Define switch_variant, helper for switch on variants.

switch_variant.h

/* Use switch_variant like this:
   
       switch_variant(someVariant,
           [](AlternativeWithoutData) { ... },
           [](const AlternativeWithData& x) { ... },
           ...
           [](auto) {} // default branch
       };
*/

template <typename... Ts>
struct overloaded : Ts... {
  using Ts::operator()...;
};

template <typename... Ts>
overloaded(Ts...) -> overloaded<Ts...>;

template <typename... Ts, typename Variant>
auto switch_variant(Variant&& variant, Ts&&... ts) {
  return std::visit(overloaded{std::forward<Ts>(ts)...}, std::forward<Variant>(variant));
}