I found a problem, that is, the file main.mm is not included in the xcode project->build phases->compile source for some reason, but when I add main.mm, it works fine. What's the reason for this, and why is the debug normal.

yes it's an iOS app. I don't know where usage is printed, I searched the project and there is no string associated with it. I've pinpointed build config as the cause, and there's a preprocessor macro that removes it and it works fine, but this macro was always present in previous versions. It feels very strange.