Revised 2025-10-17 at 14:57:41 UTC

Tentative Issues

3627⁽ⁱ⁾. Inconsistent specifications for std::make_optional overloads

Section: 22.5.10 [optional.specalg] Status: Tentatively Ready Submitter: Jiang An Opened: 2021-10-23 Last modified: 2025-10-16

Priority: 3

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Discussion:

Three std::make_optional overloads are specified in 22.5.10 [optional.specalg]. The first one is specified by "Returns:" and the other two are specified by "Effects: Equivalent to:". According to 16.3.2.4 [structure.specifications]/4, such uses of "Effects: Equivalent to:" propagate the Constraints specified for constructors. As the selected constructor for the first overload has "Constraints:" (22.5.3.2 [optional.ctor]/22), it seems that inconsistency is introduced here.

Existing implementations are inconsistent: libstdc++ constrains all three overloads, while libc++ and MSVC STL do not constrain any of them.

IMO all three overloads should be constrained.

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll.

[2025-10-16; Status changed: New → Tentatively Ready.]

Reflector poll in 2024-07 with eight supporting votes.

Proposed resolution:

This wording is relative to N4901.

1. Modify 22.5.10 [optional.specalg] as indicated:

   template<class T> constexpr optional<decay_t<T>> make_optional(T&& v);
   

   -3- ReturnsEffects: Equivalent to: return optional<decay_t<T>>(std::forward<T>(v));.

3908⁽ⁱ⁾. enumerate_view::iterator constructor is explicit

Section: 25.7.24.3 [range.enumerate.iterator] Status: Tentatively NAD Submitter: Jonathan Wakely Opened: 2023-03-23 Last modified: 2024-06-24

Priority: Not Prioritized

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Discussion:

enumerate_view::iterator has this constructor:

    constexpr explicit
      iterator(iterator_t<Base> current, difference_type pos);  // exposition only

In P2164R9 the detailed description of the function showed a default argument for the second parameter, which would justify it being explicit. However, that default argument was not present in the class synopsis and was removed from the detailed description when applying the paper to the draft.

[2023-06-01; Reflector poll]

Set status to Tentatively NAD after four votes in favour during reflector poll. The constructor is exposition-only, it doesn't make any difference to anything whether it's explicit or not.

Proposed resolution:

This wording is relative to N4944.

1. Modify the class synopsis in 25.7.24.3 [range.enumerate.iterator] as shown:

   
       constexpr explicit
         iterator(iterator_t<Base> current, difference_type pos);  // exposition only
   

2. Modify the detailed description in 25.7.24.3 [range.enumerate.iterator] as shown:

     constexpr explicit iterator(iterator_t<Base> current, difference_type pos);
   

   -2- Effects: Initializes current_ with std::move(current) and pos_ with pos.

3909⁽ⁱ⁾. Issues about viewable_range

Section: 99 [ranges.refinements], 25.7.2 [range.adaptor.object] Status: Tentatively NAD Submitter: Jiang An Opened: 2023-03-27 Last modified: 2023-06-01

Priority: Not Prioritized

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Discussion:

After LWG 3724⁽ⁱ⁾, views::all is well-constrained for view types, and the constraints are stronger than viewable_range. The difference is that given an expression such that decltype gives R, when decay_t<R> is a view type and the implicit conversion of R to decay_t<R> is forbidden, views::all rejects the expression, but viewable_range may accept R. So I think we should remove the additional constraints on views::all_t.

While viewable_range is probably not introducing any additional constraint within the standard library, I think it is still useful to express the constraints on views::all, so it should be slightly adjusted to match views::all.

Furthermore, viewable_range is currently used in 25.7.2 [range.adaptor.object], but given P2378R3 relaxed the requirements for range adaptor closure objects, I think we should also apply similar relaxation for range adaptor objects. This should have no impact on standard range adaptor objects.

[2023-06-01; Reflector poll]

Set status to Tentatively NAD after three votes in favour during reflector poll.

"First change is pointless. Second change is a duplicate of 3896⁽ⁱ⁾. Range adaptors return a view over their first argument, so they need to require it's a viewable_range."

Proposed resolution:

This wording is relative to N4944.

1. Change the definition of views::all_t in 25.2 [ranges.syn] as indicated:

   
      template<viewable_rangeclass R>
         using all_t = decltype(all(declval<R>()));          // freestanding
   

2. Change the definition of viewable_range in 25.4.6 [range.refinements] as indicated:

   -6- The viewable_range concept specifies the requirements of a range type that can be converted to a view safely.

   
   template<class T>
     concept viewable_range =
       range<T> &&
       ((view<remove_cvref_t<T>> && constructible_from<remove_cvref_t<T>, T> convertible_to<T, remove_cvref_t<T>>) ||
        (!view<remove_cvref_t<T>> &&
         (is_lvalue_reference_v<T> || (movable<remove_reference_t<T>> && !is-initializer-list<T>))));
   

3. Change 25.7.2 [range.adaptor.object] as indicated:

   -6- A range adaptor object is a customization point object (16.3.3.3.5 [customization.point.object]) that accepts a viewable_rangerange as its first argument and returns a view.

   […]

   -8- If a range adaptor object adaptor accepts more than one argument, then let range be an expression such that decltype((range)) models viewable_rangerange, let args... be arguments such that adaptor(range, args...) is a well-formed expression as specified in the rest of subclause 25.7 [range.adaptors], and let BoundArgs be a pack that denotes decay_t<decltype((args))>.... The expression adaptor(args...) produces a range adaptor closure object f that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties: [...]

3958⁽ⁱ⁾. ranges::to should prioritize the "reserve" branch

Section: 25.5.7.2 [range.utility.conv.to] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2023-07-17 Last modified: 2024-01-29

Priority: Not Prioritized

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Discussion:

When the constructed range object has no range version constructor, ranges::to falls into a branch designed specifically for C++17-compliant containers, which calls the legacy constructor that accepts an iterator pair with C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...).

However, this kind of initialization may bring some performance issues, because we split the original range into pairs of iterators, which may lose information contained in the original range, for example:

#include <boost/container/vector.hpp>
#include <sstream>
#include <ranges>

int main() {
  std::istringstream ints("1 2 3 4 5");
  std::ranges::subrange s(std::istream_iterator<int>(ints),
                          std::istream_iterator<int>(),
                          5);
  auto r = std::ranges::to<boost::container::vector>(s); // discard size info
}

Above, subrange saves the size information of the stream, but ranges::to only extracts its iterator pair to create the object, so that the original size information is discarded, resulting in unnecessary allocations.

I believe we should prefer to use the "reserve" branch here because it is really designed for this situation.

[2023-10-30; Reflector poll]

Set status to Tentatively NAD after reflector poll. "This optimizes exotic cases at the expense of realistic cases."

Proposed resolution:

This wording is relative to N4950.

1. Modify 25.5.7.2 [range.utility.conv.to] as indicated:

   template<class C, input_range R, class... Args> requires (!view<C>)
     constexpr C to(R&& r, Args&&... args);
   

   -1- Mandates: C is a cv-unqualified class type.

   -2- Returns: An object of type C constructed from the elements of r in the following manner:

   1. (2.1) — If C does not satisfy input_range or convertible_to<range_reference_t<R>, range_value_t<C>> is true:

      1. (2.1.1) — If constructible_from<C, R, Args...> is true:

         C(std::forward<R>(r), std::forward<Args>(args)...)

      2. (2.1.2) — Otherwise, if constructible_from<C, from_range_t, R, Args...> is true:

         C(from_range, std::forward<R>(r), std::forward<Args>(args)...)

      3. (2.1.3) — Otherwise, if

         1. (2.1.3.1) — common_range<R> is true,

         2. (2.1.3.2) — the qualified-id iterator_traits<iterator_t<R>>::iterator_category is valid and denotes a type that models derived_from<input_iterator_tag>, and

         3. (2.1.3.3) — constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...> is true:

            C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)

      4. (2.1.4) — Otherwise, if

         1. (2.1.4.1) — constructible_from<C, Args...> is true, and

         2. (2.1.4.2) — container-insertable<C, range_reference_t<R>> is true:

            C c(std::forward<Args>(args)...);
            if constexpr (sized_range<R> && reservable-container<C>)
              c.reserve(static_cast<range_size_t<C>>(ranges::size(r)));
            ranges::copy(r, container-inserter<range_reference_t<R>>(c));
            

      5. (?.?.?) — Otherwise, if

         1. (?.?.?.?) — common_range<R> is true,

         2. (?.?.?.?) — the qualified-id iterator_traits<iterator_t<R>>::iterator_category is valid and denotes a type that models derived_from<input_iterator_tag>, and

         3. (?.?.?.?) — constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...> is true:

            C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)

   2. (2.2) — Otherwise, if input_range<range_reference_t<R>> is true:

      to<C>(r | views::transform([](auto&& elem) {
        return to<range_value_t<C>>(std::forward<decltype(elem)>(elem));
      }), std::forward<Args>(args)...);
      

   3. (2.3) — Otherwise, the program is ill-formed.

3980⁽ⁱ⁾. The read exclusive ownership of an atomic read-modify-write operation and whether its read and write are two operations are unclear

Section: 32.5.4 [atomics.order] Status: Tentatively NAD Submitter: jim x Opened: 2023-08-22 Last modified: 2023-11-03

Priority: Not Prioritized

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Discussion:

Such two questions are sourced from StackOverflow:

1. Can the read operations in compare_exchange_strong in different two thread read the same value?

2. For purposes of ordering, is atomic read-modify-write one operation or two?

Given this example:

#include <iostream>
#include <atomic>
#include <thread>

struct SpinLock{
  std::atomic<bool> atomic_;
  void lock(){
    bool expected = false;
    while (!atomic_.compare_exchange_strong(expected,true,std::memory_order_release,std::memory_order_relaxed)) {
    }
  }
  void unlock(){
    atomic_.store(false, std::memory_order_release);
  }
};

int main(){
  SpinLock spin{false};
  auto t1 = std::thread([&](){
    spin.lock();
    spin.unlock();
  });
  auto t2 = std::thread([&](){
    spin.lock();
    spin.unlock();
  });
  t1.join();
  t2.join();
}

In the current draft, the relevant phrasing that can interpret that only one read-modify-write operation reads the initial value false is 32.5.4 [atomics.order] p10:

Atomic read-modify-write operations shall always read the last value (in the modification order) written before the write associated with the read-modify-write operation.

However, the wording can have two meanings, each kind of read can result in different explanations for the example

1. The check of the violation is done before the side effect of the RMW is in the modification order, i.e. the rule is just checked at the read point.

2. The check of the violation is done after the side effect of the RMW is in the modification order, i.e. the rule is checked when RMW tries to add the side effect that is based on the read-value to the modification order, and that side effect wouldn't be added to the modification order if the rule was violated.

With the first interpretation, the two RMW operations can read the same initial value because that value is indeed the last value in the modification order before such two RMW operations produce the side effect to the modification order.

With the second interpretation, there is only one RMW operation that can read the initial value because the latter one in the modification order would violate the rule if it read the initial value.

Such two interpretations arise from that the wording doesn't clearly specify when that check is performed.

So, my proposed wording is:

Atomic read-modify-write operations shall always read the value from a side effect X, where X immediately precedes the side effect of the read-modify-write operation in the modification order.

This wording keeps a similar utterance to 6.10.2.2 [intro.races], and it can clearly convey the meaning that we say the value read by RWM is associated with the side effect of RMW in the modification order.

Relevant discussion can be seen CWG/issues/423 here.

[2023-11-03; Reflector poll]

NAD. The first reading isn't plausible.

Proposed resolution:

This wording is relative to N4958.

1. Modify 32.5.4 [atomics.order] as indicated:

   -10- Atomic read-modify-write operations shall always read the last value from a side effect X, where X immediately precedes the side effect of the read-modify-write operation (in the modification order) written before the write associated with the read-modify-write operation.

   -11- Implementations should make atomic stores visible to atomic loads within a reasonable amount of time.

3981⁽ⁱ⁾. Range adaptor closure object is underspecified for its return type

Section: 25.7.2 [range.adaptor.object] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2023-08-22 Last modified: 2024-06-24

Priority: Not Prioritized

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Discussion:

In order to provide pipe support for user-defined range adaptors, P2387R3 removed the specification that the adaptor closure object returns a view, which conforms to the wording of ranges::to.

However, the current wording seems to be too low-spec so that the range adaptor closure object can return any type or even void. This makes it possible to break the previous specification when returning types that don't make sense, for example:

#include <ranges>

struct Closure : std::ranges::range_adaptor_closure<Closure> {
  struct NonCopyable {
    NonCopyable(const NonCopyable&) = delete;
  };

  const NonCopyable& operator()(std::ranges::range auto&&);
};

auto r = std::views::iota(0) | Closure{}; // hard error in libstdc++ and MSVC-STL

Above, since the return type of the pipeline operator is declared as auto, this causes the deleted copy constructor to be invoked in the function body and produces a hard error.

The proposed resolution adds a specification for the range adaptor closure object to return a cv-unqualified class type.

[2023-10-30; Reflector poll]

Set status to Tentatively NAD. "The wording says R | C is equivalent to C(R), not auto(C(R))."

Proposed resolution:

This wording is relative to N4958.

1. Modify 25.7.2 [range.adaptor.object] as indicated:

   -1- A range adaptor closure object is a unary function object that accepts a range argument. For a range adaptor closure object C and an expression R such that decltype((R)) models range, the following expressions are equivalent:

   […]

   -2- Given an object t of type T, where

   1. (2.1) — t is a unary function object that accepts a range argument and returns a cv-unqualified class object,

   2. […]

   then the implementation ensures that t is a range adaptor closure object.

