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 a() x
a()
a ()
#define b(p) p
x/**/b(1)/**/x
x/**/b (1)/**/x
x/**/b()/**/x
#define c(p,q) p/**/q
x/**/c(1,2)/**/x
x/**/c(1)/**/x
x/**/c()/**/x