message: std::visit => mpark:: in mpark variant serialisation
diff: @@ -79,7 +79,7 @@ inline void CEREAL_SAVE_FUNCTION_NAME(Archive& ar,
  std::int32_t index = static_cast<std::int32_t>(variant.index());
  ar(CEREAL_NVP_("index", index));
  variant_detail::variant_save_visitor<Archive> visitor(ar);
  - std::visit(visitor, variant);
  + mpark::visit(visitor, variant);
  }
  //! Loading for mpark::variant
  