3982⁽ⁱ⁾. is-derived-from-view-interface should require that T is derived from view_interface<T>

Section: 25.4.5 [range.view] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2023-08-22 Last modified: 2023-10-30

Priority: Not Prioritized

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Discussion:

Currently, the wording of is-derived-from-view-interface only detects whether type T is unambiguously derived from one base class view_interface<U> where U is not required to be T, which is not the intention of CRTP.

[2023-10-30; Reflector poll]

Set status to Tentatively NAD. The wording correctly handles the case where T derives from Base which derives from view_interface<Base>. We don't want it to only be satisfied for direct inheritance from view_interface<T>, but from any specialization of view_interface. Previously the concept only checked for inheritance from view_base but it was changed when view_interface stopped inheriting from view_base.

Proposed resolution:

This wording is relative to N4958.

1. Modify 25.4.5 [range.view] as indicated:

   template<class T>
     constexpr bool is-derived-from-view-interface = see below;            // exposition only
   template<class T>
     constexpr bool enable_view =
       derived_from<T, view_base> || is-derived-from-view-interface<T>;
   

   -6- For a type T, is-derived-from-view-interface<T> is true if and only if T has exactly one public base class view_interface<TU> for some type U and T has no base classes of type view_interface<UV> for any other type UV.

4003⁽ⁱ⁾. view_interface::back is overconstrained

Section: 25.5.3 [view.interface] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2023-10-28 Last modified: 2024-06-24

Priority: Not Prioritized

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Discussion:

Currently, view_interface only provides the back member when the derived class satisfies both bidirectional_range and common_range, which ensures that ranges::prev can act its sentinel.

However, requiring common_range seems to be too strict because when the derived class satisfies both random_access_range and sized_range, its end iterator can still be calculated in constant time, which is what some range adaptors currently do to greedily become common ranges.

I think we should follow similar rules to eliminate this inconsistency (demo):

#include <ranges>

constexpr auto r = std::ranges::subrange(std::views::iota(0), 5);
constexpr auto z = std::views::zip(r);
static_assert(r.back() == 4); // ill-formed
static_assert(std::get<0>(z.back()) == 4); // ok

[2023-11-07; Reflector poll]

NAD. "During the concat discussion LEWG decided not to support the corner case of random-access sized but not-common ranges." "If we did want to address such ranges, would be better to enforce commonness for random-access sized ranges by having ranges::end return ranges::begin(r) + ranges::size(r)."

Proposed resolution:

This wording is relative to N4964.

1. Modify 25.5.3 [view.interface], class template view_interface synopsis, as indicated:

   namespace std::ranges {
     template<class D>
       requires is_class_v<D> && same_as<D, remove_cv_t<D>>
     class view_interface {
       […]
     public:
       […]
       constexpr decltype(auto) back() requires (bidirectional_range<D> && common_range<D>) ||
                                                (random_access_range<D> && sized_range<D>);
       constexpr decltype(auto) back() const
         requires (bidirectional_range<const D> && common_range<const D>) ||
                  (random_access_range<const D> && sized_range<const D>);
       […]
     };
   }
   

2. Modify 25.5.3.2 [view.interface.members] as indicated:

   constexpr decltype(auto) back() requires (bidirectional_range<D> && common_range<D>) ||
                                            (random_access_range<D> && sized_range<D>);
   constexpr decltype(auto) back() const
     requires (bidirectional_range<const D> && common_range<const D>) ||
              (random_access_range<const D> && sized_range<const D>);
   

   -3- Preconditions: !empty() is true.

   -4- Effects: Equivalent to:

   auto common-arg-end = []<class R>(R& r) {
     if constexpr (common_range<R>) {
       return ranges::end(r);
     } else {
       return ranges::begin(r) + ranges::distance(r);
     }
   };
   return *ranges::prev(common-arg-endranges::end(derived()));
   

4006⁽ⁱ⁾. chunk_view::outer-iterator::value_type should provide empty

Section: 25.7.29.4 [range.chunk.outer.value] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2023-11-05 Last modified: 2024-03-11

Priority: Not Prioritized

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Discussion:

chunk_view::outer-iterator::value_type can determine whether it is empty by simply checking whether the chunk_view's remainder_ is 0, which makes it valuable to explicitly provide a noexcept empty member.

Otherwise, the view_interface::empty is synthesized only through the size member when the original sentinel and iterator type model sized_sentinel_for, which seems overkill:

#include <cassert>
#include <iostream>
#include <sstream>
#include <ranges>

int main() {
  auto ints = std::istringstream{"1 2 3 4 5 6 7 8 9 10"};
  for (auto chunk : std::views::istream<int>(ints) | std::views::chunk(3)) {
    for (auto elem : chunk) {
      assert(!chunk.empty()); // no matching function for call to 'empty()'
      std::cout << elem << " ";
    }
    assert(chunk.empty()); // ditto
    std::cout << "\n";
  }
}

[2024-03-11; Reflector poll]

Set status to Tentatively NAD after reflector poll in November 2023.

"The example shows you could use it if it existed, but not why that would be useful."

"This is a bad idea - the fact that the chunk 'shrinks' as it is iterated over is an implementation detail and not supposed to be observable."

Proposed resolution:

This wording is relative to N4964.

1. Modify 25.7.29.4 [range.chunk.outer.value] as indicated:

     namespace std::ranges {
       template<view V>
         requires input_range<V>
       struct chunk_view<V>::outer-iterator::value_type : view_interface<value_type> {
       private:
         chunk_view* parent_;                                        // exposition only
   
         constexpr explicit value_type(chunk_view& parent);          // exposition only
   
       public:
         constexpr inner-iterator begin() const noexcept;
         constexpr default_sentinel_t end() const noexcept;
   
         constexpr bool empty() const noexcept;
         constexpr auto size() const
           requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
       };
     }
   

   […]

   constexpr default_sentinel_t end() const noexcept;
   

   -3- Returns: default_sentinel.

   constexpr bool empty() const noexcept;
   

   -?- Effects: Equivalent to: return parent_->remainder_ == 0;

4020⁽ⁱ⁾. extents::index-cast weirdness

Section: 23.7.3.3.2 [mdspan.extents.expo] Status: Tentatively Ready Submitter: Casey Carter Opened: 2023-11-29 Last modified: 2025-10-17

Priority: Not Prioritized

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Discussion:

The exposition-only static member index-cast of extents is specified as (23.7.3.3.2 [mdspan.extents.expo]/9):

template<class OtherIndexType>
static constexpr auto index-cast(OtherIndexType&& i) noexcept;

-9- Effects:

1. (9.1) — If OtherIndexType is an integral type other than bool, then equivalent to return i;,

2. (9.2) — otherwise, equivalent to return static_cast<index_type>(i);.

[Note 1: This function will always return an integral type other than bool. Since this function's call sites are constrained on convertibility of OtherIndexType to index_type, integer-class types can use the static_cast branch without loss of precision. — end note]

This function returns T when passed an rvalue of cv-unqualified integral type T, but index_type when passed a cv-qualified and/or lvalue argument of any integral type. It would seem more consistent and easier to reason about if 9.1 was instead conditional on remove_cvref_t<OtherIndexType>.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

"Doesn't matter in this case, but logically decay_t seems like a better fit."

Proposed resolution:

This wording is relative to N4964.

1. Modify 23.7.3.3.2 [mdspan.extents.expo] as indicated:

   template<class OtherIndexType>
     static constexpr auto index-cast(OtherIndexType&& i) noexcept;
   

   -9- Effects:

   1. (9.1) — If remove_cvref_t<OtherIndexType> is an integral type other than bool, then equivalent to return i;,

   2. (9.2) — otherwise, equivalent to return static_cast<index_type>(i);.

   [Note 1: This function will always return an integral type other than bool. Since this function's call sites are constrained on convertibility of OtherIndexType to index_type, integer-class types can use the static_cast branch without loss of precision. — end note]

4095⁽ⁱ⁾. ranges::fold_meow should explicitly spell out the return type

Section: 26.4 [algorithm.syn], 26.6.18 [alg.fold] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2024-05-03 Last modified: 2024-06-24

Priority: Not Prioritized

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Discussion:

Unlike other algorithms, the return types of ranges::fold_meow are specified in terms of auto and see below, and its implementation details depend on the return types of other overloads through decltype(fold_meow(...)).

This makes determining the return type of a certain overload (such as fold_right_last) extremely difficult even for experts, requiring several trips back and forth to different overloads to finally understand what the actual return type is. The situation is even worse for newbies because such a form of specifying the return type makes it impossible for the IDE to deduce the real return type, which is extremely user-unfriendly.

I think that explicitly specifying the return type for these overloads not only greatly improves readability but also offloads the compiler from deducing the return type, which can definitely be considered an improvement.

The proposed resolution does not touch the Effects clause and only changes the function signature to seek minimal changes.

[2024-06-24; Reflector poll: NAD]

Implementations are free to spell this out if desired.

Proposed resolution:

This wording is relative to N4981.

1. Modify 26.4 [algorithm.syn], header <algorithm> synopsis, as indicated:

   #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   
   namespace std {
     […]
     namespace ranges {
       […]
       template<input_iterator I, sentinel_for<I> S, class T = iter_value_t<I>,
                indirectly-binary-left-foldable<T, I> F>
         constexpr auto fold_left(I first, S last, T init, F f) ->
           decay_t<invoke_result_t<F&, T, iter_reference_t<I>>>;
   
       template<input_range R, class T = range_value_t<R>,
                indirectly-binary-left-foldable<T, iterator_t<R>> F>
         constexpr auto fold_left(R&& r, T init, F f) ->
           decay_t<invoke_result_t<F&, T, range_reference_t<R>>>;
   
       template<input_iterator I, sentinel_for<I> S,
                indirectly-binary-left-foldable<iter_value_t<I>, I> F>
         requires constructible_from<iter_value_t<I>, iter_reference_t<I>>
         constexpr auto fold_left_first(I first, S last, F f) ->
           optional<decay_t<invoke_result_t<F&, iter_value_t<I>, iter_reference_t<I>>>>;
   
       template<input_range R, indirectly-binary-left-foldable<range_value_t<R>, iterator_t<R>> F>
         requires constructible_from<range_value_t<R>, range_reference_t<R>>
         constexpr auto fold_left_first(R&& r, F f) ->
           optional<decay_t<invoke_result_t<F&, range_value_t<R>, range_reference_t<R>>>>;
   
       template<bidirectional_iterator I, sentinel_for<I> S, class T = iter_value_t<I>,
                indirectly-binary-right-foldable<T, I> F>
         constexpr auto fold_right(I first, S last, T init, F f) ->
           decay_t<invoke_result_t<F&, iter_reference_t<I>, T>>;
   
       template<bidirectional_range R, class T = range_value_t<R>,
                indirectly-binary-right-foldable<T, iterator_t<R>> F>
         constexpr auto fold_right(R&& r, T init, F f) ->
           decay_t<invoke_result_t<F&, range_reference_t<R>, T>>;
   
       template<bidirectional_iterator I, sentinel_for<I> S,
                indirectly-binary-right-foldable<iter_value_t<I>, I> F>
         requires constructible_from<iter_value_t<I>, iter_reference_t<I>>
       constexpr auto fold_right_last(I first, S last, F f) ->
         optional<decay_t<invoke_result_t<F&, iter_reference_t<I>, iter_value_t<I>>>>;
   
       template<bidirectional_range R,
                indirectly-binary-right-foldable<range_value_t<R>, iterator_t<R>> F>
         requires constructible_from<range_value_t<R>, range_reference_t<R>>
       constexpr auto fold_right_last(R&& r, F f) ->
         optional<decay_t<invoke_result_t<F&, range_reference_t<R>, range_value_t<R>>>>;
   
       template<class I, class T>
         using fold_left_with_iter_result = in_value_result<I, T>;
       template<class I, class T>
         using fold_left_first_with_iter_result = in_value_result<I, T>;
   
       template<input_iterator I, sentinel_for<I> S, class T = iter_value_t<I>,
                indirectly-binary-left-foldable<T, I> F>
         constexpr see belowauto fold_left_with_iter(I first, S last, T init, F f) ->
           fold_left_with_iter_result<I, decay_t<invoke_result_t<F&, T, iter_reference_t<I>>>>;
   
       template<input_range R, class T = range_value_t<R>,
                indirectly-binary-left-foldable<T, iterator_t<R>> F>
         constexpr see belowauto fold_left_with_iter(R&& r, T init, F f) ->
           fold_left_with_iter_result<borrowed_iterator_t<R>,
                                      decay_t<invoke_result_t<F&, T, range_reference_t<R>>>>;
   
       template<input_iterator I, sentinel_for<I> S,
                indirectly-binary-left-foldable<iter_value_t<I>, I> F>
         requires constructible_from<iter_value_t<I>, iter_reference_t<I>>
         constexpr see belowauto fold_left_first_with_iter(I first, S last, F f) ->
           fold_left_first_with_iter_result<
             I, optional<decay_t<invoke_result_t<F&, iter_value_t<I>, iter_reference_t<I>>>>>;
   
       template<input_range R,
                indirectly-binary-left-foldable<range_value_t<R>, iterator_t<R>> F>
         requires constructible_from<range_value_t<R>, range_reference_t<R>>
         constexpr see belowauto fold_left_first_with_iter(R&& r, F f) ->
           fold_left_first_with_iter_result<
             borrowed_iterator_t<R>,
             optional<decay_t<invoke_result_t<F&, range_value_t<R>, range_reference_t<R>>>>>;
     }
     […]
   }
   

