Why does std::memory_order_acq_rel always trigger warnings in C++11?

Question

My compiler is clang 18.1.0-rc1; and the following code triggers two warnings:

#include <atomic>

std::atomic<int> n;

int main() {
    // Warning: Memory order argument to atomic operation is invalid
    n.load(std::memory_order_acq_rel); 

    // Warning: Memory order argument to atomic operation is invalid
    n.store(1, std::memory_order_acq_rel);
}

What's the legal usage of std::memory_order_acq_rel?

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Update

n.fetch_add(1, std::memory_order_acq_rel); // now is ok

Answer 1

It's valid with RMWs like fetch_add that both load and store in one operation.

The acquire part of acq_rel is valid for the load side, the release part is valid for the store side.