std::flat_map allocator-extended copy/move constructor

[duanqn]

Hi,
I am trying to compile the following code and it fails

{
    using flat_map_t = std::flat_map<int, int, std::less<void>, std::vector<int, std::allocator<int>>, std::vector<int, std::allocator<int>>>;
    flat_map_t map0;
    flat_map_t map1(map0);
    auto allocator = std::allocator<int>();
    flat_map_t map2(map0, allocator);
    flat_map_t map3(std::move(map0));
    flat_map_t map4(std::move(map1), allocator);
}

msvc compiler version: 19.41.33923
STL: 73f9694

Error message: C2448 'std::flat_map<int,int,std::less,std::vector<int,std::allocator>,std::vector<int,std::allocator>>::_Key_compare': cannot access private member declared in class 'std::flat_map<int,int,std::less,std::vector<int,std::allocator>,std::vector<int,std::allocator>>'

Apparently, the compiler is complaining about this part

    template <_Valid_allocator_for_flat_map<key_container_type, mapped_container_type> _Allocator>
    _Flat_map_base(const _Derived& _Other, const _Allocator& _Alloc)
        : _Key_compare(_Other._Key_compare),
          _Data{.keys = _STD make_obj_using_allocator<key_container_type>(_Alloc, _Other._Data.keys),
              .values = _STD make_obj_using_allocator<mapped_container_type>(_Alloc, _Other._Data.values)} {}

used here

flat_map_t map2(map0, allocator);

According to the compiler, we are not allowed to access _Derived::Key_compare and _Derived::Data from _Flat_map_base because they are declared with using _MyBase::Data under the private section in flat_map.
However, there is no compilation error for the regular copy constructor:

_Flat_map_base(const _Derived& _Other) : _Key_compare(_Other._Key_compare), _Data(_Other._Data) {}

Is this a compiler bug, or an implementation bug of <flat_map>? How should we solve it?

[duanqn]

@frederick-vs-ja @AlexGuteniev FYI

[frederick-vs-ja]

I think this is a bug. _Flat_map_base doesn't have access to the members in derived classes (where the access control is changed to private).

I'll fix this by static_cast-ing the parameters to (const) _Flat_map_base& first.