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&&
...