swap.h

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#pragma once

#ifndef SCL_CPP17
    #include <type_traits>
    #include <utility>
    #include <scl/tools/meta/enable_if.h>
    #include <scl/tools/meta/exists.h>
    #include <scl/tools/meta/is_same.h>
    #include <scl/tools/meta/type_mod.h>

    #define SCL_t std::declval<T&>()
    #define SCL_u std::declval<U&>()

    namespace scl{
        namespace tools{
            namespace meta{
                namespace details{
                    template <class T, class U, class = void>
                    struct __is_swappable_with : std::false_type{};

                    template <class T, class U>
                    struct __is_swappable_with<T, U, void_t<enable_if_t<
                                                     exists<decltype(std::swap(SCL_t, SCL_u))>()
                                                     && exists<decltype(std::swap(SCL_u, SCL_t))>()
                    >>> : std::true_type{};

                    template <class T, class U>
                    constexpr bool is_swappable_with(){
                        return __is_swappable_with<T, U>::value;
                    }

                    template <class T, class = void>
                    struct __is_swappable : std::false_type{};

                    template <class T>
                    struct __is_swappable<T, enable_if_t<
                                             !is_same<T, add_lvalue_reference_t<T>>()
                    && is_swappable_with<T&, T&>()
                    >> : std::true_type{};
                }

                template <class T>
                constexpr bool is_swappable(){
                    return details::__is_swappable<T>::value;
                }
            }
        }
    }

    #undef SCL_t
    #undef SCL_u
#endif