;
                    case 2 :
                        // "other" is valueless
                        impl_one_valueless(PLG_MOV(*this), o);
                        return;
                    case 3 :
                        // both are valueless, do nothing
                        return;
                }
            }
 
            assert(not(valueless_by_exception() && o.valueless_by_exception()));
 
            detail::visit_with_index(o, [&o, this](auto&& elem, auto index_cst) {
                if (index() == index_cst) {
                    using std::swap;
                    swap(unsafe_get<index_cst>(), elem);
                    return;
                }
 
                using idx_t = decltype(index_cst);
                detail::visit_with_index(*this, [this, &o, &elem](auto&& this_elem, auto this_index) {
                    auto tmp { PLG_MOV(this_elem) };
 
                    // destruct the element
                    detail::destruct<alternative<this_index>>(this_elem);
 
                    // ok, we just destroyed the element in this, don't call the dtor again
                    emplace_no_dtor<idx_t::value>(PLG_MOV(elem));
 
                    // we could refactor this
                    detail::destruct<alternative<idx_t::value>>(elem);
                    o.template emplace_no_dtor<(unsigned)(this_index) >(PLG_MOV(tmp));
                });
            });
        }
 
        // +================================== methods for internal use
        // these methods performs no errors checking at all
 
        template<detail::union_index_t Idx>
        [[nodiscard]] constexpr auto& unsafe_get() & noexcept {
            static_assert(Idx < size);
            assert(_current == Idx);
            return _storage.template get<Idx>();
        }
 
        template<detail::union_index_t Idx>
        [[nodiscard]] constexpr auto&& unsafe_get() && noexcept {
            static_assert(Idx < size);
            assert(_current == Idx);
            return PLG_MOV(_storage.template get<Idx>());
        }
 
        template<detail::union_index_t Idx>
        [[nodiscard]] constexpr const auto& unsafe_get() const & noexcept {
            static_assert(Idx < size);
            assert(_current == Idx);
            return _storage.template get<Idx>();
        }
 
        template<detail::union_index_t Idx>
        [[nodiscard]] constexpr const auto&& unsafe_get() const && noexcept {
            static_assert(Idx < size);
            assert(_current == Idx);
            return PLG_MOV(_storage.template get<Idx>());
        }
 
    private:
        // assign from another variant
        template<class Other, class Fn>
        constexpr void assign_from(Other&& o, Fn&& fn) {
            if constexpr (can_be_valueless) {
                if (o.index() == npos) {
                    if (_current != npos) {
                        reset_no_check();
                        _current = npos;
                    }
                    return;
                }
            }
            assert(not o.valueless_by_exception());
            detail::visit_with_index(PLG_FWD(o), PLG_FWD(fn));
        }
 
        template<unsigned Idx, class... Args>
        constexpr auto& emplace_impl(Args&&... args) {
            reset();
            emplace_no_dtor<Idx>(PLG_FWD(args)...);
            return unsafe_get<Idx>();
        }
 
        // can be used directly only when the variant is in a known empty state
        template<unsigned Idx, class... Args>
        constexpr void emplace_no_dtor(Args&&... args) {
            using T = alternative<Idx>;
 
            if constexpr (not std::is_nothrow_constructible_v<T, Args&&...>)
            {
                if constexpr (std::is_nothrow_move_constructible_v<T>)
                {
                    do_emplace_no_dtor<Idx>(T{PLG_FWD(args)...});
                }
                else if constexpr (std::is_nothrow_copy_constructible_v<T>)
                {
                    T tmp {PLG_FWD(args)...};
                    do_emplace_no_dtor<Idx>(tmp);
                }
                else
                {
                    static_assert(can_be_valueless,
                                  "Internal error : the possibly valueless branch of emplace was taken despite |can_be_valueless| being false");
                    _current = npos;
                    do_emplace_no_dtor<Idx>(PLG_FWD(args)...);
                }
            }
            else
                do_emplace_no_dtor<Idx>(PLG_FWD(args)...);
        }
 
        template<unsigned Idx, class... Args>
        constexpr void do_emplace_no_dtor(Args&&... args) {
            _current = static_cast<index_type>(Idx);
 
            auto* ptr = detail::addressof(unsafe_get<Idx>());
 
#ifdef PLUGIFY_VARIANT_NO_CONSTEXPR_EMPLACE
            using T = alternative<Idx>;
            new ((void*)(ptr)) t(PLG_FWD(args)...);
#else
            PLUGIFY_CONSTRUCT_AT(ptr, PLG_FWD(args)...);
#endif // PLUGIFY_VARIANT_NO_CONSTEXPR_EMPLACE
        }
 
