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&&
...