Don't shortcut macro expansion of non-parameter macros. It won't work once we have more kinds of expansion items than strings and arguments, viz., magic tokens for __FILE__ and __LINE__ and so on.

#define C(x) //**/* x */**//
C(3)
C(abc /* def */ ghi)

#define D(x) ///**/**/**///**/* x */**///**/**/**///
D(3)
D(abc /* def */ ghi)