variant_or_empty.h

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#pragma once
 
#include "di/meta/algorithm.h"
#include "di/meta/core.h"
#include "di/vocab/variant/variant_forward_declaration.h"
 
namespace di::meta {
namespace detail {
    struct EmptyVariant {
        EmptyVariant() = delete;
    };
 
    template<typename List>
    struct VariantOrEmptyHelper;
 
    template<typename... Types>
    struct VariantOrEmptyHelper<List<Types...>> : TypeConstant<vocab::Variant<Types...>> {};
 
    template<>
    struct VariantOrEmptyHelper<List<>> : TypeConstant<EmptyVariant> {};
}
 
template<typename... Types>
using VariantOrEmpty = detail::VariantOrEmptyHelper<meta::Unique<meta::List<Types...>>>::Type;
}