Reported by Jim Nelson <jim@yorba.org> on the Vala mailing list, see the message at http://article.gmane.org/gmane.comp.programming.vala/4245

I have confirmed it.  The following simple program demonstrates the problem:

namespace Test {
  public static int main(string[] args) {
    int[] a = new int[3];
    int current = 0;

    a[current] = 0;
    (a[current])++;

    return(a[current]);
  }
}

The result is:

mbt@zest:~/test/vala$ valac test_array.vala 
test_array.vala:7.5-7.18: error: unsupported lvalue in postfix expression
    (a[current])++;
    ^^^^^^^^^^^^^^
Compilation failed: 1 error(s), 0 warning(s)

This problem is found in Vala 0.7.7.  Likewise, it seems the prefix "++" does not work either.