[Bug c++/82758] New: Allocators declared `final` are not acceptible by containers

["wmx16835 at 126 dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82758

            Bug ID: 82758
           Summary: Allocators declared `final` are not acceptible by
                    containers
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wmx16835 at 126 dot com
  Target Milestone: ---

I found that any allocator type declared `final` are not acceptable by the
implementation of the containers that require allocators, including `vector`,
`deque`, `list`, `forward_list`, `set`, `map`, `multiset`, `multimap`,
`unordered_set`, `unordered_map`, `unordered_multiset`, `unordered_multimap`.

For example, providing there is a user-defined allocator type:

template <class T>
class MyAllocator final : public std::allocator<T> {
 public:
  template <class U>
  struct rebind { using other = MyAllocator<U>; };
};

Each line of the following expressions shall be well-formed. However, they do
not compile.

std::vector<int, MyAllocator<int>> v{};
std::deque<int, MyAllocator<int>> d{};
std::list<int, MyAllocator<int>> l{};
std::forward_list<int, MyAllocator<int>> f_l{};
std::set<int, std::less<int>, MyAllocator<int>> s{};
std::map<int, int, std::less<int>, MyAllocator<std::pair<int, int>>> mp{};
std::multiset<int, std::less<int>, MyAllocator<int>> m_s{};
std::multimap<int, int, std::less<int>, MyAllocator<std::pair<int, int>>>
m_mp{};
std::unordered_set<int, std::hash<int>, std::equal_to<int>, MyAllocator<int>>
u_s{};
std::unordered_map<int, int, std::hash<int>, std::equal_to<int>,
MyAllocator<std::pair<const int, int>>> u_mp{};
std::unordered_multiset<int, std::hash<int>, std::equal_to<int>,
MyAllocator<int>> u_m_s {};
std::unordered_multimap<int, int, std::hash<int>, std::equal_to<int>,
MyAllocator<std::pair<const int, int>>> u_m_mp{};