Provides declarations of apply_visitor for three or more variant parameters. Allows compile-time checked type-safe application of the given visitor to the content of the given variant, ensuring that all types are handled by the visitor. See apply_visitor for more information. typename MultiVisitor::result_type OR decltype(auto) MultiVisitor & Variant1&& Variant2&& Variant3&& ... typename MultiVisitor::result_type OR decltype(auto) const MultiVisitor & Variant1&& Variant2&& Variant3&& ...