First, it's not a C++ feature. In C++ you tell the compiler how to lay out objects in memory. Here you declare what properties your class has (e.g. whether it n

First, it's not a C++ feature. In C++ you tell the compiler how to lay out objects in memory. Here you declare what properties your class has (e.g. whether it needs identity or not), and the compiler decides how to lay out each of its instances in memory, and may automatically do it in different ways in different places. So the principle that "you tell us what semantics you want and let the compiler figure out the implementation" remains in effect. Second, the reason why the compiler cannot infer on its own that a class does or does not need identity without you declaring it is that the use of identity can be in a different module.