C++ happens-before 关系是不可传递的

P0668R4 对此进行了解释

The definition of plain happens-before became unpleasantly complicated with the introduction of memory_order_consume. And it is not transitive, which remains counterintuitive. This proposal changes neither of those. And if the user refrains from using memory_order_consume it can continue to be entirely ignored, as before. Until we have a usable version of memory_order_consume, I would expect teaching materials to ignore these issues, and pretend that happens-befors is defined as our simply-happens-before relation, which is clearly transitive. In the presence of memory_order_consume, this problem is unavoidable, since consume can order two accesses without also ordering the first with respect to an access that immediately follows the second; happens-before cannot compose with sequenced-before, and thus happens-before cannot be transitive.

在不使用 memory_order_consume 的情况下，happens-before 可以视为可传递，否则为不可传递。C++17 提出了 strongly happens before，排除了 memory_order_consume，是可传递的，可以和 sequenced-before 进行组合。