Write if the following code fragment doesn't compile or write  if the code compiles but has undefined behavior when executed. Otherwise, write the exact text written to the standard output stream.

void mystery(int const &x, int const& y) {
  int temp {x};
  x = y;
  y = temp;
}
// suppose the following statements comprise function main ...
int a {10}, b {20};
mystery(a, b);
std::cout << a << ',' << b;