common_type.hpp

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#ifndef BSL_COMMON_TYPE_HPP
#define BSL_COMMON_TYPE_HPP

#include "decay.hpp"
#include "declval.hpp"

namespace bsl
{
    template<typename...>
    struct common_type;

    template<typename... T>
    using common_type_t = typename common_type<T...>::type;


    template<typename T>
    struct common_type<T> final
    {
        using type = decay_t<T>;
    };

    template<typename T1, typename T2>
    struct common_type<T1, T2> final
    {
        using type = decay_t<decltype(true ? declval<T1>() : declval<T2>())>;    // NOLINT
    };

    template<typename T1, typename T2, typename... R>
    struct common_type<T1, T2, R...> final
    {
        using type = common_type_t<common_type_t<T1, T2>, R...>;
    };

}

#endif