invalid offsetof from non-POD type

[John Quigley]

The code below triggers an "invalid offsetof from non-POD type" warning:

In this case, and in every case I've seen, the warning is a false alarm and 
the compiler produces the correct result.  Is there a case where the warning 
fires and the compiler produces an incorrect result?  (i.e.: returns the 
wrong offset)?  If there is no such case, is there anything wrong with 
providing a command line flag in gcc that allows the user to disable this 
warning?

Thanks,
- jq

#include <cstddef>
#include <iostream>
using namespace std;

class Foo
{
   public:
      int x;
      char fillerdata[256];
      int y;

      Foo()
      {
         x = 0;
         y = 0;
      }
};

int main()
{
   int yoffset = offsetof(Foo, y); // triggers warning
   cout << "yoffset: " << yoffset << endl;

   Foo foo;
   
   // get a pointer to y using the computed offset
   int* yptr = (int*)(((unsigned char*)&foo) + yoffset);
   // test it be assigning 
   *yptr = 15;
   cout << "foo values: " << foo.x << ", " << foo.y << endl;

   return 0;
}