26.5.9 Directives Within Macro Arguments

GNU C permits and handles preprocessing directives in the text provided as arguments for a macro. That case is undefined in the C standard. but in GNU C conditional directives in macro arguments are clear and valid.

A paradoxical case is to redefine a macro within the call to that same macro. What happens is, the new definition takes effect in time for pre-expansion of all the arguments, then the original definition is expanded to replace the call. Here is a pathological example:

#define f(x) x x
f (first f second
#undef f
#define f 2
f)

which expands to

first 2 second 2 first 2 second 2

with the semantics described above. We suggest you avoid writing code which does this sort of thing.