The four calls should work, but #1 doesn't work.

struct S
{
    int v;
    alias v this;
}
void foo(ref int n){}

void main()
{
    auto s = S(1);
    // variable s is lvalue, so
    // alias this expanded expression s.v is also lvalue and matches to ref.

    foo(s);         // 1.NG -> OK
    s.foo();        // 2.OK, ufcs
    foo(s.v);       // 3.OK
    s.v.foo();      // 4.OK, ufcs
}