2. Modify 26.6.18 [alg.fold] as indicated:

   template<input_iterator I, sentinel_for<I> S, class T = iter_value_t<I>,
            indirectly-binary-left-foldable<T, I> F>
   constexpr auto ranges::fold_left(I first, S last, T init, F f) ->
     decay_t<invoke_result_t<F&, T, iter_reference_t<I>>>;
   
   template<input_range R, class T = range_value_t<R>,
            indirectly-binary-left-foldable<T, iterator_t<R>> F>
   constexpr auto ranges::fold_left(R&& r, T init, F f) ->
     decay_t<invoke_result_t<F&, T, range_reference_t<R>>>;
   

   -1- Returns:

   ranges::fold_left_with_iter(std::move(first), last, std::move(init), f).value
   

   template<input_iterator I, sentinel_for<I> S,
            indirectly-binary-left-foldable<iter_value_t<I>, I> F>
     requires constructible_from<iter_value_t<I>, iter_reference_t<I>>
     constexpr auto ranges::fold_left_first(I first, S last, F f) ->
       optional<decay_t<invoke_result_t<F&, iter_value_t<I>, iter_reference_t<I>>>>;
   
   template<input_range R, indirectly-binary-left-foldable<range_value_t<R>, iterator_t<R>> F>
     requires constructible_from<range_value_t<R>, range_reference_t<R>>
     constexpr auto ranges::fold_left_first(R&& r, F f) ->
       optional<decay_t<invoke_result_t<F&, range_value_t<R>, range_reference_t<R>>>>;
   

   -2- Returns:

   ranges::fold_left_first_with_iter(std::move(first), last, f).value
   

   template<bidirectional_iterator I, sentinel_for<I> S, class T = iter_value_t<I>,
            indirectly-binary-right-foldable<T, I> F>
     constexpr auto ranges::fold_right(I first, S last, T init, F f) ->
       decay_t<invoke_result_t<F&, iter_reference_t<I>, T>>;
   
   template<bidirectional_range R, class T = range_value_t<R>,
           indirectly-binary-right-foldable<T, iterator_t<R>> F>
     constexpr auto ranges::fold_right(R&& r, T init, F f) ->
       decay_t<invoke_result_t<F&, range_reference_t<R>, T>>;  
   

   -3- Effects: Equivalent to:

   using U = decay_t<invoke_result_t<F&, iter_reference_t<I>, T>>;
   if (first == last)
     return U(std::move(init));
   I tail = ranges::next(first, last);
   U accum = invoke(f, *--tail, std::move(init));
   while (first != tail)
     accum = invoke(f, *--tail, std::move(accum));
   return accum;
   

   template<bidirectional_iterator I, sentinel_for<I> S,
           indirectly-binary-right-foldable<iter_value_t<I>, I> F>
     requires constructible_from<iter_value_t<I>, iter_reference_t<I>>
   constexpr auto ranges::fold_right_last(I first, S last, F f) ->
     optional<decay_t<invoke_result_t<F&, iter_reference_t<I>, iter_value_t<I>>>>;
   
   template<bidirectional_range R,
            indirectly-binary-right-foldable<range_value_t<R>, iterator_t<R>> F>
    requires constructible_from<range_value_t<R>, range_reference_t<R>>
   constexpr auto ranges::fold_right_last(R&& r, F f) ->
     optional<decay_t<invoke_result_t<F&, range_reference_t<R>, range_value_t<R>>>>;
   

   -4- Let U be decltype(ranges::fold_right(first, last, iter_value_t<I>(*first), f)).

   -5- Effects: Equivalent to:

   if (first == last)
     return optional<U>();
   I tail = ranges::prev(ranges::next(first, std::move(last)));
   return optional<U>(in_place,
     ranges::fold_right(std::move(first), tail, iter_value_t<I>(*tail), std::move(f)));
   

   template<input_iterator I, sentinel_for<I> S, class T = iter_value_t<I>,
            indirectly-binary-left-foldable<T, I> F>
     constexpr see belowauto ranges::fold_left_with_iter(I first, S last, T init, F f) ->
       fold_left_with_iter_result<I, decay_t<invoke_result_t<F&, T, iter_reference_t<I>>>>;
   
   template<input_range R, class T = range_value_t<R>,
            indirectly-binary-left-foldable<T, iterator_t<R>> F>
     constexpr see belowauto ranges::fold_left_with_iter(R&& r, T init, F f) ->
       fold_left_with_iter_result<borrowed_iterator_t<R>,
                                  decay_t<invoke_result_t<F&, T, range_reference_t<R>>>>;
   

   -6- Let U be decay_t<invoke_result_t<F&, T, iter_reference_t<I>>>.

   -7- Effects: Equivalent to:

   if (first == last)
     return {std::move(first), U(std::move(init))};
   U accum = invoke(f, std::move(init), *first);
   for (++first; first != last; ++first)
     accum = invoke(f, std::move(accum), *first);
   return {std::move(first), std::move(accum)};
   

   -8- Remarks: The return type is fold_left_with_iter_result<I, U> for the first overload and fold_left_with_iter_result<borrowed_iterator_t<R>, U> for the second overload.

   template<input_iterator I, sentinel_for<I> S,
            indirectly-binary-left-foldable<iter_value_t<I>, I> F>
     requires constructible_from<iter_value_t<I>, iter_reference_t<I>>
     constexpr see belowauto ranges::fold_left_first_with_iter(I first, S last, F f) ->
       fold_left_first_with_iter_result<
         I, optional<decay_t<invoke_result_t<F&, iter_value_t<I>, iter_reference_t<I>>>>>;
   
   template<input_range R,
            indirectly-binary-left-foldable<range_value_t<R>, iterator_t<R>> F>
     requires constructible_from<range_value_t<R>, range_reference_t<R>>
     constexpr see belowauto ranges::fold_left_first_with_iter(R&& r, F f) ->
       fold_left_first_with_iter_result<
         borrowed_iterator_t<R>,
         optional<decay_t<invoke_result_t<F&, range_value_t<R>, range_reference_t<R>>>>>;
   

   -9- Let U be

   decltype(ranges::fold_left(std::move(first), last, iter_value_t<I>(*first), f))
   

   -10- Effects: Equivalent to:

   if (first == last)
     return {std::move(first), optional<U>()};
   optional<U> init(in_place, *first);
   for (++first; first != last; ++first)
     *init = invoke(f, std::move(*init), *first);
   return {std::move(first), std::move(init)};
   

   -11- Remarks: The return type is fold_left_first_with_iter_result<I, optional<U>> for the first overload and fold_left_first_with_iter_result<borrowed_iterator_t<R>, optional<U>> for the second overload.

4163⁽ⁱ⁾. Can the overload of std::num_get::do_get for bool call the overload for long?

Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: Tentatively NAD Submitter: Jiang An Opened: 2024-09-29 Last modified: 2025-02-07

Priority: Not Prioritized

View other active issues in [facet.num.get.virtuals].

View all other issues in [facet.num.get.virtuals].

View all issues with Tentatively NAD status.

Discussion:

28.3.4.3.2.3 [facet.num.get.virtuals]/6 currently says:

Effects: If (str.flags()&ios_base::boolalpha) == 0 then input proceeds as it would for a long except that if a value is being stored into val, […]

It is unclear whether an implementation is allowed to call the overload for long in this case. Currently, libc++'s version calls that overload, while libstdc++ and MSVC STL's don't (example).

As the divergence implementation strategies is observable, perhaps we should clarify on this.

[2025-02-07; Reflector poll: NAD]

I think this is just a libc++ bug. The wording says it "proceeds as it would for long", which is not the same as actually making a virtual call to do_get for long. It can either duplicate the code from do_get for long, or make a non-virtual (i.e. qualified) call to num_get::do_get.

Proposed resolution:

4184⁽ⁱ⁾. Domain of ranges::cmeow doesn't match ranges::meow

Section: 25.3 [range.access] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2024-12-17 Last modified: 2025-02-07

Priority: Not Prioritized

View all issues with Tentatively NAD status.

Discussion:

ranges::begin/rbegin/data can be used on non-ranges as long as the object has a begin/rbegin/data member, this is also true for their const versions before C++23.

However, in C++23 the const version always applied possibly-const-range to the object, which no longer worked for non-ranges due to this function requiring input_range, which seems to be a breaking change (demo):

#include <ranges>

struct NotRange {
        int* begin();
  const int* begin() const;
        int* rbegin();
  const int* rbegin() const;
        int* data();
  const int* data() const;
};

int main() {
  NotRange r;

  (void) std::ranges::begin(r);
  (void) std::ranges::rbegin(r);
  (void) std::ranges::data(r);

  // The following works in C++20, fails in C++23
  (void) std::ranges::cbegin(r);
  (void) std::ranges::crbegin(r);
  (void) std::ranges::cdata(r);
}

[2025-02-07; Reflector poll: NAD]

"We don't need to support ranges::cbegin on non-ranges."

"Seems to be very similar to LWG 3913⁽ⁱ⁾ which LWG closed as NAD."

Proposed resolution:

4194⁽ⁱ⁾. atomic<void*> should use generic class template

Section: 32.5.8.5 [atomics.types.pointer] Status: Tentatively NAD Submitter: Gonzalo Brito Opened: 2025-01-16 Last modified: 2025-02-07

Priority: Not Prioritized

View other active issues in [atomics.types.pointer].

View all other issues in [atomics.types.pointer].

View all issues with Tentatively NAD status.

Discussion:

32.5.8.5 [atomics.types.pointer] p1 states (emphasis mine):

There is a partial specialization of the atomic class template for pointers.

which requires atomic<void*> to use the atomic class template for pointers. However, the fetch_add/_sub member functions add a difference_type to a T* which requires a pointer-to-object type (these member functions are constexpr, so trying to support this seems unimplementable).

For atomic_ref, the 32.5.7.5 [atomics.ref.pointer] p1 states (emphasis mine):

There are specializations of the atomic_ref` class template for all pointer-to-object types.

which avoids this issue and applying the same form to 32.5.8.5 [atomics.types.pointer] would make atomic<void*> and atomic_ref<void*> consistent.

Technically this would be a breaking change, but all C++ standard library implementations surveyed are broken, and the proposed fix would make them compliant: see libstdc++, libc++ and MSVC STL errors here. These standard libraries require a pointer-to-object type, atomic<void*> uses the general template. Therefore, no user code seems to be impacted.

[2025-02-07; Reflector poll: NAD]

The fetch_OP members have "Mandates: T is a complete object type." and a note explaining that this means arithmetic on void* is ill-formed. So implementations are expected to use the partial specialization for void* but to reject attempts at arithmetic. They all do this correctly today.

Proposed resolution:

This wording is relative to N5001.

1. Modify 32.5.8.5 [atomics.types.pointer] as indicated:

   -1- There is a partial specialization of the atomic class template for pointerspointer-to-object types. Specializations of this partial specialization are standard-layout structs. They each have a trivial destructor.

4228⁽ⁱ⁾. Does vector<bool, Allocator> mandate that Allocator::value_type is bool?

Section: 23.3.14.1 [vector.bool.pspc] Status: Tentatively NAD Submitter: Stephan T. Lavavej Opened: 2025-03-18 Last modified: 2025-06-13

Priority: Not Prioritized

View all other issues in [vector.bool.pspc].

View all issues with Tentatively NAD status.

Discussion:

N5008 23.3.14.1 [vector.bool.pspc]/2 says:

Unless described below, all operations have the same requirements and semantics as the primary vector template, except that operations dealing with the bool value type map to bit values in the container storage and allocator_traits::construct (20.2.9.3 [allocator.traits.members]) is not used to construct these values.

23.2.2.5 [container.alloc.reqmts]/5 says:

Mandates: allocator_type::value_type is the same as X::value_type.

Is vector<bool, allocator<int>> forbidden? There's implementation divergence: MSVC's STL enforces the mandate, while libc++ and libstdc++ accept this code, discovered while running libc++'s tests with MSVC's STL.

Preferred resolution: I would be satisfied with resolving this as NAD, with a record that LWG believes that "all operations have the same requirements" means that the Mandate applies. I suppose that an editorial note could also be added to this effect, although I don't believe it's necessary.

Alternate resolution: If LWG believes that the Mandate does not apply, and that vector<bool> should be unique among containers in accepting allocator<Anything>, then I believe that a normative sentence should be added to 23.3.14.1 [vector.bool.pspc]/2, specifically creating an exemption to 23.2.2.5 [container.alloc.reqmts]/5.

[2025-06-13; Reflector poll]

Set status to Tentatively NAD. This is just a bug in some implementations (now fixed in libstdc++).

Proposed resolution:

4246⁽ⁱ⁾. Redundant constraint in range_formatter::format

Section: 28.5.7.2 [format.range.formatter] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2025-04-18 Last modified: 2025-06-12

Priority: Not Prioritized

View all other issues in [format.range.formatter].

View all issues with Tentatively NAD status.

Discussion:

Currently, the signature of range_formatter::format is as follows:

template<ranges::input_range R, class FormatContext>
  requires formattable<ranges::range_reference_t<R>, charT> &&
           same_as<remove_cvref_t<ranges::range_reference_t<R>>, T>
typename FormatContext::iterator
  format(R&& r, FormatContext& ctx) const;

which requires that the reference type of the range parameter must be formattable, and such type must be exactly T after removing the cvref-qualifiers.

However, satisfying the latter always implies satisfying the former, as the range_formatter class already requires that T must be formattable.

There is no need to perform a redundant check here.

[2025-06-12; Reflector poll]

Set status to Tentatively NAD. This is not redundant, it might check that const T is formattable, which is not the same as checking that T is formattable.

Proposed resolution:

This wording is relative to N5008.

