Yes, a program can have a second-party dependency in the form of invoking some implementation-defined behaviors or undefined behaviors that de facto work, not j

Yes, a program can have a second-party dependency in the form of invoking some implementation-defined behaviors or undefined behaviors that de facto work, not just a dependence on the simple existence of something without which the program doesn't build. We can't infer a dependence of that kind from seeing standard header includes. That does not raise the suspicion above background levels.