clang++ error

In file included from test.cpp:2:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/memory:82:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr.h:52:
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr_base.h:1106:50: error:
      cannot cast 'List' to its private base class 'const enable_shared_from_this<Shared>'
          __enable_shared_from_this_helper(_M_refcount, _M_ptr, _M_ptr);
                                                        ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr.h:319:4: note:
      in instantiation of function template specialization 'std::__shared_ptr<List,
      __gnu_cxx::_Lock_policy::_S_atomic>::__shared_ptr<std::allocator<List>>' requested here
        : __shared_ptr<_Tp>(__tag, __a, std::forward<_Args>(__args)...)
          ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr.h:619:14: note:
      in instantiation of function template specialization
      'std::shared_ptr<List>::shared_ptr<std::allocator<List>>' requested here
      return shared_ptr<_Tp>(_Sp_make_shared_tag(), __a,
             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr.h:635:19: note:
      in instantiation of function template specialization 'std::allocate_shared<List,
      std::allocator<List>>' requested here
      return std::allocate_shared<_Tp>(std::allocator<_Tp_nc>(),
                  ^
test.cpp:27:19: note: in instantiation of function template specialization 'std::make_shared<List>'
      requested here
    auto c = std::make_shared<List>();
                  ^
test.cpp:20:14: note: constrained by private inheritance here
class List : private Container, virtual public Widget{
             ^~~~~~~~~~~~~~~~~
In file included from test.cpp:2:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/memory:82:
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr.h:583:27: error:
      cannot cast 'List' to its private base class 'Shared'
        { _M_weak_this._M_assign(__p, __n); }
                                 ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr.h:601:8: note:
      in instantiation of function template specialization
      'std::enable_shared_from_this<Shared>::_M_weak_assign<List>' requested here
        __pe->_M_weak_assign(const_cast<_Tp2*>(__px), __pn);
              ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr_base.h:1106:4: note:
      in instantiation of function template specialization
      'std::__enable_shared_from_this_helper<Shared, List>' requested here
          __enable_shared_from_this_helper(_M_refcount, _M_ptr, _M_ptr);
          ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr.h:319:4: note:
      in instantiation of function template specialization 'std::__shared_ptr<List,
      __gnu_cxx::_Lock_policy::_S_atomic>::__shared_ptr<std::allocator<List>>' requested here
        : __shared_ptr<_Tp>(__tag, __a, std::forward<_Args>(__args)...)
          ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr.h:619:14: note:
      in instantiation of function template specialization
      'std::shared_ptr<List>::shared_ptr<std::allocator<List>>' requested here
      return shared_ptr<_Tp>(_Sp_make_shared_tag(), __a,
             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/shared_ptr.h:635:19: note:
      in instantiation of function template specialization 'std::allocate_shared<List,
      std::allocator<List>>' requested here
      return std::allocate_shared<_Tp>(std::allocator<_Tp_nc>(),
                  ^
test.cpp:27:19: note: in instantiation of function template specialization 'std::make_shared<List>'
      requested here
    auto c = std::make_shared<List>();
                  ^
test.cpp:20:14: note: constrained by private inheritance here
class List : private Container, virtual public Widget{
             ^~~~~~~~~~~~~~~~~
2 errors generated.