1. Modify 28.5.7.2 [format.range.formatter] as indicated:

   namespace std {
     template<class T, class charT = char>
       requires same_as<remove_cvref_t<T>, T> && formattable<T, charT>
     class range_formatter {
       […]
       template<ranges::input_range R, class FormatContext>
           requires formattable<ranges::range_reference_t<R>, charT> &&
                    same_as<remove_cvref_t<ranges::range_reference_t<R>>, T>
         typename FormatContext::iterator
           format(R&& r, FormatContext& ctx) const;
     };
   }
   

   […]

   template<ranges::input_range R, class FormatContext>
     requires formattable<ranges::range_reference_t<R>, charT> &&
              same_as<remove_cvref_t<ranges::range_reference_t<R>>, T>
   typename FormatContext::iterator
     format(R&& r, FormatContext& ctx) const;
   

   -11- Effects: Writes the following into ctx.out(), adjusted according to the range-format-spec:

4253⁽ⁱ⁾. basic_const_iterator should provide iterator_type

Section: 24.5.3.3 [const.iterators.iterator] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2025-04-29 Last modified: 2025-06-13

Priority: Not Prioritized

View other active issues in [const.iterators.iterator].

View all other issues in [const.iterators.iterator].

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Discussion:

Currently, iterator adaptors in <iterator> that wrap a single iterator such as reverse_iterator, move_iterator, and counted_iterator all provide a public iterator_type member for users to access the underlying iterator type, except for basic_const_iterator (demo):

#include <iterator>

using I  = int*;
using RI = std::reverse_iterator<I>;
using MI = std::move_iterator<I>;
using CI = std::counted_iterator<I>;
using BI = std::basic_const_iterator<I>;

static_assert(std::same_as<RI::iterator_type, I>);
static_assert(std::same_as<MI::iterator_type, I>);
static_assert(std::same_as<CI::iterator_type, I>);
static_assert(std::same_as<BI::iterator_type, I>); // error

It seems reasonable to add one for basic_const_iterator for consistency.

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 24.5.3.3 [const.iterators.iterator] as indicated:

   namespace std {
     […]
     template<input_iterator Iterator>
     class basic_const_iterator {
       Iterator current_ = Iterator();                             // exposition only
       using reference = iter_const_reference_t<Iterator>;         // exposition only
       using rvalue-reference =                                    // exposition only
         iter-const-rvalue-reference-t<Iterator>;
             
       public:
         using iterator_type = Iterator;
         using iterator_concept = see below;
         using iterator_category = see below;  // not always present
         using value_type = iter_value_t<Iterator>;
         using difference_type = iter_difference_t<Iterator>;
         […]
     };
   }
   

4257⁽ⁱ⁾. Stream insertion for chrono::local_time should be constrained

Section: 30.7.9 [time.clock.local] Status: Tentatively Ready Submitter: Jonathan Wakely Opened: 2025-05-16 Last modified: 2025-08-26

Priority: Not Prioritized

View all issues with Tentatively Ready status.

Discussion:

Stream insertion for chrono::local_time is defined in terms of conversion to chrono::sys_time, but not all chrono::sys_time specializations can be inserted into an ostream, because one of the overloads is constrained and the other requires convertibility to chrono::sys_days (see 30.7.2.3 [time.clock.system.nonmembers]).

This means the following code fails to compile:

#include <iostream>
#include <chrono>

template<typename T>
concept ostream_insertable = requires (std::ostream& o, const T& t) { o << t; };

using D = std::chrono::duration<double>;

int main() {
  if constexpr (ostream_insertable<std::chrono::sys_time<D>>)
    std::cout << std::chrono::sys_time<D>{};
  if constexpr (ostream_insertable<std::chrono::local_time<D>>)
    std::cout << std::chrono::local_time<D>{}; // FAIL
}

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 30.7.9 [time.clock.local] as indicated:

   
   template<class charT, class traits, class Duration>
     basic_ostream<charT, traits>&
       operator<<(basic_ostream<charT, traits>& os, const local_time<Duration>& lt);
   

   -?- Constraints: os << sys_time<Duration>{lt.time_since_epoch()} is a valid expression.

   -2- Effects:

    os << sys_time<Duration>{lt.time_since_epoch()};
   

   -3- Returns: os.

4266⁽ⁱ⁾. layout_stride::mapping should treat empty mappings as exhaustive

Section: 23.7.3.4.7 [mdspan.layout.stride] Status: Tentatively Ready Submitter: Tomasz Kamiński Opened: 2025-05-22 Last modified: 2025-06-13

Priority: Not Prioritized

View all other issues in [mdspan.layout.stride].

View all issues with Tentatively Ready status.

Discussion:

Mapping over an empty multidimensional index space is always exhaustive according to the corresponding definitions from 23.7.3.4.2 [mdspan.layout.reqmts] p16.

However, the current specification of layout_stride::mapping does not consider whether some of the empty multidimensional index spaces are unique or exhaustive. For illustration, the mapping with the following configuration is not considered exhaustive according to the current specification of 23.7.3.4.7.4 [mdspan.layout.stride.obs] bullet 5.2:

extents: 2, 2, 0
strides: 2, 6, 20

This prevents the implementation from implementing sm.is_exhaustive() as sm.fwd-prod-of-extents(sm::extents_type::rank()) == sm.required_span_size(). For all mappings with size greater than zero, such an expression provides an answer consistent with the standard. However, it always returns true for an empty mapping, such as shown in the example.

We should make such implementation conforming, and require is_exhaustive() to return true for empty mappings.

For consistency, we could update is_always_exhaustive() to recognize mapping with rank() == 0, and one for which at least one of the static extents is equal to zero (i.e., they always represent a multidimensional index space).

[2025-06-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.3.4.7.1 [mdspan.layout.stride.overview] as indicated:

   namespace std {
     template<class Extents>
     class layout_stride::mapping {
       […]
       static constexpr bool is_always_unique() noexcept { return true; }
       static constexpr bool is_always_exhaustive() noexcept; { return false; }
       static constexpr bool is_always_strided() noexcept { return true; }
       […]
     };
   }
   

2. Modify 23.7.3.4.7.4 [mdspan.layout.stride.obs] as indicated:

   […]

   static constexpr bool is_always_exhaustive() noexcept;
   

   -?- Returns: true if rank_ is 0 or if there is a rank index r of extents() such that extents_type::static_extent(r) is 0, otherwise false.

   constexpr bool is_exhaustive() const noexcept;
   

   -5- Returns:

   1. (5.1) — true if rank_ or the size of the multidimensional index space m.extents() is 0.

   2. (5.2) — […]

   3. (5.3) — […]

4269⁽ⁱ⁾. unique_copy passes arguments to its predicate backwards

Section: 26.7.9 [alg.unique] Status: Tentatively Ready Submitter: Jonathan Wakely Opened: 2025-05-29 Last modified: 2025-10-17

Priority: Not Prioritized

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Discussion:

For the unique algorithms, 26.7.9 [alg.unique] p1 says:

1. Let pred be equal_to{} for the overloads with no parameter pred, and let E be

1. (1.1) — bool(pred(*(i - 1), *i)) for the overloads in namespace std;
2. (1.2) — bool(invoke(comp, invoke(proj, *(i - 1)), invoke(proj, *i))) for the overloads in namespace ranges.

However for the unique_copy algorithms, 26.7.9 [alg.unique] p6 says that the arguments *i and *(i-1) should be reversed:

6. Let pred be equal_to{} for the overloads with no parameter pred, and let E be

1. (6.1) — bool(pred(*i, *(i - 1))) for the overloads in namespace std;
2. (6.2) — bool(invoke(comp, invoke(proj, *i), invoke(proj, *(i - 1)))) for the overloads in namespace ranges.

This reversed order is consistent with the documentation for SGI STL unique_copy, although the docs for SGI STL unique show reversed arguments too, and the C++ standard doesn't match that.

A survey of known implementations shows that all three of libstdc++, libc++, and MSVC STL use the pred(*(i - 1), *i) order for all of std::unique, std::unique_copy, ranges::unique, and ranges::unique_copy. The range-v3 library did the same, and even the SGI STL did too (despite what its docs said). Only two implementations were found which match the spec and use a different argument order for unique and unique_copy, Casey Carter's (cmcstl2) and Fraser Gordon's.

In the absence of any known rationale for unique and unique_copy to differ, it seems sensible to make unique_copy more consistent with unique (and with the majority of implementations stretching back three decades).

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Fixed misplaced ) in the (6.1) change as pointed out on reflector, and rebased on N5014.

"I remain inconvinced that this actually matters given the equivalence relation requirement."

Proposed resolution:

This wording is relative to N5014.

1. Modify 26.7.9 [alg.unique] as indicated:

   6. Let pred be equal_to{} for the overloads with no parameter pred, and let E(i) be

   1. (6.1) — bool(pred(*i, *(i - 1), *i)) for the overloads in namespace std;
   2. (6.2) — bool(invoke(comp, invoke(proj, *i), invoke(proj, *(i - 1)), invoke(proj, *i))) for the overloads in namespace ranges.

4274⁽ⁱ⁾. The chrono::hh_mm_ss constructor is ill-formed for unsigned durations

Section: 30.9.2 [time.hms.members] Status: Tentatively Ready Submitter: Michael Welsh Duggan Opened: 2025-06-04 Last modified: 2025-06-13

Priority: Not Prioritized

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Discussion:

In 30.9.2 [time.hms.members], paragraph 3, the current wording for the constructor of hh_mm_ss expresses some of its requirements in terms of abs(d), which is assumed to be chrono::abs(chrono::duration). chrono::abs is not defined, however, for durations with an unsigned representation. I believe that not being able to create hh_mm_ss objects from unsigned durations is unintentional.

Moreover, is_constructible_v<hh_mm_ss<ud>, ud> is required to be true by the standard for any duration, so making it actually work makes a lot of sense.

[2025-06-13; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 30.9.2 [time.hms.members] as indicated:

   constexpr explicit hh_mm_ss(Duration d);
   

   -3- Effects: Constructs an object of type hh_mm_ss which represents the Duration d with precision precision.

   1. (3.1) — Initializes is_neg with d < Duration::zero(). Let ABS_D represent -d if is_neg is true and d otherwise.

   2. (3.2) — Initializes h with duration_cast<chrono::hours>(abs(d)ABS_D).

   3. (3.3) — Initializes m with duration_cast<chrono::minutes>(abs(d)ABS_D - hours()).

   4. (3.4) — Initializes s with duration_cast<chrono::seconds>(abs(d)ABS_D - hours() - minutes()).

   5. (3.5) — If treat_as_floating_point_v<precision::rep> is true, initializes ss with abs(d)ABS_D - hours() - minutes() - seconds(). Otherwise, initializes ss with duration_cast<precision>(abs(d)ABS_D - hours() - minutes() - seconds()).

4275⁽ⁱ⁾. std::dynamic_extent should also be defined in <mdspan>

Section: 23.7.3.2 [mdspan.syn] Status: Tentatively Ready Submitter: Aiden Grossman Opened: 2025-06-06 Last modified: 2025-10-17

Priority: 3

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Discussion:

std::dynamic_extent can be used in certain circumstances in std::mdspan, such as with padded layouts. However, std::dynamic_extent is currently only defined in <span> which necessitates including <span> solely for the std::dynamic_extent definition.

Previous resolution [SUPERSEDED]:

This wording is relative to N5008.

1. Modify 23.7.3.2 [mdspan.syn], header <span> synopsis, as indicated:

   // all freestanding
   namespace std {
     // constants
     inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
     
     // 23.7.3.3 [mdspan.extents], class template extents
     template<class IndexType, size_t... Extents>
     class extents;
   
     […]
   }
   

[2025-06-10; Jonathan provides improved wording]

[2025-10-15; Reflector poll]

Set priority to 3 after reflector poll.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.1 [views.general] as indicated:

   The header <span> (23.7.2.1 [span.syn]) defines the view span. The header <mdspan> (23.7.3.2 [mdspan.syn]) defines the class template mdspan and other facilities for interacting with these multidimensional views.

   -?- In addition to being available via inclusion of the <span> header, dynamic_extent is available when the header <mdspan> is included.

4276⁽ⁱ⁾. front() and back() are not hardened for zero-length std::arrays

Section: 23.3.3.5 [array.zero] Status: Tentatively Ready Submitter: Jan Schultke Opened: 2025-06-08 Last modified: 2025-08-26

Priority: Not Prioritized

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Discussion:

The intent of P3471 "Standard library hardening" is clearly to provide hardened preconditions for members of sequence containers, including std::array. However, a zero-length std::array dodges this hardening by having undefined behavior for front() and back() explicitly specified in 23.3.3.5 [array.zero] paragraph 3.

Without this paragraph, front() and back() would be hardened as well, as specified in 23.2.4 [sequence.reqmts].

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.3.3.5 [array.zero] as indicated:

   -3- The effect of calling front() or back() for a zero-sized array is undefined.

4280⁽ⁱ⁾. simd::partial_load uses undefined identifier T

Section: 29.10.8.7 [simd.loadstore] Status: Tentatively Ready Submitter: Tim Song Opened: 2025-06-21 Last modified: 2025-08-26

Priority: Not Prioritized

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Discussion:

The Effects: element of std::simd::partial_load (after the latest rename) uses T but that is not defined anywhere. It should be V::value_type.

Also, this paragraph should be a Returns: element.

