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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/108619] Compilation error if the construct method of
 the allocator isn't implemented and the detructor of value_type is private
Date: Wed, 10 Jul 2024 22:07:19 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108619

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu=
.org

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This fails for all modes from C++11 to C++20:

#include <memory>
#include <vector>
#include <type_traits>

class a {

  ~a() =3D default;

  friend class b;

};

class b {

  template<typename C>
    struct alloc {
      std::allocator<C> a;

      alloc() { }

      template<typename U> alloc(const alloc<U>&) { }

      using value_type =3D C;

      C* allocate(std::size_t n) { return a.allocate(n); }
      void deallocate(C* p, std::size_t n) { return a.deallocate(p, n); }

      template<typename U>
        void destroy(U* p){
          if constexpr (std::is_array_v<U>){
            for(auto& elem : *p){
              (destroy)(std::addressof(elem));
            }
          } else {
            p->~U();
          }
        }

    };

  std::vector<a,alloc<a>> v;

public:

  void f(){
    v.emplace_back();
  }

};


I have a patch ...=