C++ attribute: fallthrough (since C++17)

void f(int n)
{
    void g(), h(), i();
 
    switch (n)
    {
        case 1:
        case 2:
            g();
            [[fallthrough]];
        case 3: // no warning on fallthrough
            h();
        case 4: // compiler may warn on fallthrough
            if (n < 3)
            {
                i();
                [[fallthrough]]; // OK
            }
            else
            {
                return;
            }
        case 5:
            while (false)
            {
                [[fallthrough]]; // ill-formed: next statement is not
                                 //             part of the same iteration
            }
        case 6:
            [[fallthrough]]; // ill-formed, no subsequent case or default label
    }
}