You can't infer shit, because of the way current compiler writers are interpreting the standard today. At one point in the 90's it was obvious for the entire planet including compiler authors that some undefined behaviors did not apply for a given target architecture, so obviously obvious that nobody would even require to have it specified in the compiler doc (still nice to have, but you would not be too much angry if it does not appear)
Now they just add their "optimizations" at the highest levels so of course even for targets where it should makes no sense, and without asking for your permission, and even by default, and even some they consider "aggressive". So either you have provisions to avoid all that shit, like having a guy disabling all the new ones each time you upgrade you compiler, and you better have some defenses in depth, and I agree with you that using TU as boundaries is also a good idea, if your compiler+build-sys have an option to NOT do WPO.
But it is just not in any way guaranteed by the ISO standard, and still just an implementation detail from its pov. And honestly that's a problem. Because compiler writers will takes more drugs and come up with new imaginative way to break your code more, in the name of their "strictly-conformance and nothing more" crazy ideal.
Now they just add their "optimizations" at the highest levels so of course even for targets where it should makes no sense, and without asking for your permission, and even by default, and even some they consider "aggressive". So either you have provisions to avoid all that shit, like having a guy disabling all the new ones each time you upgrade you compiler, and you better have some defenses in depth, and I agree with you that using TU as boundaries is also a good idea, if your compiler+build-sys have an option to NOT do WPO.
But it is just not in any way guaranteed by the ISO standard, and still just an implementation detail from its pov. And honestly that's a problem. Because compiler writers will takes more drugs and come up with new imaginative way to break your code more, in the name of their "strictly-conformance and nothing more" crazy ideal.