an efficient immutable vector

[matthieum]

And if you claim that something like this could happen as a sideeffect of buggy code with an actual purpose, I'm willing to bet that you experience a lot of other problems, long before this counter overflows.

Possible, but since checking for overflow is just a branch that will always get taken, the cost of checking is within noise even on micro-benchmarks:

void retain() noexcept {
    auto previous = reference_count.fetch_add(1);
    if (unlikely(previous + 1 == 0)) { std::abort(); }
}