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 29.10.8.7 [simd.loadstore] as indicated:

   
   template<class V = see below, ranges::contiguous_range R, class... Flags>
     requires ranges::sized_range<R>
     constexpr V partial_load(R&& r, flags<Flags...> f = {});
   […]
   template<class V = see below, contiguous_iterator I, sized_sentinel_for<I> S, class... Flags>
     constexpr V partial_load(I first, S last, const typename V::mask_type& mask,
                              flags<Flags...> f = {});
   

   -6- […]

   -7- Mandates: […]

   -8- Preconditions: […]

   -9- Effects: Initializes theReturns: A basic_simd object whose i^th element is initialized with mask[i] && i < ranges::size(r) ? static_cast<T>(ranges::data(r)[i]) : T() for all i in the range of [0, V::size()), where T is V::value_type.

   -10- Remarks: The default argument for template parameter V is basic_simd<ranges::range_value_t<R>>.

4286⁽ⁱ⁾. Some more feature-test macros for fully freestanding features are not marked freestanding

Section: 17.3.2 [version.syn], 20.2.2 [memory.syn] Status: Tentatively Ready Submitter: Yihe Li Opened: 2025-06-17 Last modified: 2025-10-14

Priority: Not Prioritized

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Discussion:

P1642R11 (accepted in C++23) plus LWG 4189⁽ⁱ⁾ (accepted in Hagenberg) added nearly the entire <ranges> header to freestanding. However, the only feature-test macro being added to freestanding is __cpp_lib_ranges_cache_latest, which seems weird, since views::enumerate is also added to freestanding following the blanket comment strategy, but its feature-test macro remains not in freestanding. In retrospective, since all range algorithms are in freestanding via P2976, all __cpp_lib_ranges_* FTMs (except __cpp_lib_ranges_generate_random since ranges::generate_random is not in freestanding) should probably be marked as freestanding.

Furthermore, LWG 4126⁽ⁱ⁾ left out some other FTMs for fully freestanding features. They are also added in the following wording.

A note about is_sufficiently_aligned: P2897R7 does indicate in 5.7.6.1 that the function should be freestanding, but somehow the wording didn't say so. The following wording includes the function and its FTM anyway since hopefully this is just an omission when wording the paper.

[2025-10-14; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 17.3.2 [version.syn], header <version> synopsis, as indicated:

   […]
   #define __cpp_lib_aligned_accessor             202411L // freestanding, also in <mdspan>
   […]                                     
   #define __cpp_lib_array_constexpr              201811L // freestanding, also in <iterator>, <array>
   […]                                     
   #define __cpp_lib_clamp                        201603L // freestanding, also in <algorithm>
   […]                                     
   #define __cpp_lib_constexpr_numeric            201911L // freestanding, also in <numeric>
   […]                                     
   #define __cpp_lib_function_ref                 202306L // freestanding, also in <functional>
   #define __cpp_lib_gcd_lcm                      201606L // freestanding, also in <numeric>
   […]
   #define __cpp_lib_integer_comparison_functions 202002L // freestanding, also in <utility>
   […]
   #define __cpp_lib_is_sufficiently_aligned      202411L // freestanding, also in <memory>
   […]
   #define __cpp_lib_ranges_contains              202207L // freestanding, also in <algorithm>
   #define __cpp_lib_ranges_enumerate             202302L // freestanding, also in <ranges>
   #define __cpp_lib_ranges_find_last             202207L // freestanding, also in <algorithm>
   #define __cpp_lib_ranges_fold                  202207L // freestanding, also in <algorithm>
   […]
   #define __cpp_lib_ranges_iota                  202202L // freestanding, also in <numeric>
   […]
   #define __cpp_lib_ranges_starts_ends_with      202106L // freestanding, also in <algorithm>
   […]
   #define __cpp_lib_robust_nonmodifying_seq_ops  201304L // freestanding, also in <algorithm>
   #define __cpp_lib_sample                       201603L // freestanding, also in <algorithm>
   #define __cpp_lib_saturation_arithmetic        202311L // freestanding, also in <numeric>
   […]
   

2. Modify 20.2.2 [memory.syn], header <memory> synopsis, as indicated:

   […]
   template<size_t Alignment, class T>
     bool is_sufficiently_aligned(T* ptr);      // freestanding
   […]
   

4291⁽ⁱ⁾. explicit map(const Allocator&) should be constexpr

Section: 23.4.3.1 [map.overview] Status: Tentatively Ready Submitter: Jonathan Wakely Opened: 2025-07-10 Last modified: 2025-08-26

Priority: Not Prioritized

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Discussion:

The intent of P3372R3 was for all container constructors to be constexpr, but during application of the paper to the working draft it was observed that one map constructor was missed.

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.4.3.1 [map.overview] as indicated:

   
   // 23.4.3.2, construct/copy/destroy
   constexpr map() : map(Compare()) { }
   constexpr explicit map(const Compare& comp, const Allocator& = Allocator());
   template<class InputIterator>
     constexpr map(InputIterator first, InputIterator last,
                   const Compare& comp = Compare(), const Allocator& = Allocator());
   template<container-compatible-range <value_type> R>
     constexpr map(from_range_t, R&& rg, const Compare& comp = Compare(),
                   const Allocator& = Allocator());
   constexpr map(const map& x);
   constexpr map(map&& x);
   constexpr explicit map(const Allocator&);
   constexpr map(const map&, const type_identity_t<Allocator>&);
   constexpr map(map&&, const type_identity_t<Allocator>&);
   constexpr map(initializer_list<value_type>, const Compare& = Compare(),
                 const Allocator& = Allocator());
   

4292⁽ⁱ⁾. Unordered container local iterators should be constexpr iterators

Section: 23.5.3.1 [unord.map.overview], 23.5.4.1 [unord.multimap.overview], 23.5.6.1 [unord.set.overview], 23.5.7.1 [unord.multiset.overview] Status: Tentatively Ready Submitter: Jonathan Wakely Opened: 2025-07-10 Last modified: 2025-08-29

Priority: Not Prioritized

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Discussion:

The intent of P3372R3 was for all container iterators to be constexpr iterators, but during application of the paper to the working draft it was observed that unordered containers don't say it for their local iterators.

[2025-08-29; Reflector poll]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.5.3.1 [unord.map.overview] as indicated:

   -4- The types iterator, and const_iterator, local_iterator, and const_local_iterator meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

2. Modify 23.5.4.1 [unord.multimap.overview] as indicated:

   -4- The types iterator, and const_iterator, local_iterator, and const_local_iterator meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

3. Modify 23.5.6.1 [unord.set.overview] as indicated:

   -4- The types iterator, and const_iterator, local_iterator, and const_local_iterator meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

4. Modify 23.5.7.1 [unord.multiset.overview] as indicated:

   -4- The types iterator, and const_iterator, local_iterator, and const_local_iterator meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).

4293⁽ⁱ⁾. span::subspan/first/last chooses wrong constructor when T is const-qualified bool

Section: 23.7.2.2.4 [span.sub] Status: Tentatively Ready Submitter: Yuhan Liu Opened: 2025-07-11 Last modified: 2025-08-26

Priority: Not Prioritized

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Discussion:

In section 23.7.2.2.4 [span.sub], paragraphs p12, p14, and p16 erroneously use the initializer list constructor for span instead of the intended iterator/count constructor.

Specifically, in these paragraphs, the standard states:

Effects: Equivalent to: return {data(), count};

A proposed fix (thanks to Jonathan Wakely) could look like this following:

return span<element_type>(data(), count);

[2025-07-11; Jonathan adds proposed resolution]

The meaning of those Effects: paragraphs was changed for C++26 by P2447R6 which added the span(initializer_list) constructor. A simpler demo is:

bool a[5]{};
std::span<const bool> s(a);
std::span<const bool> s2 = s.first(5);
assert(s2.size() == 5); // OK in C++23, fails in C++26
assert(s2.data() == a); // OK in C++23, fails in C++26

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.2.2.4 [span.sub] as indicated:

   
   template<size_t Count> constexpr span<element_type, Count> first() const;
   

   -1- Mandates: Count <= Extent is true.

   -2- Hardened preconditions: Count <= size() is true.

   -3- Effects: Equivalent to: return R({data(), Count}); where R is the return type.

   
   template<size_t Count> constexpr span<element_type, Count> last() const;
   

   -4- Mandates: Count <= Extent is true.

   -5- Hardened preconditions: Count <= size() is true.

   -6- Effects: Equivalent to: return R({data() + (size() - Count), Count}); where R is the return type.

   
   template<size_t Offset, size_t Count = dynamic_extent>
     constexpr span<element_type, see below> subspan() const;
   

   -7- Mandates:

   Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset)
   

   is true.

   -8- Hardened preconditions:

   Offset <= size() && (Count == dynamic_extent || Count <= size() - Offset)
   

   is true.

   -9- Effects: Equivalent to:

   return span<ElementType, see below>(
       data() + Offset, Count != dynamic_extent ? Count : size() - Offset);
   

   -10- Remarks: The second template argument of the returned span type is:

   Count != dynamic_extent ? Count
                           : (Extent != dynamic_extent ? Extent - Offset
                                                       : dynamic_extent)
   

   
   constexpr span<element_type, dynamic_extent> first(size_type count) const;
   

   -11- Hardened preconditions: count <= size() is true.

   -12- Effects: Equivalent to: return R({data(), count}); where R is the return type.

   
   constexpr span<element_type, dynamic_extent> last(size_type count) const;
   

   -13- Hardened preconditions: count <= size() is true.

   -14- Effects: Equivalent to: return R({data() + (size() - count), count}); where R is the return type.

   
   constexpr span<element_type, dynamic_extent> subspan(
     size_type offset, size_type count = dynamic_extent) const;
   

   -15- Hardened preconditions:

   offset <= size() && (count == dynamic_extent || count <= size() - offset
   

   is true.

   -16- Effects: Equivalent to:

   return R({data() + offset, count == dynamic_extent ? size() - offset :  count});
   

   where R is the return type.

4294⁽ⁱ⁾. bitset(const CharT*) constructor needs to be constrained

Section: 22.9.2.2 [bitset.cons] Status: Tentatively Ready Submitter: Jonathan Wakely Opened: 2025-07-12 Last modified: 2025-08-26

Priority: Not Prioritized

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Discussion:

This code might be ill-formed, with an error outside the immediate context that users cannot prevent:

#include <bitset>
struct NonTrivial { ~NonTrivial() { } };
static_assert( ! std::is_constructible_v<std::bitset<1>, NonTrivial*> );

The problem is that the bitset(const CharT*) constructor tries to instantiate basic_string_view<NonTrivial> to find its size_type, and that instantiation might ill-formed, e.g. if std::basic_string_view or std::char_traits has a static assert enforcing the requirement for their character type to be sufficiently char-like. 27.1 [strings.general] defines a char-like type as "any non-array trivially copyable standard-layout (6.9.1 [basic.types.general]) type T where is_trivially_default_constructible_v<T> is true."

[2025-08-21; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 22.9.2.2 [bitset.cons] as indicated:

   
   template<class charT>
     constexpr explicit bitset(
       const charT* str,
       typename basic_string_view<charT>::size_type n = basic_string_view<charT>::npos,
       charT zero = charT(’0’),
       charT one = charT(’1’));
   

   -?- Constraints:

   • is_array_v<charT> is false,
   • is_trivially_copyable_v<charT> is true,
   • is_standard_layout_v<charT> is true, and
   • is_trivially_default_constructible_v<charT> is true.

   -8- Effects: As if by:

   bitset(n == basic_string_view<charT>::npos
             ? basic_string_view<charT>(str)
             : basic_string_view<charT>(str, n),
          0, n, zero, one)
   

4299⁽ⁱ⁾. Missing Mandates: part in optional<T&>::transform

Section: 22.5.4.7 [optional.ref.monadic] Status: Tentatively Ready Submitter: Giuseppe D'Angelo Opened: 2025-07-15 Last modified: 2025-10-16

Priority: Not Prioritized

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Discussion:

In 22.5.4.7 [optional.ref.monadic] the specification of optional<T&>::transform is missing an additional part of the Mandates: element compared to the primary template's transform (in 22.5.3.8 [optional.monadic] p8); that is, is missing to enforce that the U type is a valid contained type for optional.

The definition of "valid contained type" comes from P2988R12. The paper amended the Mandates: element of the primary template's transform to use this definition. The fact that the same wording has not been applied to optional<T&>::transform as well looks like an oversight. I would suggest to apply it.

[2025-10-16; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

As optional<remove_cv_t<invoke_result_t<F, T&>>> is part of the signature (return type), we never enter the body to trigger the Mandates, so it's already implicitly ill-formed if the result of f is not a valid contained type. It's worth clarifying that though."

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.4.7 [optional.ref.monadic] as indicated:

   template<class F>
     constexpr optional<remove_cv_t<invoke_result_t<F, T&>>> transform(F&& f) const;
   

   -4- Let U be remove_cv_t<invoke_result_t<F, T&>>.

   -5- Mandates: U is a valid contained type for optional. The declaration

   U u(invoke(std::forward<F>(f), *val ));
   

   is well-formed for some invented variable u.

4300⁽ⁱ⁾. Missing Returns: element in optional<T&>::emplace

Section: 22.5.4.3 [optional.ref.assign] Status: Tentatively Ready Submitter: Giuseppe D'Angelo Opened: 2025-07-15 Last modified: 2025-08-28

Priority: Not Prioritized

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Discussion:

The specification for optional<T&>::emplace in 22.5.4.3 [optional.ref.assign] is not specifying the returned value via a Returns: element; however the function does indeed return something (a T&). Such a Returns: element is there for the primary template's emplace (cf. 22.5.3.4 [optional.assign]).

[2025-08-27; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.4.3 [optional.ref.assign] as indicated:

   template<class U>
     constexpr T& emplace(U&& u) noexcept(is_nothrow_constructible_v<T&, U>);
   

   -4- Constraints: […]

   -5- Effects: Equivalent to: convert-ref-init-val(std::forward<U>(u)).