        // destroy the current elem IFF not valueless
        constexpr void reset() {
            if constexpr (can_be_valueless)
                if (valueless_by_exception()) return;
            reset_no_check();
        }
 
        // destroy the current element without checking for valueless
        constexpr void reset_no_check() {
            assert(index() < size);
            if constexpr (not trivial_dtor) {
                detail::visit_with_index(*this, [](auto& elem, auto index_cst) {
                    detail::destruct<alternative<index_cst>>(elem);
                });
            }
        }
 
        // construct this from another variant, for constructors only
        template<class Other>
        constexpr void construct_from(Other&& o) {
            if constexpr (can_be_valueless)
                if (o.valueless_by_exception()) {
                    _current = npos;
                    return;
                }
 
            detail::visit_with_index(PLG_FWD(o), detail::emplace_no_dtor_from_elem<variant&>{*this});
        }
 
        template<class T>
        friend struct detail::emplace_no_dtor_from_elem;
 
        storage _storage;
        index_type _current;
    };
 
    // ================================= value access (20.7.5)
 
    template<class T, class... Ts>
    constexpr bool holds_alternative(const variant<Ts...>& v) noexcept {
        static_assert((std::is_same_v<T, Ts> || ...), "Requested type is not contained in the variant");
        constexpr auto Index = variant<Ts...>::template index_of<T>;
        return v.index() == Index;
    }
 
    // ========= get by index
 
    template<std::size_t Idx, class... Ts>
    [[nodiscard]] constexpr auto& get(variant<Ts...>& v) {
        static_assert(Idx < sizeof...(Ts), "Index exceeds the variant size. ");
        PLUGIFY_ASSERT(v.index() == Idx, "plg::variant:get(): Bad variant access in get.", bad_variant_access);
        return (v.template unsafe_get<Idx>());
    }
 
    template<std::size_t Idx, class... Ts>
    [[nodiscard]] constexpr const auto& get(const variant<Ts...>& v) {
        return plg::get<Idx>(const_cast<variant<Ts...>&>(v));
    }
 
    template<std::size_t Idx, class... Ts>
    [[nodiscard]] constexpr auto&& get(variant<Ts...>&& v) {
        return PLG_MOV(plg::get<Idx>(v));
    }
 
    template<std::size_t Idx, class... Ts>
    [[nodiscard]] constexpr const auto&& get(const variant<Ts...>&& v) {
        return PLG_MOV(plg::get<Idx>(v));
    }
 
    // ========= get by type
 
    template<class T, class... Ts>
    [[nodiscard]] constexpr T& get(variant<Ts...>& v) {
        return plg::get<variant<Ts...>::template index_of<T> >(v);
    }
 
    template<class T, class... Ts>
    [[nodiscard]] constexpr const T& get(const variant<Ts...>& v) {
        return plg::get<variant<Ts...>::template index_of<T> >(v);
    }
 
    template<class T, class... Ts>
    [[nodiscard]] constexpr T&& get(variant<Ts...>&& v) {
        return plg::get<variant<Ts...>::template index_of<T> >(PLG_FWD(v));
    }
 
    template<class T, class... Ts>
    [[nodiscard]] constexpr const T&& get(const variant<Ts...>&& v) {
        return plg::get<variant<Ts...>::template index_of<T> >(PLG_FWD(v));
    }
 
    // ===== get_if by index
 
    template<std::size_t Idx, class... Ts>
    [[nodiscard]] constexpr const auto* get_if(const variant<Ts...>* v) noexcept {
        using rtype = typename variant<Ts...>::template alternative<Idx>*;
        if (v == nullptr || v->index() != Idx)
            return rtype{nullptr};
        else
            return detail::addressof(v->template unsafe_get<Idx>());
    }
 
    template<std::size_t Idx, class... Ts>
    [[nodiscard]] constexpr auto* get_if(variant<Ts...>* v) noexcept {
        using rtype = typename variant<Ts...>::template alternative<Idx>;
        return const_cast<rtype*>(
            plg::get_if<Idx>(static_cast<const variant<Ts...>*>(v))
        );
    }
 
