Is `C == C++` undefined behaviour?

Question

A friend tells me that after:

int C = anything;

C == C++ will have the value true. This is intended as a joke, a

Answer 1

You are correct. Even the compiler says so: compiling

#include 

int main()
{
  int C = 0;
  if (C == C++) {
  ...

results in:

main.cpp: In function 'int main()':
main.cpp:6:17: warning: operation on 'C' may be undefined [-Wsequence-point]
     if (C == C++) {
                 ^

(Coliru)