C メンバー関数宣言における 1 つのアンパサンド (&) は何を意味しますか?

class wrap {
   operator obj() const & { ... }   //Copy from me.
   operator obj() && { ... }  //Move from me.
};

struct foo
{
   void bar() {}
   void bar1() & {}
   void bar2() && {}
};

int main()
{
   foo().bar();  // (always fine)
   foo().bar1(); // doesn't compile because `bar1()` requires an lvalue
   foo().bar2(); // doesn't compile because `bar2()` requires an rvalue

   foo f;
   f.bar();      // (always fine)
   f.bar1();
   f.bar2();     // doesn't compile because `bar2()` requires an rvalue
}