   -?- Returns: *val.

4301⁽ⁱ⁾. condition_variable{_any}::wait_{for, until} should take timeout by value

Section: 32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany] Status: Tentatively Ready Submitter: Hui Xie Opened: 2025-07-19 Last modified: 2025-08-29

Priority: Not Prioritized

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Discussion:

At the moment, both condition_variable and condition_variable_any's wait_for and wait_until take the timeout time_point/duration by const reference. This can cause surprising behaviour. Given the following example (thanks to Tim Song):

struct Task {
  system_clock::time_point deadline;
  // stuff
};

std::mutex mtx;
std::condition_variable cv;
std::priority_queue<Task, vector<Task>, CompareDeadlines> queue;

// thread 1
std::unique_lock lck(mtx);
if (queue.empty()) { cv.wait(lck); }
else { cv.wait_until(lck, queue.top().deadline); }

// thread 2
std::lock_guard lck(mtx);
queue.push(/* some task */);
cv.notify_one();

From the user's point of view, it is sufficiently locked on both threads. However, due to the fact that the time_point is taken by reference, and that both libc++ and libstdc++'s implementation will read the value again after waking up, this will read a dangling reference of the time_point.

Another example related to this issue:

We (libc++) recently received a bug report on condition_variable{_any}::wait_{for, until}.

Basically the user claims that these functions take time_point/duration by const reference, if the user modifies the time_point/duration on another thread with the same mutex, they can get unexpected return value for condition_variable, and data race for conditional_variable_any.

Bug report here.

Reproducer (libstdc++ has the same behaviour as ours) on godbolt.

std::mutex mutex;
std::condition_variable cv;
auto timeout = std::chrono::steady_clock::time_point::max();

// Thread 1:
std::unique_lock lock(mutex);
const auto status = cv.wait_until(lock, timeout);

// Thread 2:
std::unique_lock lock(mutex);
cv.notify_one();
timeout = std::chrono::steady_clock::time_point::min();

So basically the problem was that when we return whether there is no_timeout or timeout at the end of the function, we read the const reference again, which can be changed since the beginning of the function. For condition_variable, it is "unexpected results" according to the user. And in conditional_variable_any, we actually unlock the user lock and acquire our internal lock, then read the input again, so this is actually a data race.

For wait_for, the spec has

Effects: Equivalent to: return wait_until(lock, chrono::steady_clock::now() + rel_time);

So the user can claim our implementation is not conforming because the spec says there needs to be a temporary time_point (now + duration) created and since it should operate on this temporary time_point. There shouldn't be any unexpected behaviour or data race .

For wait_until it is unclear whether the spec has implications that implementations are allowed to read abs_time while the user's lock is unlocked.

it is also unclear if an implementation is allowed to return timeout if cv indeed does not wait longer than the original value of timeout. If it is not allowed, implementations will have to make a local copy of the input rel_time or abs_time, which defeats the purpose of taking arguments by const reference.

For both of the examples, Ville has a great comment in the reflector:

It seems like a whole bag of problems goes away if these functions just take the timeout by value?

libc++ implementers have strong preference just changing the API to take these arguments by value, and it is not an ABI break for us as the function signature has changed.

[2025-08-29; Reflector poll]

Set status to Tentatively Ready after nine votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 32.7.4 [thread.condition.condvar] as indicated:

   namespace std {
     class condition_variable {
     public:
       […]
       template<class Predicate>
         void wait(unique_lock<mutex>& lock, Predicate pred);
       template<class Clock, class Duration>
         cv_status wait_until(unique_lock<mutex>& lock,
                              const chrono::time_point<Clock, Duration>& abs_time);
       template<class Clock, class Duration, class Predicate>
         bool wait_until(unique_lock<mutex>& lock,
                         const chrono::time_point<Clock, Duration>& abs_time,
                         Predicate pred);
       template<class Rep, class Period>
         cv_status wait_for(unique_lock<mutex>& lock,
                            const chrono::duration<Rep, Period>& rel_time);
       template<class Rep, class Period, class Predicate>
         bool wait_for(unique_lock<mutex>& lock,
                       const chrono::duration<Rep, Period>& rel_time,
                       Predicate pred);    
       […]
     };
   }
   

   […]

   template<class Clock, class Duration>
     cv_status wait_until(unique_lock<mutex>& lock,
                          const chrono::time_point<Clock, Duration>& abs_time);
   

   -17- Preconditions: […]

   […]

   template<class Rep, class Period>
     cv_status wait_for(unique_lock<mutex>& lock,
                        const chrono::duration<Rep, Period>& rel_time);
   

   -23- Preconditions: […]

   […]

   template<class Clock, class Duration, class Predicate>
     bool wait_until(unique_lock<mutex>& lock,
                     const chrono::time_point<Clock, Duration>& abs_time,
                     Predicate pred);
   

   -29- Preconditions: […]

   […]

   template<class Rep, class Period, class Predicate>
     bool wait_for(unique_lock<mutex>& lock,
                   const chrono::duration<Rep, Period>& rel_time,
                   Predicate pred);    
   

   -35- Preconditions: […]

   […]

2. Modify 32.7.5.1 [thread.condition.condvarany.general] as indicated:

   namespace std {
     class condition_variable_any {
     public:
       […]
       // 32.7.5.2 [thread.condvarany.wait], noninterruptible waits
       template<class Lock>
         void wait(Lock& lock);
       template<class Lock, class Predicate>
         void wait(Lock& lock, Predicate pred);
       
       template<class Lock, class Clock, class Duration>
         cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);
       template<class Lock, class Clock, class Duration, class Predicate>
         bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time,
                         Predicate pred);
       template<class Lock, class Rep, class Period>
         cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);
       template<class Lock, class Rep, class Period, class Predicate>
         bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);
      
       // 32.7.5.3 [thread.condvarany.intwait], interruptible waits
       template<class Lock, class Predicate>
         bool wait(Lock& lock, stop_token stoken, Predicate pred);
       template<class Lock, class Clock, class Duration, class Predicate>
         bool wait_until(Lock& lock, stop_token stoken,
                         const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);
       template<class Lock, class Rep, class Period, class Predicate>
         bool wait_for(Lock& lock, stop_token stoken,
                       const chrono::duration<Rep, Period>& rel_time, Predicate pred);
     };
   }
   

3. Modify 32.7.5.2 [thread.condvarany.wait] as indicated:

   […]

   template<class Lock, class Clock, class Duration>
     cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);
   

   -6- Effects: […]

   […]

   template<class Lock, class Rep, class Period>
     cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);
   

   -11- Effects: […]

   […]

   template<class Lock, class Clock, class Duration, class Predicate>
     bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time,
                     Predicate pred);
   

   -16- Effects: […]

   […]

   template<class Lock, class Rep, class Period, class Predicate>
     bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);
   

   -19- Effects: […]

4. Modify 32.7.5.3 [thread.condvarany.intwait] as indicated:

   […]

   template<class Lock, class Clock, class Duration, class Predicate>
     bool wait_until(Lock& lock, stop_token stoken,
                     const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);
   

   -7- Effects: […]

   […]

   template<class Lock, class Rep, class Period, class Predicate>
     bool wait_for(Lock& lock, stop_token stoken,
                   const chrono::duration<Rep, Period>& rel_time, Predicate pred);
   

   -13- Effects: […]

4305⁽ⁱ⁾. Missing user requirements on type_order template

Section: 17.12.7 [compare.type] Status: Tentatively Ready Submitter: Daniel Krügler Opened: 2025-07-27 Last modified: 2025-10-14

Priority: Not Prioritized

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Discussion:

The recently approved paper P2830R10 proposes to add the new type_order type traits to 17.12 [cmp] (and thus outside of 21.3 [type.traits]), which has the subtle and most likely unintended effect, that it doesn't fall under the general requirement expressed in 21.3.2 [meta.rqmts] p4,

Unless otherwise specified, the behavior of a program that adds specializations for any of the templates specified in 21.3.2 [meta.rqmts] is undefined.

and so in principle the explicit allowance specified in 16.4.5.2.1 [namespace.std] p2,

Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std […]

holds. So we need to add extra wording to the type_order specification in 17.12.7 [compare.type] to prohibit such program specializations.

This was reported shortly before the Sofia meeting during reflector discussion but seems to be forgotten before the final paper appeared on plenary.

During the reflector discussion two possible ways to solve this issue were pointed out:

1. The most simple one would mimic the wording in 21.3.2 [meta.rqmts] p4 quoted above.

2. Instead of introducing just another undefined opportunity to run into undefined behaviour it has been pointed out that we could follow the approach taken by std::initializer_list and make the program ill-formed in this case, as specified in 17.11.2 [initializer.list.syn] p2:

   If an explicit specialization or partial specialization of initializer_list is declared, the program is ill-formed.

I think ill-formed would be better. It shouldn't be difficult for implementations to have special cases that are disallowed.

Given the already existing experience with std::initializer_list the proposed wording below therefore follows the ill-formed program approach.

[2025-10-14; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 17.12.7 [compare.type] as indicated:

   template<class T, class U>
     struct type_order;
   

   -2- The name type_order denotes a Cpp17BinaryTypeTrait (21.3.2 [meta.rqmts]) with a base characteristic of integral_constant<strong_ordering, TYPE-ORDER(T, U)>.

   -?- If an explicit specialization or partial specialization of type_order is declared, the program is ill-formed.

   -3- Recommended practice: The order should be lexicographical on parameter-type-lists and template argument lists.

4312⁽ⁱ⁾. Const and value category mismatch for allocator_arg_t/allocator_arg in the description of uses-allocator construction

Section: 20.2.8.2 [allocator.uses.construction] Status: Tentatively Ready Submitter: Jiang An Opened: 2025-08-06 Last modified: 2025-10-14

Priority: Not Prioritized

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Discussion:

Currently, 20.2.8.2 [allocator.uses.construction] bullet 2.2 states:

Otherwise, if T has a constructor invocable as T(allocator_arg, alloc, args...) (leading-allocator convention), […]

However, when forming construction arguments in the utility functions, we're actually using cv-unqualified rvalue of allocator_arg_t, which can be inferred from using plain allocator_arg_t but not const allocator_arg_t& in 20.2.8.2 [allocator.uses.construction] bullet 5.2.

It seems that such mismatch was present even since C++11 (per N3337 [allocator.uses.construction]/1.2). If the use of plain allocator_arg_t is considered correct, I think we should fix the description.

[2025-10-14; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Unless the std::allocator_arg tag object is not supposed to be used, wouldn't it make more sense to preserve the "if T has a constructor invocable as T(allocator_arg, alloc, args...)" wording and change every allocator_arg_t into const allocator_arg_t&, so that we check for construction from the const tag object, and then actually use a const value in the constructor arguments. Strongly don't care though.

Proposed resolution:

This wording is relative to N5014.

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

   -2- The following utility functions support three conventions for passing alloc to a constructor:

   1. (2.1) — […]

   2. (2.2) — Otherwise, if T has a constructor invocable as T(allocator_argallocator_arg_t{}, alloc, args...) (leading-allocator convention), then uses-allocator construction chooses this constructor form.

   3. (2.3) — […]

4317⁽ⁱ⁾. The meaning of "resource" in the Cpp17Destructible requirements is undefined

Section: 16.4.4.2 [utility.arg.requirements] Status: Tentatively Ready Submitter: Jiang An Opened: 2025-08-15 Last modified: 2025-10-14

Priority: Not Prioritized

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Discussion:

The meaning of "resource" in the Cpp17Destructible requirements cannot be inferred from the standard wording and it seems unlikely that the standard will determine its meaning in the future. What are considered as resources generally depends on users' intent, so the standard shouldn't determine the well-definedness of a program execution due to it. Moreover, the wording doesn't seem to consider shared ownership, which can be represented by shared_ptr.

[2025-10-14; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 16.4.4.2 [utility.arg.requirements], Table 35 [tab:cpp17.destructible] as indicated:

   Table 35 — Cpp17Destructible requirements [tab:cpp17.destructible]

   Expression	Post-condition
   u.~T()	All resources owned by u are reclaimed, nNo exception is propagated.
   [Note 3: Array types and non-object types are not Cpp17Destructible. — end note]

4318⁽ⁱ⁾. Have hive::erase_if reevaluate end() to avoid UB

Section: 23.3.9.6 [hive.erasure] Status: Tentatively Ready Submitter: Frank Birbacher Opened: 2025-08-16 Last modified: 2025-08-29

Priority: Not Prioritized

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Discussion:

Background: https://github.com/cplusplus/draft/pull/8162

For 23.3.9.6 [hive.erasure] p2, the defining code must not cache the end-iterator. In case the last element of the sequence is removed, the past-the-end iterator will be invalidated. This will trigger UB in the loop condition. Instead, re-evaluate end() each time.

[2025-08-29; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

[Drafting note: There are other ways to fix this code while keeping the caching behaviour, but I don't see any particular reason to do so for the definition of the effects.]

1. Modify 23.3.9.6 [hive.erasure] as indicated:

   template<class T, class Allocator, class Predicate>
     typename hive<T, Allocator>::size_type
       erase_if(hive<T, Allocator>& c, Predicate pred);
   

   -2- Effects: Equivalent to:

   auto original_size = c.size();
   for (auto i = c.begin(), last = c.end(); i != lastc.end(); ) {
     if (pred(*i)) {
       i = c.erase(i);
     } else {
       ++i;
     }
   }
   return original_size - c.size();
   

4341⁽ⁱ⁾. Missing rvalue reference qualification for task::connect()

Section: 33.13.6.2 [task.class] Status: Tentatively Ready Submitter: Dietmar Kühl Opened: 2025-08-31 Last modified: 2025-10-17

Priority: Not Prioritized

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Discussion:

Coroutines can't be copied. Thus, a task can be connect() just once. To represent that task::connect() should be rvalue reference qualified but currently it isn't.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

"It's nice to rvalue qualify such a function, but it is not strictly necessary."

