为什么不用这个std :: list移动​​语义？

Question

以下代码无法编译，但我不确定为什么。我原本以为编译器会在任何地方都使用移动构造函数，因为一切都是右值，但它似乎试着复制构造函数。

#include <memory>
#include <list>

using ptr_t = std::unique_ptr< int >;
using list_t = std::list< ptr_t >;

list_t foo(void) {
    return list_t {
        ptr_t( new int( 42 ) )
    };
}

int main( void ) {
    list_t list = foo( );
    return * ( list.front( ) );
}

错误：

In file included from x.cc:2:
In file included from /usr/lib/gcc/i686-pc-linux-gnu/4.8.0/../../../../include/c++/4.8.0/list:62:
/usr/lib/gcc/i686-pc-linux-gnu/4.8.0/../../../../include/c++/4.8.0/bits/stl_list.h:114:33: error: call to deleted constructor of 'std::unique_ptr<int, std::default_delete<int> >'
        : __detail::_List_node_base(), _M_data(std::forward<_Args>(__args)...)
                                       ^       ~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/i686-pc-linux-gnu/4.8.0/../../../../include/c++/4.8.0/ext/new_allocator.h:120:23: note: in instantiation of function template specialization 'std::_List_node<std::unique_ptr<int,
      std::default_delete<int> > >::_List_node<const std::unique_ptr<int, std::default_delete<int> > &>' requested here
        { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
                             ^
/usr/lib/gcc/i686-pc-linux-gnu/4.8.0/../../../../include/c++/4.8.0/bits/stl_list.h:505:32: note: in instantiation of function template specialization
      '__gnu_cxx::new_allocator<std::_List_node<std::unique_ptr<int, std::default_delete<int> > > >::construct<std::_List_node<std::unique_ptr<int, std::default_delete<int> > >, const std::unique_ptr<int,
      std::default_delete<int> > &>' requested here
              _M_get_Node_allocator().construct(__p,
                                      ^
/usr/lib/gcc/i686-pc-linux-gnu/4.8.0/../../../../include/c++/4.8.0/bits/stl_list.h:1561:18: note: in instantiation of function template specialization 'std::list<std::unique_ptr<int, std::default_delete<int> >,
      std::allocator<std::unique_ptr<int, std::default_delete<int> > > >::_M_create_node<const std::unique_ptr<int, std::default_delete<int> > &>' requested here
         _Node* __tmp = _M_create_node(std::forward<_Args>(__args)...);
                    ^
/usr/lib/gcc/i686-pc-linux-gnu/4.8.0/../../../../include/c++/4.8.0/bits/stl_list.h:1026:17: note: in instantiation of function template specialization 'std::list<std::unique_ptr<int, std::default_delete<int> >,
      std::allocator<std::unique_ptr<int, std::default_delete<int> > > >::_M_insert<const std::unique_ptr<int, std::default_delete<int> > &>' requested here
        { this->_M_insert(end(), std::forward<_Args>(__args)...); }
                ^
/usr/lib/gcc/i686-pc-linux-gnu/4.8.0/../../../../include/c++/4.8.0/bits/stl_list.h:1491:6: note: in instantiation of function template specialization 'std::list<std::unique_ptr<int, std::default_delete<int> >,
      std::allocator<std::unique_ptr<int, std::default_delete<int> > > >::emplace_back<const std::unique_ptr<int, std::default_delete<int> > &>' requested here
            emplace_back(*__first);
            ^
/usr/lib/gcc/i686-pc-linux-gnu/4.8.0/../../../../include/c++/4.8.0/bits/stl_list.h:608:9: note: in instantiation of function template specialization 'std::list<std::unique_ptr<int, std::default_delete<int> >,
      std::allocator<std::unique_ptr<int, std::default_delete<int> > > >::_M_initialize_dispatch<const std::unique_ptr<int, std::default_delete<int> > *>' requested here
      { _M_initialize_dispatch(__l.begin(), __l.end(), __false_type()); }
        ^
x.cc:8:12: note: in instantiation of member function 'std::list<std::unique_ptr<int, std::default_delete<int> >, std::allocator<std::unique_ptr<int, std::default_delete<int> > > >::list' requested here
    return list_t {
           ^
/usr/lib/gcc/i686-pc-linux-gnu/4.8.0/../../../../include/c++/4.8.0/bits/unique_ptr.h:273:7: note: function has been explicitly marked deleted here
  unique_ptr(const unique_ptr&) = delete;
      ^
1 error generated.