[Bug libstdc++/122289] New: std::tuple allocator-extended constructors should use std::make_obj_using_allocator for each element

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122289

            Bug ID: 122289
           Summary: std::tuple allocator-extended constructors should use
                    std::make_obj_using_allocator for each element
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

See the example in https://cplusplus.github.io/LWG/issue4313

I agree with Pablo's comment there: tuple's allocator-extended constructors
should be using std::make_obj_using_allocator, so that uses-allocator
construction works properly for std::pair elements in the std::tuple.

#include <tuple>
#include <utility>
#include <testsuite_hooks.h>
#include <testsuite_allocator.h>

template<typename T>
using Alloc = __gnu_test::uneq_allocator<T>;

struct X
{
  using allocator_type = Alloc<int>;

  X(int i) : alloc(), i(i) { }

  X(std::allocator_arg_t, const allocator_type& alloc, int i)
  : alloc(alloc), i(i)
  { }

  allocator_type alloc;
  int i;
};

using Pair = std::pair<X, X>;
using Tuple = std::tuple<Pair>;

int main()
{
  Alloc<int> a(99);

  Pair p = std::make_obj_using_allocator<Pair>(a, 1, 2);
  VERIFY(p.first.alloc == a);
  VERIFY(p.second.alloc == a);

  Tuple t(std::allocator_arg, a, {1, 2});
  VERIFY(std::get<0>(t).first.alloc == a);
  VERIFY(std::get<0>(t).second.alloc == a);
}

This fails:
4313.cc:35: int main(): Assertion 'std::get<0>(t).first.alloc == a' failed.

But if we do uses-allocator construction for each element then we should
construct each element of the pair with the allocator, as happens for the Pair
p.