Proposed resolution:

In the synopsis in 33.13.6.2 [task.class] add rvalue reference qualification to task::connect():

namespace std::execution {
  template<class T, class Environment>
  class task {
    ...
    template<receiver Rcvr>
        state<Rcvr> connect(Rcvr&& rcvr) &&;
    ...
  }
}

In the specification in 33.13.6.3 [task.members] paragraph 3 add rvalue reference qualification to task::connect():

template<receiver Rcvr>
    state<Rcvr> connect(Rcvr&& rcvr) &&;

-3- Precondition: bool(handle) is true.

-4- Effects: Equivalent to:

    return state<Rcvr>(exchange(handle, {}), std::forward<Rcvr>(recv));

4342⁽ⁱ⁾. Missing rvalue reference qualification for task_scheduler::ts-sender::connect()

Section: 33.13.5 [exec.task.scheduler] Status: Tentatively Ready Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2025-10-17

Priority: Not Prioritized

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Discussion:

The result of schedule(sched) for a scheduler sched is only required to be movable. An object of this type may need to be forwarded to an operation state constructor by task_scheduler::ts-sender::connect. Thus, this function should be qualified with an rvalue reference.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Proposed resolution:

Add an rvalue qualifier to the declaration of connect in 33.13.5 [exec.task.scheduler] paragraph 8:

namespace std::execution {
  class task_scheduler::ts-sender {     // exposition only
  public:
    using sender_concept = sender_t;

    template<receiver Rcvr>
      state<Rcvr> connect(Rcvr&& rcvr) &&;
  };
}

In the specification in 33.13.5 [exec.task.scheduler] paragraph 10 add an rvalue qualifier to connect:

template<receiver Rcvr>
  state<Rcvr> connect(Rcvr&& rcvr) &&;

-10- Effects: Let r be an object of a type that models receiver and whose completion handlers result in invoking the corresponding completion handlers of rcvr or copy thereof. Returns an object of type state<Rcvr> containing an operation state object initialized with connect(SENDER(*this), std::move(r)).

4343⁽ⁱ⁾. Missing default template arguments for task

Section: 33.13.6.2 [task.class] Status: Tentatively Ready Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2025-10-17

Priority: Not Prioritized

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Discussion:

The design discussion of task describes defaults for the two template parameters T and Environment of task but these defaults are not reflected in the synopsis of 33.13.6.2 [task.class]. This is an oversight and should be fixed. The default for T should be void and the default for Environment should be env<> (the design paper used empty_env but this struct was replaced by the class template env by P3325R5).

There could be a counter argument to defining a default for the Environment template parameter: this type is used to determine various customizations of task, e.g., the allocator_type, the scheduler_type, and the stop_source_type. Leaving the type a required argument means that a future standard could choose a possibly better default than the types determined when the Environment doesn't define them. On the other hand, a future standard could provide a suitable alias with modified types under a different name and/or a different namespace. Based on the LEWG discussion on 2025-08-26 the direction is to add the default arguments.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Proposed resolution:

Add default template arguments for task for T = void and Environment = env<> in the synopsis of 33.13.6.2 [task.class]:

namespace std::execution {
  template<class T = void, class Environment = env<>>
  class task {
     ...
  };
}

4345⁽ⁱ⁾. task::promise_type::return_value default template parameter

Section: 33.13.6.5 [task.promise] Status: Tentatively Ready Submitter: Dietmar Kühl Opened: 2025-09-01 Last modified: 2025-10-17

Priority: Not Prioritized

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Discussion:

The template parameter V of task::promise_type::return_value doesn't have a default template argument specified. Specifying a default template argument of T would enable use of co_return { ... } which would be consistent with normal return statements. This feature was not discussed in the design paper but based on the LEWG discussion on 2025-08-26 it is considered to be more a bug fix than a new feature.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Proposed resolution:

Add a default template argument of T to the template parameter V of task::promise_type::return_value in the synopsis of 33.13.6.5 [task.promise]:

namespace std::execution {
  template<class T, class Environment>
  class task<T, Environment>::promise_type {
     ...
    template<typename V = T>
    void return_value(V&& value);
    ...
  };

}

4351⁽ⁱ⁾. integral-constant-like needs more remove_cvref_t

Section: 23.7.2.1 [span.syn] Status: Tentatively Ready Submitter: Jonathan Wakely Opened: 2025-09-05 Last modified: 2025-10-17

Priority: Not Prioritized

View all issues with Tentatively Ready status.

Discussion:

P2781R9 tweaked the definition of integral-constant-like to work with constant_wrapper, like so:

template<class T>
    concept integral-constant-like =                    // exposition only
      is_integral_v<remove_cvref_t<decltype(T::value)>> &&
      !is_same_v<bool, remove_const_t<decltype(T::value)>> &&
      convertible_to<T, decltype(T::value)> &&
      equality_comparable_with<T, decltype(T::value)> &&
      bool_constant<T() == T::value>::value &&
      bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;

This was done so that cw<5> models the concept, but it needs an additional tweak so that cw<true> does not model it.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.7.2.1 [span.syn] as indicated:

   template<class T>
       concept integral-constant-like =                    // exposition only
         is_integral_v<remove_cvref_t<decltype(T::value)>> &&
         !is_same_v<bool, remove_cvrefconst_t<decltype(T::value)>> &&
         convertible_to<T, decltype(T::value)> &&
         equality_comparable_with<T, decltype(T::value)> &&
         bool_constant<T() == T::value>::value &&
         bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;
   

4362⁽ⁱ⁾. Inconsistent usage of constexpr for inplace_stop_token and inplace_stop_source

Section: 32.3.8 [stoptoken.inplace] Status: Tentatively NAD Submitter: Lewis Baker Opened: 2025-08-28 Last modified: 2025-10-17

Priority: Not Prioritized

View all issues with Tentatively NAD status.

Discussion:

The inplace_stop_source::get_token() member function is declared constexpr, but there are no constexpr member-functions declared on inplace_stop_token, making the utility of being able to call this member function during constant evaluation limited.

Should the member functions of inplace_stop_token also be declared constexpr? i.e. operator==, swap(), stop_possible() and stop_requested().

The operator== and stop_possible() and swap() member functions should be able to be made constexpr trivially as they are just required to compare/modify pointers to the associated stop source.

The stop_requested() member function is specified to be equivalent to calling stop_requested() on the associated inplace_stop_source (if any), which is not currently declared constexpr primarily because its implementation requires synchronisation/atomic operations.

Now that std::atomic operations are now constexpr, it may be possible/appropriate for stop_requested() on both inplace_stop_source and inplace_stop_token to also be declared constexpr.

[2025-10-17; Reflector poll. Status changed: New → Tentatively NAD.]

This allows constant-initializing a token, it's basically a constructor. Other member functions don't need to be constexpr, similar to how std::mutex::lock() doesn't need to be constexpr for constant-init of std::mutex to be useful.

Proposed resolution:

This wording is relative to N5014.

[Drafting note:: This is the minimum proposed wording change. Additionally, consider adding constexpr to the declaration of inplace_stop_token::stop_requested() (in 32.3.8.1 [stoptoken.inplace.general] and 32.3.8.2 [stoptoken.inplace.mem]) and to inplace_stop_source::stop_requested() (in 32.3.9.1 [stopsource.inplace.general] and 32.3.9.3 [stopsource.inplace.mem])]

1. Modify 32.3.8.1 [stoptoken.inplace.general], class inplace_stop_token synopsis, as indicated:

   namespace std {
     class inplace_stop_token {
     public:
       template<class CallbackFn>
         using callback_type = inplace_stop_callback<CallbackFn>;
       
       constexpr inplace_stop_token() = default;
       constexpr bool operator==(const inplace_stop_token&) const = default;
       
       // 32.3.8.2 [stoptoken.inplace.mem], member functions
       bool stop_requested() const noexcept;
       constexpr bool stop_possible() const noexcept;
       constexpr void swap(inplace_stop_token&) noexcept;
       
     private:
       const inplace_stop_source* stop-source = nullptr; // exposition only
     };
   }
   

2. Modify 32.3.8.2 [stoptoken.inplace.mem] as indicated:

   [Drafting note:: As a drive-by fix this adds the missing return type bool to the stop_possible() prototype]

   constexpr void swap(inplace_stop_token& rhs) noexcept;
   

   -1- Effects: Exchanges the values of stop-source and rhs.stop-source.

   […]

   constexpr bool stop_possible() const noexcept;
   

   -4- Returns: stop-source != nullptr.

4366⁽ⁱ⁾. Heterogeneous comparison of expected may be ill-formed

Section: 22.8.6.8 [expected.object.eq], 22.8.7.8 [expected.void.eq] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2025-09-06 Last modified: 2025-10-16

Priority: Not Prioritized

View all issues with Tentatively Ready status.

Discussion:

These comparison functions all explicitly static_cast the result of the underlying comparison to bool. However, the Constraints only require the implicit conversion, not the explicit one (i.e., "convertible to bool" rather than "models boolean-testable").

This means that in some pathological cases it will lead to hard errors (demo):

#include <expected>

struct E1 {};
struct E2 {};

struct Bool {
  operator bool() const;
  explicit operator bool() = delete;
};
Bool operator==(E1, E2);

int main() {
  std::unexpected e1{E1{}};
  std::unexpected e2{E2{}};
  return std::expected<int, E1>{e1} == e2; // fire
}

It is reasonable to specify return consistency with actual Constraints.

[2025-10-16; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Related to LWG 4366⁽ⁱ⁾, but the wording styles are inconsistent. optional uses "Effects: Equivalent to ..." and expected just uses Returns:.

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.8.6.8 [expected.object.eq] as indicated:

   template<class T2> friend constexpr bool operator==(const expected& x, const T2& v);
   

   -3- Constraints: T2 is not a specialization of expected. The expression *x == v is well-formed and its result is convertible to bool.

   [Note 1: T need not be Cpp17EqualityComparable. — end note]

   -4- Returns: If x.has_value() is true, && static_cast<bool>(*x == v); otherwise false.

   template<class E2> friend constexpr bool operator==(const expected& x, const unexpected<E2>& e);
   

   -5- Constraints: The expression x.error() == e.error() is well-formed and its result is convertible to bool.

   -6- Returns: If !x.has_value() is true, && static_cast<bool>(x.error() == e.error()); otherwise false.

2. Modify 22.8.7.8 [expected.void.eq] as indicated:

   template<class T2, class E2> requires is_void_v<T2>
     friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);
   

   -1- Constraints: The expression x.error() == y.error() is well-formed and its result is convertible to bool.

   -2- Returns: If x.has_value() does not equal y.has_value(), false; otherwise if x.has_value() is true, true; otherwise || static_cast<bool>(x.error() == y.error()).

   template<class E2>
     friend constexpr bool operator==(const expected& x, const unexpected<E2>& e);
   

   -3- Constraints: The expression x.error() == e.error() is well-formed and its result is convertible to bool.

   -4- Returns: If !x.has_value() is true, && static_cast<bool>(x.error() == e.error()) ; otherwise false.

4370⁽ⁱ⁾. Comparison of optional<T> to T may be ill-formed

Section: 22.5.9 [optional.comp.with.t] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2025-09-06 Last modified: 2025-10-16

Priority: Not Prioritized

View all other issues in [optional.comp.with.t].

View all issues with Tentatively Ready status.

Discussion:

When comparing an optional with its value type, the current wording specifies that the result is the ternary expression of x.has_value() ? *x == v : false, where *x == v returns a result that can be implicitly converted to bool.

However, when the result can also be constructed using bool (which is common), the ternary operation will be ill-formed due to ambiguity (demo):

#include <optional>

struct Bool {
  Bool(bool);
  operator bool() const;
};

struct S {
  Bool operator==(S) const;
};

int main() {
  return std::optional<S>{} == S{}; // fire
}

[2025-10-16; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

"Alternatively could keep the conditional operator but cast one side to bool, but that would do an explicit conversion, which might not be what we want."

"Should just require boolean-testable."

Related to LWG 4366⁽ⁱ⁾.

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.9 [optional.comp.with.t] as indicated:

   template<class T, class U> constexpr bool operator==(const optional<T>& x, const U& v);
   

   -1- Constraints: U is not a specialization of optional. The expression *x == v is well-formed and its result is convertible to bool.

   [Note 1: T need not be Cpp17EqualityComparable. — end note]

   -2- Effects: Equivalent to: return x.has_value() ? *x == v : false;

   if (x.has_value())
     return *x == v;
   return false;
   

   template<class T, class U> constexpr bool operator==(const T& v, const optional<U>& x);
   

   -3- Constraints: T is not a specialization of optional. The expression v == *x is well-formed and its result is convertible to bool.

   -4- Effects: Equivalent to: return x.has_value() ? v == *x : false;

   if (x.has_value())
     return v == *x;
   return false;
   

   template<class T, class U> constexpr bool operator!=(const optional<T>& x, const U& v);
   

   -5- Constraints: U is not a specialization of optional. The expression *x != v is well-formed and its result is convertible to bool.

   -6- Effects: Equivalent to: return x.has_value() ? *x != v : true;

   if (x.has_value())
     return *x != v;
   return true;
   

   template<class T, class U> constexpr bool operator!=(const T& v, const optional<U>& x);
   

   -7- Constraints: T is not a specialization of optional. The expression v != *x is well-formed and its result is convertible to bool.

