Well, the `traits` / `caller_traits` are default just like `static bool const is_pass_through_unary = false;`. The logic is: if the test of `traits<Subject>::xxx` fails, it fallbacks to the default mechanism as defined in the traits/ directory, and if the test of `caller_traits<Subject, Ts...>::xxx` fails, it fallbacks to `traits<Subject>::xxx`, and so on...