为什么会出现以下情况?
const int i0 = 5;
//int i1 = const_cast<int>(i0); // compilation error
int i2 = (int)i0; // okay
int i3 = 5;
//const int i4 = const_cast<const int>(i3); // compilation error
const int i5 = (const int)i3; // okay