   -8- Effects: Equivalent to: return x.has_value() ? v != *x : true;

   if (x.has_value())
     return v != *x;
   return true;
   

   template<class T, class U> constexpr bool operator<(const optional<T>& x, const U& v);
   

   -9- Constraints: U is not a specialization of optional. The expression *x < v is well-formed and its result is convertible to bool.

   -10- Effects: Equivalent to: return x.has_value() ? *x < v : true;

   if (x.has_value())
     return *x < v;
   return true;
   

   template<class T, class U> constexpr bool operator<(const T& v, const optional<U>& x);
   

   -11- Constraints: T is not a specialization of optional. The expression v < *x is well-formed and its result is convertible to bool.

   -12- Effects: Equivalent to: return x.has_value() ? v < *x : false;

   if (x.has_value())
     return v < *x;
   return false;
   

   template<class T, class U> constexpr bool operator>(const optional<T>& x, const U& v);
   

   -13- Constraints: U is not a specialization of optional. The expression *x > v is well-formed and its result is convertible to bool.

   -14- Effects: Equivalent to: return x.has_value() ? *x > v : false;

   if (x.has_value())
     return *x > v;
   return false;
   

   template<class T, class U> constexpr bool operator>(const T& v, const optional<U>& x);
   

   -15- Constraints: T is not a specialization of optional. The expression v > *x is well-formed and its result is convertible to bool.

   -16- Effects: Equivalent to: return x.has_value() ? v > *x : true;

   if (x.has_value())
     return v > *x;
   return true;
   

   template<class T, class U> constexpr bool operator<=(const optional<T>& x, const U& v);
   

   -17- Constraints: U is not a specialization of optional. The expression *x <= v is well-formed and its result is convertible to bool.

   -18- Effects: Equivalent to: return x.has_value() ? *x <= v : true;

   if (x.has_value())
     return *x <= v;
   return true;
   

   template<class T, class U> constexpr bool operator<=(const T& v, const optional<U>& x);
   

   -19- Constraints: T is not a specialization of optional. The expression v <= *x is well-formed and its result is convertible to bool.

   -20- Effects: Equivalent to: return x.has_value() ? v <= *x : false;

   if (x.has_value())
     return v <= *x;
   return false;
   

   template<class T, class U> constexpr bool operator>=(const optional<T>& x, const U& v);
   

   -21- Constraints: U is not a specialization of optional. The expression *x >= v is well-formed and its result is convertible to bool.

   -22- Effects: Equivalent to: return x.has_value() ? *x >= v : false;

   if (x.has_value())
     return *x >= v;
   return false;
   

   template<class T, class U> constexpr bool operator>=(const T& v, const optional<U>& x);
   

   -23- Constraints: T is not a specialization of optional. The expression v >= *x is well-formed and its result is convertible to bool.

   -24- Effects: Equivalent to: return x.has_value() ? v >= *x : true;

   if (x.has_value())
     return v >= *x;
   return true;
   

4372⁽ⁱ⁾. Weaken Mandates: for dynamic padding values in padded layouts

Section: 23.7.3.4.8.1 [mdspan.layout.leftpad.overview], 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] Status: Tentatively Ready Submitter: Luc Grosheintz Opened: 2025-09-09 Last modified: 2025-10-17

Priority: Not Prioritized

View all issues with Tentatively Ready status.

Discussion:

Two new layouts were added to <mdspan> in C++26. Both have a template parameter size_t PaddingValue. This value is allowed to be std::dynamic_extent to signal that the padding value isn't known at compile time.

A class Mandates: element (in 23.7.3.4.8.1 [mdspan.layout.leftpad.overview] (5.2) and 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] (5.2), respectively) requires (unconditionally) that

• PaddingValue is representable as a value of index_type.

Since std::dynamic_extent is defined as size_t(-1) (in 23.7.2.1 [span.syn]) this immediately prohibits all dynamically padded layout mappings for any index_type for which:

numeric_limit<index_type>::max() < numeric_limit<size_t>::max()

One example is int on a 64-bit system.

The proposed resolution states that the modified representability Mandates: element holds for rank <= 1, even though in that case the PaddingValue has no other effect. Hence, the Mandates: element could be weakened further.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

"This matches the wording in 23.7.3.3.1 [mdspan.extents.overview] 1.2"

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.7.3.4.8.1 [mdspan.layout.leftpad.overview] as indicated:

   -5- Mandates:

   1. (5.1) — […]

   2. (5.2) — if padding_value is not equal to dynamic_extent, then padding_value is representable as a value of type index_type.

   3. (5.3) — […]

   4. (5.4) — […]

2. Modify 23.7.3.4.9.1 [mdspan.layout.rightpad.overview] as indicated:

   -5- Mandates:

   1. (5.1) — […]

   2. (5.2) — if padding_value is not equal to dynamic_extent, then padding_value is representable as a value of type index_type.

   3. (5.3) — […]

   4. (5.4) — […]

4391⁽ⁱ⁾. Ambiguities of simd::basic_vec constructor

Section: 29.10.7.2 [simd.ctor] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2025-09-29 Last modified: 2025-10-17

Priority: Not Prioritized

View other active issues in [simd.ctor].

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Discussion:

The broadcasting, generator-based, and range constructors of simd::basic_vec all take a single argument, and their constraints are not mutually exclusive.

This means that when a type satisfies both characteristics, such as a range that can be converted to a value_type, this will lead to ambiguity:

#include <simd>

struct S {
  operator double() const;       // basic_vec(U&& value)
  
  double operator()(int) const;  // basic_vec(G&& gen)

  double* begin() const;         // basic_vec(R&& r, flags<Flags...> = {});
  double* end() const;
  constexpr static int size() { return 2; }
};

int main() {
  std::simd::vec<double> simd(S{}); // error: call of overloaded 'basic_simd(S)' is ambiguous
}

Do we need more constraints, similar to the one in string_view(R&& r) that requires R not to be convertible to const char*, to make the above work, i.e., only invoke the broadcasting constructor?

[2025-10-17; Reflector poll. Status changed: New → Tentatively NAD.]

Users of such types should do disambiguation explicitly, basic_vec should not guess what they mean.

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.7.2 [simd.ctor] as indicated:

   template<class G> constexpr explicit basic_vec(G&& gen);
   

   -8- Let Fromi denote the type decltype(gen(integral_constant<simd-size-type, i>())).

   -9- Constraints:

   1. (9.?) — constructible_from<value_type, G> is false.

   2. (9.?) — Fromi satisfies convertible_to<value_type> for all i in the range of [0, size()). In addition, for all i in the range of [0, size()), if Fromi is an arithmetic type, conversion from Fromi to value_type is value-preserving.

   […]

   template<class R, class... Flags>
     constexpr basic_vec(R&& r, flags<Flags...> = {});
   template<class R, class... Flags>
     constexpr basic_vec(R&& r, const mask_type& mask, flags<Flags...> = {});
   

   -12- Let mask be mask_type(true) for the overload with no mask parameter.

   -13- Constraints:

   1. (13.1) — R models ranges::contiguous_range and ranges::sized_range,

   2. (13.2) — ranges::size(r) is a constant expression, and

   3. (13.3) — ranges::size(r) is equal to size().,

   4. (13.?) — constructible_from<value_type, R> is false, and

   5. (13.?) — r(integral_constant<simd-size-type, 0>()) is not a valid expression.

4398⁽ⁱ⁾. enable_nonlocking_formatter_optimization should be disabled for container adaptors

Section: 23.6.2 [queue.syn], 23.6.5 [stack.syn] Status: Tentatively Ready Submitter: Tomasz Kamiński Opened: 2025-10-02 Last modified: 2025-10-17

Priority: 2

View all issues with Tentatively Ready status.

Discussion:

As the standard currently defines formatters for queue, prioriy_queue, and stack enable_nonlocking_formatter_optimization is specialized to true for these adaptors per 28.5.6.4 [format.formatter.spec] p3:

Unless specified otherwise, for each type T for which a formatter specialization is provided by the library, each of the headers provides the following specialization:

template<> inline constexpr bool enable_nonlocking_formatter_optimization<T> = true;

However, formatting an adaptor requires formatting of the underlying range in terms of ranges::ref_view, and we disable the nonlocking_optimizations for all ranges, including ranges::ref_view.

This problem does not occur for the flat_set, flat_map adaptors, which are also ranges, but unlike stack etc. they do not have a specialized formatter. They use the formatter specialization for ranges and we already disable the optimization for that formatter.

The proposed wording has recently been implemented in gcc's libstdc++.

[2025-10-14; Reflector poll]

Set priority to 2 after reflector poll.

This is a duplicate of LWG 4146⁽ⁱ⁾, with a different proposed resolution.

[2025-10-17; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.6.2 [queue.syn], header <queue> synopsis, as indicated:

   […]
   // 23.6.13 [container.adaptors.format], formatter specialization for queue
   template<class charT, class T, formattable<charT> Container>
     struct formatter<queue<T, Container>, charT>;
     
   template<class T, class Container>
     constexpr bool enable_nonlocking_formatter_optimization<queue<T, Container>> = false;
   
   // 23.6.4 [priority.queue], class template priority_queue
   template<class T, class Container = vector<T>,
            class Compare = less<typename Container::value_type>>
     class priority_queue;
   […]
   // 23.6.13 [container.adaptors.format], formatter specialization for priority_queue
   template<class charT, class T, formattable<charT> Container, class Compare>
     struct formatter<priority_queue<T, Container, Compare>, charT>;
     
   template<class T, class Container, class Compare>
     constexpr bool enable_nonlocking_formatter_optimization<priority_queue<T, Container, Compare>> = false;
   […]
   

2. Modify 23.6.5 [stack.syn], header <stack> synopsis, as indicated:

   […]
   
   // 23.6.13 [container.adaptors.format], formatter specialization for stack
   template<class charT, class T, formattable<charT> Container>
     struct formatter<stack<T, Container>, charT>;
   
   template<class T, class Container>
     constexpr bool enable_nonlocking_formatter_optimization<stack<T, Container>> = false;
   
   […]
   

4399⁽ⁱ⁾. enable_nonlocking_formatter_optimization for pair and tuple needs remove_cvref_t

Section: 28.5.9 [format.tuple] Status: Tentatively Ready Submitter: Tomasz Kamiński Opened: 2025-10-02 Last modified: 2025-10-17

Priority: Not Prioritized

View all issues with Tentatively Ready status.

Discussion:

The enable_nonlocking_formatter_optimization variable template is specialized only for cv-unqualified types. However, the specialization for pair and tuple does not remove the references and cv-qualifiers from the elements:

template<class... Ts> 
  constexpr bool enable_nonlocking_formatter_optimization<pair-or-tuple<Ts...>> = 
    (enable_nonlocking_formatter_optimization<Ts> && ...);

As consequence pair<const std::string, int> or pair<const std::string&, int&> (map and flat_map reference types) will not use unbuffered prints.

The proposed wording has recently been implemented in gcc's libstdc++.

[2025-10-17; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 28.5.9 [format.tuple] as indicated:

   -1- For each of pair and tuple, the library provides the following formatter specialization where pair-or-tuple is the name of the template:

   namespace std {
     […]
     
     template<class... Ts> 
       constexpr bool enable_nonlocking_formatter_optimization<pair-or-tuple<Ts...>> = 
         (enable_nonlocking_formatter_optimization<remove_cvref_t<Ts>> && ...);
   }
   

4403⁽ⁱ⁾. simd::basic_vec CTAD misses difference type casting

Section: 29.10.7.2 [simd.ctor] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2025-10-04 Last modified: 2025-10-17

Priority: Not Prioritized

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Discussion:

Currently, basic_vec can take an object r of range type R whose size is a constant expression and deduced to vec<ranges::range_value_t<R>, ranges::size(r)>.

However, such a deduced type is ill-formed when R has a an integer-class type size which cannot be implicitly converted to simd-size-type, which is a signed integer type.

It is necessary to perform difference type casting here, and the narrowing conversion will still correctly be rejected due to the constructor's constraints.

[2025-10-17; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.7.2 [simd.ctor] as indicated:

   template<class R, class... Ts>
     basic_vec(R&& r, Ts...) -> see below;
   

   -17- Constraints:

   1. (17.1) — R models ranges::contiguous_range and ranges::sized_range, and

   2. (17.2) — ranges::size(r) is a constant expression.

   -18- Remarks: The deduced type is equivalent to vec<ranges::range_value_t<R>, static_cast<simd-size-type>(ranges::size(r))>

4407⁽ⁱ⁾. constexpr-wrapper-like needs remove_cvref_t in simd::basic_vec constructor

Section: 29.10.7.2 [simd.ctor] Status: Tentatively Ready Submitter: Hewill Kang Opened: 2025-10-05 Last modified: 2025-10-17

Priority: Not Prioritized

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Discussion:

decltype(From::value) would be const int& if From is a type of std::cw<42>, so the reference also needs to be removed for checking the arithmetic type.

[2025-10-17; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.7.2 [simd.ctor] as indicated:

   template<class U> constexpr explicit(see below) basic_vec(U&& value) noexcept;
   

   -1- Let From denote the type remove_cvref_t<U>.

   […]

   -4- Remarks: The expression inside explicit evaluates to false if and only if U satisfies convertible_to<value_type>, and either

   1. (4.1) — From is not an arithmetic type and does not satisfy constexpr-wrapper-like,

   2. (4.2) — From is an arithmetic type and the conversion from From to value_type is value-preserving (29.10.1 [simd.general]), or

   3. (4.3) — From satisfies constexpr-wrapper-like, remove_cvref_tremove_const_t<decltype(From::value)> is an arithmetic type, and From::value is representable by value_type.