[tialaramex]

I was not aware. I wonder if anybody tried asking Apple? That's what happened for SRWLOCK in Windows. Mara asked Microsoft if they can promise it actually does what it seems like it does, formally, and they did document that promise so then Rust used SRWLOCK [today your Rust does not use SRWLOCK, this year it was replaced for performance reasons, which is timely because it turns out SRWLOCK is faulty and so "Don't use SRWLOCK" is the most practical fix in the medium term, awkward for poor C++ though as they used SRWLOCK too].

Apple are even less talkative than Microsoft, but it is possible that the reason libc++ uses this is that it's actually guaranteed and if properly asked Apple would say so. It would be nice for (newer) macOS to get the same promises as the mainstream platforms with respect to these primitives.