[PATCH v2] libstdc++: Implement P2408R5 Ranges iterators as inputs to non-Ranges algorithms
Tomasz Kaminski
tkaminsk@redhat.com
Mon Nov 24 13:57:37 GMT 2025
On Mon, Nov 24, 2025 at 5:44 AM Patrick Palka <ppalka@redhat.com> wrote:
> changes v2: more tests, and the one-argument version of
> __iter_concept_or_category is now constexpr instead of
> consteval
>
> -- >8 --
>
> From the paper's abstract:
>
> Change the iterator requirements for non-Ranges algorithms. For
> forward iterators and above that are constant iterators, instead of
> requiring that iterators meet certain Cpp17...Iterator requirements,
> require that the iterators model certain iterator concepts. This makes
> iterators from several standard views usable with non-Ranges
> algorithms that require forward iterators or above, such as the
> parallel overloads of most algorithms.
>
> This patch narrowly implements P2408R5 in C++23 mode and C++20 mode
> (as an extension). "Narrowly" because just as in the paper, we don't
> attempt to relax the requirements of mutable iterators even though it's
> possible in theory. Note that the PSTL algorithm requirements have
> already been relaxed in r15-3650. And we don't bother touching the
> deprecated parallel mode algorithms under include/parallel.
>
>From my reading of the paper the requirements were relaxed per iterator
parameters, and not algorithm, so in particular input range of unique_copy
should be relaxed (__unique_copy_1 uses iterator_traits<T>::value_type),
but c++20 may use readable_traits.
I think I would handle this second batch (iterators with input and output
range),
in a separate commit.
>
> The main workhorse of this paper is a new helper
> __iterator_concept_or_category that replaces existing uses of
> __iterator_category and iterator_traits::iterator_category with constant
> iterators. This new helper considers both the iterator_concept and
> iterator_category of the given iterator and returns the former if it's
> at least as strong as the latter. It's implemented in terms of the
> __promotable_iterator concept added in r16-2588 that made std::advance
> etc aware of C++20 iterators. Note that this helper doesn't check the
> actual C++20 iterator concepts (which check syntactic requirements along
> with iterator_concept if it's defined) and instead just checks for, and
> fully trusts, the iterator_concept defined by the iterator type. This
> is a slight deviation from the paper but IMHO it's consistent with the
> existing trusting of iterator_category and should be good enough in
> practice, though it means C++20 iterators that don't define
> iterator_concept will not be recognized as such by this helper even if
> they otherwise model the std::foo_iterator concept. (An undefined
> iterator_concept effectively defaults to random_access_iterator_tag.)
>
I think that user defined iterators may just specify the concept as
random_access_iterator,
and really on concept check to reduce the concept as appropriate. We try
hard to not
lie in the interator_concept tag, but the paper clearly states that concept
is what is checked.
But this also affects the previous prev/next optimization, so I think we
should handle it in separate patch.
>
> Most of the changes made here are effectively optimizations that don't
> have a semantic impact, e.g. for std::reduce. I added tests for a
> couple of algorithms where these changes are observable.
>
> The new __iterator_concept_or_category helper can probably also be used
> to fix PR100070 "Standard library container iterator-pair constructors
> should check C++20 iterator concepts".
>
> As a follow-up to this patch we should either remove the Boost-style
> concept checks, or relax them accordingly. It seems we're leaning
> towards removing them outright; see this thread:
> https://gcc.gnu.org/pipermail/libstdc++/2025-May/061568.html
>
> PR libstdc++/113299
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/deque.tcc (__copy_move_a1): Constrain with
> __is_any_random_access_iter instead of __is_random_access_iter.
> (__copy_move_backward_a1): Likewise.
> (__equal_aux1): Likewise.
> * include/bits/stl_algo.h (__search_n): Use
> __iter_concept_or_category instead of __iterator_category
> or iterator_traits::iterator_category.
> (find_end): Likewise.
> (__is_permutation): Likewise.
> (for_each_n): Likewise.
> (unique_copy): Likewise, for constant iterators.
> (sample): Likewise, for constant iterators.
> * include/bits/stl_algobase.h (__copy_move_a1): Adjust
> deque-based forward declaration accordingly.
> (__copy_move_backward_a1): Likewise.
> (__equal_aux1): Likewise.
> (__lexicographical_compare_impl): Use
> __iter_concept_or_category instead of __iterator_category or
> iterator_traits::iterator_category.
