AsyncMergeSort

#include <algorithm>
#include <execution>
#include <future>
#include <numeric>
#include <vector>

using namespace std;

constexpr int MAX_ASYNC_DEPTH = 2;

template <typename RandomIt>
void MergeSort(RandomIt range_begin, RandomIt range_end, int depth = 0) {
    const int range_length = range_end - range_begin;
    if (range_length < 2) {
        return;
    }

    vector elements(range_begin, range_end);

    const auto mid = elements.begin() + range_length / 2;

    auto left_task = [start = elements.begin(), mid, depth] {
        MergeSort(start, mid, depth + 1);
    };
    auto right_task = [mid, finish = elements.end(), depth] {
        MergeSort(mid, finish, depth + 1);
    };

    if (depth <= MAX_ASYNC_DEPTH) {
        auto left_future = async(left_task);
        right_task();
        left_future.get();
    } else {
        left_task();
        right_task();
    }

    merge(execution::par, elements.begin(), mid, mid, elements.end(), range_begin);
}