3187. P0591R4 reverted DR 2586 fixes to scoped_allocator_adaptor::construct()

Section: 20.2.8.2 [allocator.uses.construction] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-02-14 Last modified: 2021-02-25

Priority: Not Prioritized

View all other issues in [allocator.uses.construction].

View all issues with C++20 status.

Discussion:

2586 fixed the value category in the uses-allocator checks done by scoped_allocator_adaptor. When we made that use uses_allocator_construction_args we reintroduced the problem, because that function has the same bug.

#include <memory>

struct X {
  using allocator_type = std::allocator<X>;
  X(std::allocator_arg_t, allocator_type&&) { }
  X(const allocator_type&) { }
};

int main() {
  std::allocator<X> a;
  std::make_obj_using_allocator<X>(a);
}

This will fail to compile, because uses_allocator_construction_args will check is_constructible using an rvalue allocator, but then return tuple<allocator_arg_t, const allocator<X>&> containing an lvalue allocator. Those args cannot be used to construct an X.

[2019-02; Kona Wednesday night issue processing]

Status to Ready

Proposed resolution:

This wording is relative to N4800.

1. Change 20.2.8.2 [allocator.uses.construction] as indicated:

   [Drafting Note: Arguably the uses_allocator specialization should also use const Alloc& but in practice that doesn't matter, except for even more contrived cases than the very contrived example above.]

   template <class T, class Alloc, class... Args>
     auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) -> see below;
   

   […]

   -5- Returns: A tuple value determined as follows:

   1. (5.1) — If uses_allocator_v<T, Alloc> is false and is_constructible_v<T, Args...> is true, return forward_as_tuple(std::forward<Args>(args)...).

   2. (5.2) — Otherwise, if uses_allocator_v<T, Alloc> is true and is_constructible_v<T, allocator_arg_t, const Alloc&, Args...> is true, return

      tuple<allocator_arg_t, const Alloc&, Args&&...>(
        allocator_arg, alloc, std::forward<Args>(args)...)
      

   3. (5.3) — Otherwise, if uses_allocator_v<T, Alloc> is true and is_constructible_v<T, Args..., const Alloc&> is true, return forward_as_tuple(std::forward<Args>(args)..., alloc).

   4. (5.4) — Otherwise, the program is ill-formed.

   […]