    // ====== get_if by type
 
    template<class T, class... Ts>
    [[nodiscard]] constexpr T* get_if(variant<Ts...>* v) noexcept {
        static_assert((std::is_same_v<T, Ts> || ...), "Requested type is not contained in the variant");
        return plg::get_if<variant<Ts...>::template index_of<T> >(v);
    }
 
    template<class T, class... Ts>
    [[nodiscard]] constexpr const T* get_if(const variant<Ts...>* v) noexcept {
        static_assert((std::is_same_v<T, Ts> || ...), "Requested type is not contained in the variant");
        return plg::get_if<variant<Ts...>::template index_of<T> >(v);
    }
 
    // =============================== visitation (20.7.7)
 
    template<class Fn, class... Vs>
    constexpr decltype(auto) visit(Fn&& fn, Vs&&... vs) {
        if constexpr ((std::decay_t<Vs>::can_be_valueless || ...))
            PLUGIFY_ASSERT(!(vs.valueless_by_exception() || ...), "plg::variant:visit(): Bad variant access in visit.", bad_variant_access);
    
        if constexpr (sizeof...(Vs) == 1)
            return detail::visit(PLG_FWD(fn), PLG_FWD(vs)...);
        else
            return detail::multi_visit(PLG_FWD(fn), PLG_FWD(vs)...);
    }
 
    template<class Fn>
    constexpr decltype(auto) visit(Fn&& fn) {
        return PLG_FWD(fn)();
    }
 
    template<class R, class Fn, class... Vs>
#ifdef __cpp_concepts
        requires (is_variant_v<Vs> && ...)
#endif // __cpp_concepts
    constexpr R visit(Fn&& fn, Vs&&... vars) {
        return static_cast<R>(plg::visit(PLG_FWD(fn), PLG_FWD(vars)...));
    }
 
    // ============================== relational operators (20.7.6)
 
    template<class... Ts>
#ifdef __cpp_concepts
        requires (detail::has_eq_comp<Ts> && ...)
#endif // __cpp_concepts
    [[nodiscard]] constexpr bool operator==(const variant<Ts...>& v1, const variant<Ts...>& v2) {
        if (v1.index() != v2.index())
            return false;
        if constexpr (variant<Ts...>::can_be_valueless)
            if (v1.valueless_by_exception()) return true;
        return detail::visit_with_index(v2, [&v1](auto& elem, auto index) -> bool {
            return (v1.template unsafe_get<index>() == elem);
        });
    }
 
    template<class... Ts>
    [[nodiscard]] constexpr bool operator!=(const variant<Ts...>& v1, const variant<Ts...>& v2)
#ifdef __cpp_concepts
        requires requires { v1 == v2; }
#endif // __cpp_concepts
    {
        return not(v1 == v2);
    }
 
    template<class... Ts>
#ifdef __cpp_concepts
        requires (detail::has_lesser_comp<const Ts&> && ...)
#endif // __cpp_concepts
    [[nodiscard]] constexpr bool operator<(const variant<Ts...>& v1, const variant<Ts...>& v2) {
        if constexpr (variant<Ts...>::can_be_valueless) {
            if (v2.valueless_by_exception()) return false;
            if (v1.valueless_by_exception()) return true;
        }
        if (v1.index() == v2.index()) {
            return detail::visit_with_index(v1, [&v2](auto& elem, auto index) -> bool {
                return (elem < v2.template unsafe_get<index>());
            });
        }
        else
            return (v1.index() < v2.index());
    }
 
    template<class... Ts>
    [[nodiscard]] constexpr bool operator>(const variant<Ts...>& v1, const variant<Ts...>& v2)
#ifdef __cpp_concepts
        requires requires { v2 < v1; }
#endif // __cpp_concepts
    {
        return v2 < v1;
    }
 
    template<class... Ts>
#ifdef __cpp_concepts
        requires (detail::has_less_or_eq_comp<const Ts&> && ...)
#endif // __cpp_concepts
    [[nodiscard]] constexpr bool operator<=(const variant<Ts...>& v1, const variant<Ts...>& v2) {
        if constexpr (variant<Ts...>::can_be_valueless) {
            if (v1.valueless_by_exception()) return true;
            if (v2.valueless_by_exception()) return false;
        }
        if (v1.index() == v2.index()) {
            return detail::visit_with_index(v1, [&v2](auto& elem, auto index) -> bool {
                return (elem <= v2.template unsafe_get<index>());
            });
        }
        else
            return (v1.index() < v2.index());
    }
 
    template<class... Ts>
    [[nodiscard]] constexpr bool operator>=(const variant<Ts...>& v1, const variant<Ts...>& v2)
#ifdef __cpp_concepts
        requires requires { v2 <= v1; }
#endif // __cpp_concepts
    {
        return v2 <= v1;
    }
 
