holds_alternative.h

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#pragma once
 
#include "di/meta/algorithm.h"
#include "di/vocab/variant/variant_like.h"
#include "di/vocab/variant/variant_types.h"
 
namespace di::vocab {
namespace detail {
    template<typename T>
    struct HoldsAlternativeFunction {
        template<concepts::VariantLike Var, typename List = meta::VariantTypes<Var>>
        requires(meta::UniqueType<T, List>)
        constexpr auto operator()(Var const& variant) const -> bool {
            constexpr auto expected_index = meta::Lookup<T, List>;
            return variant_index(variant) == expected_index;
        }
    };
}
 
template<typename T>
constexpr inline auto holds_alternative = detail::HoldsAlternativeFunction<T> {};
}
 
namespace di {
using vocab::holds_alternative;
}