std::list

The code

#include <list>
#include <memory>

class B;
class A {
    std::list<std::unique_ptr<B>> bs;
public:
    A();
    ~A();
};

int main()
{
    A x;
    return 0;
}

obviously compiles. It doesn't link because A::A() and A::~A() are missing, but that is expected and alright. Changing

std::list<std::unique_ptr<B>> bs;

which is supposed to call the std::list's standard constructor

list() : list(Allocator()) {}

(C++14 and up) to

std::list<std::unique_ptr<B>> bs{};

which is supposed to call

list(std::initializer_list<T>, const Allocator & = Allocator());

i.e. calling the initializer_list constructor with an empty initializer_list, gives the following error with c++ (Ubuntu 5.2.1-22ubuntu2) 5.2.1 20151010 and the --std=c++17 parameter:

/usr/include/c++/5/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = B]’:
/usr/include/c++/5/bits/unique_ptr.h:236:17:   required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = B; _Dp = std::default_delete<B>]’
/usr/include/c++/5/bits/stl_list.h:106:12:   required from ‘void __gnu_cxx::new_allocator<_Tp>::destroy(_Up*) [with _Up = std::_List_node<std::unique_ptr<B> >; _Tp = std::_List_node<std::unique_ptr<B> >]’
/usr/include/c++/5/bits/list.tcc:75:4:   required from ‘void std::__cxx11::_List_base<_Tp, _Alloc>::_M_clear() [with _Tp = std::unique_ptr<B>; _Alloc = std::allocator<std::unique_ptr<B> >]’
/usr/include/c++/5/bits/stl_list.h:446:17:   required from ‘std::__cxx11::_List_base<_Tp, _Alloc>::~_List_base() [with _Tp = std::unique_ptr<B>; _Alloc = std::allocator<std::unique_ptr<B> >]’
/usr/include/c++/5/bits/stl_list.h:507:11:   required from here
/usr/include/c++/5/bits/unique_ptr.h:74:22: error: invalid application of ‘sizeof’ to incomplete type ‘B’
  static_assert(sizeof(_Tp)>0,
                      ^

Which barks about the type of B being incomplete. My question is:

Why does the initializer_list constructor need the complete type of B for an empty initializer list?