    // ===================================== monostate (20.7.8, 20.7.9)
 
    struct monostate{};
    [[nodiscard]] constexpr bool operator==(monostate, monostate) noexcept { return true; }
    [[nodiscard]] constexpr bool operator> (monostate, monostate) noexcept { return false; }
    [[nodiscard]] constexpr bool operator< (monostate, monostate) noexcept { return false; }
    [[nodiscard]] constexpr bool operator<=(monostate, monostate) noexcept { return true; }
    [[nodiscard]] constexpr bool operator>=(monostate, monostate) noexcept { return true; }
 
    // ===================================== specialized algorithms (20.7.10)
 
    template<class... Ts>
    constexpr void swap(variant<Ts...>& a, variant<Ts...>& b)
        noexcept(noexcept(a.swap(b)))
#ifdef __cpp_concepts
        requires requires { a.swap(b); }
#endif // __cpp_concepts
    {
        a.swap(b);
    }
 
    // ===================================== helper classes (20.7.4)
 
    template<class T>
#ifdef __cpp_concepts
        requires is_variant_v<T>
#endif // __cpp_concepts
    inline constexpr std::size_t variant_size_v = std::decay_t<T>::size;
 
    // not sure why anyone would need this, i'm adding it anyway
    template<class T>
#ifdef __cpp_concepts
        requires is_variant_v<T>
#endif // __cpp_concepts
    struct variant_size : std::integral_constant<std::size_t, variant_size_v<T>> {};
 
    namespace detail {
        // ugh, we have to take care of volatile here
        template<bool IsVolatile>
        struct var_alt_impl {
            template<std::size_t Idx, class T>
            using type = std::remove_reference_t<decltype(std::declval<T>().template unsafe_get<Idx>())>;
        };
 
        template<>
        struct var_alt_impl<true> {
            template<std::size_t Idx, class T>
            using type = volatile typename var_alt_impl<false>::template type<Idx, std::remove_volatile_t<T>>;
        };
    }
 
    template<std::size_t Idx, class T>
#ifdef __cpp_concepts
        requires (Idx < variant_size_v<T>)
#endif // __cpp_concepts
    using variant_alternative_t = typename detail::var_alt_impl<std::is_volatile_v<T>>::template type<Idx, T>;
 
    template<std::size_t Idx, class T>
#ifdef __cpp_concepts
        requires is_variant_v<T>
#endif // __cpp_concepts
    struct variant_alternative {
        using type = variant_alternative_t<Idx, T>;
    };
 
    // ===================================== extensions (unsafe_get)
 
    template<std::size_t Idx, class Var>
#ifdef __cpp_concepts
        requires is_variant_v<Var>
#endif // __cpp_concepts
    [[nodiscard]] constexpr auto&& unsafe_get(Var&& var) noexcept {
        static_assert(Idx < std::decay_t<Var>::size, "Index exceeds the variant size.");
        return PLG_FWD(var).template unsafe_get<Idx>();
    }
 
    template<class T, class Var>
#ifdef __cpp_concepts
        requires is_variant_v<Var>
#endif // __cpp_concepts
    [[nodiscard]] constexpr auto&& unsafe_get(Var&& var) noexcept {
        return plg::unsafe_get<std::decay_t<Var>::template index_of<T>>(PLG_FWD(var));
    }
 
} // namespace plg
 
// ====================================== hash support(20.7.12)
#ifndef PLUGIFY_VARIANT_NO_STD_HASH
namespace std {
    template<class... Ts>
#ifdef __cpp_concepts
        requires (plg::detail::has_std_hash<Ts> && ...)
#endif // __cpp_concepts
    struct hash<plg::variant<Ts...>> {
        std::size_t operator()(const plg::variant<Ts...>& v) const {
            if constexpr (plg::variant<Ts...>::can_be_valueless)
                if (v.valueless_by_exception())
                    return static_cast<std::size_t>(-1);
 
            return plg::detail::visit_with_index(v, [](auto& elem, auto index_) {
                using type = std::remove_cvref_t<decltype(elem)>;
                return std::hash<type>{}(elem) + index_;
            });
        }
    };
 
    template<>
    struct hash<plg::monostate> {
        constexpr std::size_t operator()(plg::monostate) const noexcept { return static_cast<std::size_t>(-1); }
    };
}  // namespace std
#endif // PLUGIFY_VARIANT_NO_STD_HASH
 
#undef PLG_FWD
#undef PLG_MOV