This is actually extracted from my module (Pre-processor in C)

The conditional expression could contain any C operator except for the assignment operators,increment, and decrement operators.

I am not sure if I am getting this statement or not since I tried using this and it worked.Also for other manipulation a probable work around would be to simply declare macro or function inside the conditional expression,something like this to be precise.

Also I don't understand what is the rationale behind this rule. Could somebody explain?

Thanks