#include <execution>
#include <algorithm>
#include <mutex>

int main()
{
  // example from https://en.cppreference.com/w/cpp/algorithm/execution_policy_tag_t
  int x = 0;
  std::mutex m;
  int a[] = {1, 2};
  std::for_each(std::execution::par, std::begin(a), std::end(a), [&](int) {
    std::lock_guard<std::mutex> guard(m);
    ++x; // correct
  });
}