Declared in <absl/container/btree_set.h>
template<typename T>
requires std::conjunction_v<
std::is_same<value_type, typename T::value_type>,
std::is_same<allocator_type, typename T::allocator_type>,
std::is_same<typename params_type::is_map_container,
typename T::params_type::is_map_container>>
void
merge(btree_container<T>&& src);