I now think it's actually this part of the standard that prevents it: http://eel.is/c++draft/intro.multithread#intro.progress-7 http://eel.is/c++draft/intro.mul

I now think it's actually this part of the standard that prevents it: http://eel.is/c++draft/intro.multithread#intro.progress-7 http://eel.is/c++draft/intro.multithread#intro.progress-7 Basically a compiler can not optimize a non-deadlocking program into a potentially deadlocking one.