Assignment between union members

Is this code well-defined?

int main()    
{    
    union     
    {    
        int i;    
        float f;    
    } u;    

    u.f = 5.0;    
    u.i = u.f;       // ?????
}    

It accesses two different union members in one expression so I am wondering if it falls foul of the [class.union]/1 provisions about active member of a union.

The C++ Standard seems to underspecify which operations change the active member for builtin types, and what happens if an inactive member is read or written.

via Chebli Mohamed