The func can be implemented like this:

    float func(int *i, float *f) {
      *i = 42;
      return *f;
    }

Since the compiler assumes that the pointers i and f do not alias, it has a right to generate the code that is equivalent to:

    float func(int *i, float *f) {
      float result = *f;
      *i = 42;
      return result;
    }

But this will change the result of the function. So in practice if you want to do type punning using unions, the compiler must know about it.