> (__equal4): Likewise.
> * include/bits/stl_iterator_base_funcs.h
> (__detail::__iter_category_converts_to_concept)
> (__detail::__promotable_iterator): Move to ...
> * include/bits/stl_iterator_base_types.h: ... here.
> (__iter_concept_or_category): New.
> (__is_any_random_access_iter): New.
> * include/bits/stl_uninitialized.h (uninitialized_copy_n):
> Use __iterator_concept_or_category instead of
> __iterator_category for the constant iterator __first.
> (__uninitialized_copy_n_pair): Likewise.
> * include/bits/version.def (algorithm_iterator_requirements):
> Define.
> * include/bits/version.h: Regenerate.
> * include/std/algorithm: Provide the FTM
> __cpp_lib_algorithm_iterator_requirements.
> * include/std/memory: Likewise.
> * include/std/numeric: Likewise.
> (reduce): Use __is_any_random_access_iter instead of
> __is_random_access_iter.
> (transform_reduce): Likewise.
> (inclusive_scan) [C++20]: Use ranges::iter_move(iter) instead
> of std::move(*iter).
> * testsuite/25_algorithms/find_end/p2408r5.cc: New test.
> * testsuite/25_algorithms/sample/p2408r5.cc: New test.
> * testsuite/25_algorithms/search_n/p2408r5.cc: New test.
> * testsuite/25_algorithms/unique_copy/p2408r5.cc: New test.
>
I would really preffer for the test to be named after what is tested
(C++20 only iterator) instead of paper. I do not remember the numbers
by hearth, co naming it c++20_iter or something similar sounds better to me.
> ---
> libstdc++-v3/include/bits/deque.tcc | 8 +-
> libstdc++-v3/include/bits/stl_algo.h | 28 +++----
> libstdc++-v3/include/bits/stl_algobase.h | 20 ++---
> .../include/bits/stl_iterator_base_funcs.h | 22 ------
> .../include/bits/stl_iterator_base_types.h | 74 +++++++++++++++++++
> libstdc++-v3/include/bits/stl_uninitialized.h | 4 +-
> libstdc++-v3/include/bits/version.def | 9 +++
> libstdc++-v3/include/bits/version.h | 10 +++
> libstdc++-v3/include/std/algorithm | 1 +
> libstdc++-v3/include/std/memory | 1 +
> libstdc++-v3/include/std/numeric | 13 +++-
> .../25_algorithms/find_end/p2408r5.cc | 23 ++++++
> .../testsuite/25_algorithms/sample/p2408r5.cc | 23 ++++++
> .../25_algorithms/search_n/p2408r5.cc | 21 ++++++
> .../25_algorithms/unique_copy/p2408r5.cc | 33 +++++++++
> 15 files changed, 234 insertions(+), 56 deletions(-)
> create mode 100644
> libstdc++-v3/testsuite/25_algorithms/find_end/p2408r5.cc
> create mode 100644 libstdc++-v3/testsuite/25_algorithms/sample/p2408r5.cc
> create mode 100644
> libstdc++-v3/testsuite/25_algorithms/search_n/p2408r5.cc
> create mode 100644
> libstdc++-v3/testsuite/25_algorithms/unique_copy/p2408r5.cc
>
> diff --git a/libstdc++-v3/include/bits/deque.tcc
> b/libstdc++-v3/include/bits/deque.tcc
> index 20b23fffc9e1..e409f3a3fc57 100644
> --- a/libstdc++-v3/include/bits/deque.tcc
> +++ b/libstdc++-v3/include/bits/deque.tcc
> @@ -1225,7 +1225,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>
> template<bool _IsMove, typename _II, typename _Tp>
> typename __gnu_cxx::__enable_if<
> - __is_random_access_iter<_II>::__value,
> + __is_any_random_access_iter<_II>::__value,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Tp&, _Tp*> >::__type
> __copy_move_a1(_II __first, _II __last,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Tp&, _Tp*>
> __result)
> @@ -1347,7 +1347,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>
> template<bool _IsMove, typename _II, typename _Tp>
> typename __gnu_cxx::__enable_if<
> - __is_random_access_iter<_II>::__value,
> + __is_any_random_access_iter<_II>::__value,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Tp&, _Tp*> >::__type
> __copy_move_backward_a1(_II __first, _II __last,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Tp&, _Tp*> __result)
> @@ -1406,7 +1406,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>
> template<typename _Tp, typename _Ref, typename _Ptr, typename _II>
