operator= required for std::vector<>::push_back ??

If I have the following test file, x.cc:

    #include <vector>

    class X
    {
    public:
      X (const int &_i) : i (_i) { }
      X (const X &x) : i (x.i) { }
    #if 0
      X &operator= (const X &x)
      {
        if (&x != this)
          new (this) X (x.i);
        return *this;
      }
    #endif
      const int &i;
    };

    int test ()
    {
      std::vector<X> xv;

      int i;
      xv.push_back (X (i));
    }

I get an error during compilation, like:

    g++ -c -o x.o x.cc
    x.cc: In member function ???X& X::operator=(const X&)???:
    x.cc:4: instantiated from ???void std::vector<_Tp, _Alloc>::_M_insert_aux(__gnu_cxx::__normal_iterator<typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer, std::vector<_Tp, _Alloc> >, const _Tp&) [with _Tp = X, _Alloc = std::allocator<X>]???
    /usr/include/c++/4.4/bits/stl_vector.h:741: instantiated from ???void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = X, _Alloc = std::allocator<X>]???
    x.cc:23: instantiated from here
    x.cc:4: error: non-static reference member ???const int& X::i???, can't use default assignment operator

Changing the "#if 0" to "#if 1" to define operator= makes things work.

I understand the immediate reason for the error: the g++ vector
implementation does indeed seem to call operator= on vector elements for
push_back, and in this case, the compiler-synthesized method isn't good
enough.

But it somehow seems wrong that it's using operator= at all; shouldn't a
copy-constructor be enough?

Looking at the g++ STL headers, it seems like the problem is that it's
defining push_back in terms of insertion, and for the latter, you need
operator= to move elements subsequent elements to make room for a new
element. However in the case of push_back, that's really unnecessary,
and no copying of objects other than copy-constructing into new memory
when the vector's underlying storage is reallocated should be need.

Opinions? Is the error here reasonable?

Thanks,

-Miles

--
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