absl::btree_map::insert_or_assign

template<
    typename K = key_type,
    class M,
    int = (EnableIf<LifetimeBoundKV<K, false, M, false, IfRRef<int &&>::AddPtr<K>, IfRRef<const int &>::AddPtr<M>>>())>
decltype(auto)
insert_or_assign(
    key_arg<K>&& k,
    M const& obj);