Create a Variant_Union object holding the alternative at index (template parameter) t_INDEX in (template parameters) 't_HEAD, t_TAIL..., direct-initialized from the specified args'. Note that t_INDEX is not necessarily the absolute index of the desired alternative in the bsl::variant object. We use the tag to "unravel" the union until we get to the desired alternative type.
Declared in <bslstl_variant.h>
template<
size_t t_INDEX,
class... t_ARGS>
Variant_Union(
bsl::in_place_index_t<t_INDEX>,
t_ARGS&&... args);