i = 4, i++ + i should return 9, but returns 8

My C/C++ professor mentioned in class that many compilers cannot properly evaluate the expression i++ + i and some permutations of it, so I decided to test that out in Visual Studio with a small program:

#include

int main ( void )
{

*** int i = 4;

*** printf("%i", i++ + i);

*** return 0;

}

It returns 8. It should return 9. Why does it return 8?