C++17 variant type checker trait.

variant_checker.hpp

// Variant type checker to make sure definition and implementation are in sync.
template <typename VariantT, int Index, typename Current, typename... Args>
struct check_variant_sub {
    inline static constexpr bool value = std::is_same_v<std::decay_t<decltype(std::get<Index>(std::declval<VariantT>()))>, Current>
                                         && check_variant_sub<VariantT, Index + 1, Args...>::value;
};

template <typename VariantT, int Index, typename Last>
struct check_variant_sub<VariantT, Index, Last> {
    inline static constexpr bool value = std::is_same_v<std::decay_t<decltype(std::get<Index>(std::declval<VariantT>()))>, Last>;
};

template <typename VariantT, typename... Args>
struct check_variant {
    inline static constexpr bool value = check_variant_sub<VariantT, 0, Args...>::value;
};

template <typename VariantT, typename... Args>
inline constexpr bool check_variant_v = check_variant<VariantT, Args...>::value;

// Example:
using Data = std::variant<std::string, double, DateTime>;
static_assert(check_variant_v<Data, std::string, double, DateTime>);