Error when using std::make_move_iterator to insert a std::list<std::unique_ptr>> into another one - c++11

Question

I've been reading this answer about moving elements from a std::vector<std::unique_ptr<T>> to another one using std::make_move_iterator. It works flawlessly:
std::vector<std::unique_ptr<int>> c1;
std::vector<std::unique_ptr<int>> c2;
c1.push_back(std::unique_ptr<int>(new int));
c2.insert(c2.end(), std::make_move_iterator(c1.begin()), std::make_move_iterator(c1.end()));
Now I tried to change it to use a different container (std::list):
std::list<std::unique_ptr<int>> c1;
std::list<std::unique_ptr<int>> c2;
c1.push_back(std::unique_ptr<int>(new int));
c2.insert(c2.end(), std::make_move_iterator(c1.begin()), std::make_move_iterator(c1.end()));
But compilation fails with:
error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'
However, it does work if lists contain other objects (like std::string):
std::list<std::string> s1;
std::list<std::string> s2;
s2.insert(s2.end(), std::make_move_iterator(s1.begin()), std::make_move_iterator(s1.end()));
Now, if I use another answer from the same question, which uses std::back_inserter and std::move, it works with both std::vector and std::list:
std::list<std::unique_ptr<int>> c1;
std::list<std::unique_ptr<int>> c2;
c1.push_back(std::unique_ptr<int>(new int));
std::move(c1.begin(), c1.end(), std::back_inserter(c2));
As far as I understand, this solution basically moves each item individually when back inserting on the second container.
My question is, why using std::make_move_iterator on a std::list of std::unique_ptr doesn't work but it does with std::vector or if the list element is of a different type?
Note: I'm using Visual Studio 2010.

Answer 1

The core problem is that Visual Studio 2010's implementation of std::list::insert doesn't support moving semantics. Internally, the std::list::insert invokes a method called _Insert, which is declared as follows in VS 2010:
void _Insert(const_iterator _Where, const _Ty& _Val)
In VS 2017 (not checked for other versions), it calls the same method but it is declared as:
void _Insert(_Unchecked_const_iterator _Where, _Valty&&... _Val)
As seen, VS 2017 uses an rvalue reference, therefore calling the moving constructor of std::unique_ptr when making the insertion. On the other hand, VS 2010 uses an lvalue reference instead, so at some moment the copy constructor is called, which is declared private for std::unique_ptr, generating the compilation error.
Conclusion
In VS 2010 there is no way to use the solution based on std::make_move_iterator because how std::list::insert is implemented. Options for appending a std::list<std::unique_ptr<T>> into another one include:
Using the back_inserter:
std::move(c1.begin(), c1.end(), std::back_inserter(c2));
Using std::list::splice (credit to #T.C.), which is very useful since it can be directly used with lists returned by functions:
c2.splice(c2.end(), c1);