sdbusplus reading InterfacesAdded issue: not all variants are created equal

[Paul Fertser]

On Thu, Dec 23, 2021 at 10:24:30AM -0600, Patrick Williams wrote:
> You could pretty easily add a `merge_variant` on top of this that
> would be the union of all the variant types.

This seems to work but I have near-zero clue about templates
meta-programming, please bear with me. Does this look suitable as an
another sdbusplus utility, should I send it for review after adding
docs?

template <typename T, typename... Unused>
struct merge_variant
{   
    using type = T;
};
template <typename D, typename... Done, typename... First, typename... Rest>
struct merge_variant<std::variant<D, Done...>, std::variant<First...>, Rest...>
    : public merge_variant<
          sdbusplus::utility::dedup_variant_t<D, Done..., First...>, Rest...>
{
};
template <typename... T>
using merge_variant_t = typename merge_variant<T...>::type;

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