提供(空的)用户定义析构函数会导致编译错误

代码在没有用户定义析构函数的情况下(在GCC 4.7.2上)编译得很好，即使提供了空的用户定义析构函数也会产生错误:

#include <memory>
class Test
{
   std::unique_ptr<int> val;
};
template <typename Type>
class B
{
public:
   //destructor:
   // if I comment this out, the code compiles just fine:
   ~B() { }
private:
   Test a;
};
int main()
{
   auto s = B<int>();
}

析构函数未被注释掉时所产生的错误的重点是:

• Test的复制构造函数不存在，并且没有隐式创建，因为它将是错误的
• 有人试图使用已删除的unique_ptr复制构造函数。

完整的错误输出，对于感兴趣的人来说，在这篇文章的底部。

我知道unique_ptr不能被复制构造(除非参数是右值)，因此编译器不可能为类Test生成有效的隐式复制构造函数。

我不明白的是为什么定义一个空的析构函数突然需要这些复制工具。显然，当使用unique_ptr s之类的东西时，这是不可能提供的。

析构函数未被注释掉时的完整错误输出:

In file included from /usr/include/c++/4.7/list:64:0,
                 from ../../Dropbox/Programming/C++/test/main.cpp:2:
/usr/include/c++/4.7/bits/stl_list.h: In instantiation of 'std::_List_node<_Tp>::_List_node(_Args&& ...) [with _Args = {const Test&}; _Tp = Test]':
/usr/include/c++/4.7/ext/new_allocator.h:110:4:   required from 'void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::_List_node<Test>; _Args = {const Test&}; _Tp = std::_List_node<Test>]'
/usr/include/c++/4.7/bits/stl_list.h:503:8:   required from 'std::list<_Tp, _Alloc>::_Node* std::list<_Tp, _Alloc>::_M_create_node(_Args&& ...) [with _Args = {const Test&}; _Tp = Test; _Alloc = std::allocator<Test>; std::list<_Tp, _Alloc>::_Node = std::_List_node<Test>]'
/usr/include/c++/4.7/bits/stl_list.h:1533:63:   required from 'void std::list<_Tp, _Alloc>::_M_insert(std::list<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const Test&}; _Tp = Test; _Alloc = std::allocator<Test>; std::list<_Tp, _Alloc>::iterator = std::_List_iterator<Test>]'
/usr/include/c++/4.7/bits/stl_list.h:997:9:   required from 'void std::list<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = Test; _Alloc = std::allocator<Test>; std::list<_Tp, _Alloc>::value_type = Test]'
/usr/include/c++/4.7/bits/stl_list.h:1466:6:   required from 'void std::list<_Tp, _Alloc>::_M_initialize_dispatch(_InputIterator, _InputIterator, std::__false_type) [with _InputIterator = std::_List_const_iterator<Test>; _Tp = Test; _Alloc = std::allocator<Test>]'
/usr/include/c++/4.7/bits/stl_list.h:582:9:   required from 'std::list<_Tp, _Alloc>::list(const std::list<_Tp, _Alloc>&) [with _Tp = Test; _Alloc = std::allocator<Test>; std::list<_Tp, _Alloc> = std::list<Test>]'
../../Dropbox/Programming/C++/test/main.cpp:11:7:   required from here
/usr/include/c++/4.7/bits/stl_list.h:115:71: error: use of deleted function 'Test::Test(const Test&)'
../../Dropbox/Programming/C++/test/main.cpp:5:7: note: 'Test::Test(const Test&)' is implicitly deleted because the default definition would be ill-formed:
../../Dropbox/Programming/C++/test/main.cpp:5:7: error: use of deleted function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>; std::unique_ptr<_Tp, _Dp> = std::unique_ptr<int>]'
In file included from /usr/include/c++/4.7/memory:86:0,
                 from ../../Dropbox/Programming/C++/test/main.cpp:3:
/usr/include/c++/4.7/bits/unique_ptr.h:262:7: error: declared here