I was trying to figure out what this header actually does, as it doesn't wrap anything around std namespace, or any new typedefs etc...

GCC implements it like this:

Code:
#ifndef _GLIBCXX_CSTDDEF
#define _GLIBCXX_CSTDDEF 1

#pragma GCC system_header

#include <stddef.h>

namespace std
{
  using ::ptrdiff_t;
  using ::size_t;
}

#endif
What exactly is the point of this?

I suppose it is for more than just the joy of writing a fancier std::size_t instead of the boring size_t, right?

The only reason I could maybe imagine is for some sort of Koenig resolution, if an ill advised user defined his own size_t in his own namespace? But even in this case, I don't think this would be necessary, as stl function would still resolve with the global size_t.

I may be looking too hard into this, but I also think the comity did it for the lulz. And I'm not one to give up on a C++ mystery.