> typename __gnu_cxx::__enable_if<
> - __is_random_access_iter<_II>::__value, bool>::__type
> + __is_any_random_access_iter<_II>::__value, bool>::__type
> __equal_aux1(_GLIBCXX_STD_C::_Deque_iterator<_Tp, _Ref, _Ptr>
> __first1,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Ref, _Ptr> __last1,
> _II __first2)
> @@ -1422,7 +1422,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>
> template<typename _II, typename _Tp, typename _Ref, typename _Ptr>
> typename __gnu_cxx::__enable_if<
> - __is_random_access_iter<_II>::__value, bool>::__type
> + __is_any_random_access_iter<_II>::__value, bool>::__type
> __equal_aux1(_II __first1, _II __last1,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Ref, _Ptr> __first2)
> {
> diff --git a/libstdc++-v3/include/bits/stl_algo.h
> b/libstdc++-v3/include/bits/stl_algo.h
> index bbd1800af779..0d8f7af9d272 100644
> --- a/libstdc++-v3/include/bits/stl_algo.h
> +++ b/libstdc++-v3/include/bits/stl_algo.h
> @@ -226,7 +226,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> return std::__find_if(__first, __last, __unary_pred);
>
> return std::__search_n_aux(__first, __last, __count, __unary_pred,
> - std::__iterator_category(__first));
> + std::__iter_concept_or_category(__first));
> }
>
> // find_end for forward iterators.
> @@ -337,8 +337,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __glibcxx_requires_valid_range(__first2, __last2);
>
> return std::__find_end(__first1, __last1, __first2, __last2,
> - std::__iterator_category(__first1),
> - std::__iterator_category(__first2),
> + std::__iter_concept_or_category(__first1),
> + std::__iter_concept_or_category(__first2),
> __gnu_cxx::__ops::equal_to());
> }
>
> @@ -388,8 +388,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __glibcxx_requires_valid_range(__first2, __last2);
>
> return std::__find_end(__first1, __last1, __first2, __last2,
> - std::__iterator_category(__first1),
> - std::__iterator_category(__first2),
> + std::__iter_concept_or_category(__first1),
> + std::__iter_concept_or_category(__first2),
> __comp);
> }
>
> @@ -3495,9 +3495,9 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
> _BinaryPredicate __pred)
> {
> using _Cat1
> - = typename iterator_traits<_ForwardIterator1>::iterator_category;
> + = __decltype(std::__iter_concept_or_category<_ForwardIterator1>());
> using _Cat2
> - = typename iterator_traits<_ForwardIterator2>::iterator_category;
> + = __decltype(std::__iter_concept_or_category<_ForwardIterator2>());
> using _It1_is_RA = is_same<_Cat1, random_access_iterator_tag>;
> using _It2_is_RA = is_same<_Cat2, random_access_iterator_tag>;
> constexpr bool __ra_iters = __and_<_It1_is_RA, _It2_is_RA>::value;
> @@ -3800,7 +3800,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
> for_each_n(_InputIterator __first, _Size __n, _Function __f)
> {
> auto __n2 = std::__size_to_integer(__n);
> - using _Cat = typename
> iterator_traits<_InputIterator>::iterator_category;
> + using _Cat =
> __decltype(std::__iter_concept_or_category<_InputIterator>());
> if constexpr (is_base_of_v<random_access_iterator_tag, _Cat>)
> {
> if (__n2 <= 0)
> @@ -4450,7 +4450,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
> return __result;
> return std::__unique_copy(__first, __last, __result,
> __gnu_cxx::__ops::equal_to(),
> - std::__iterator_category(__first));
> + std::__iter_concept_or_category(__first));
> }
>
> /**
> @@ -4491,7 +4491,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
> if (__first == __last)
> return __result;
> return std::__unique_copy(__first, __last, __result, __binary_pred,
> - std::__iterator_category(__first));
> + std::__iter_concept_or_category(__first));
> }
>
> #if __cplusplus <= 201103L || _GLIBCXX_USE_DEPRECATED
> @@ -5881,10 +5881,10 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
> _SampleIterator __out, _Distance __n,
> _UniformRandomBitGenerator&& __g)
> {
> - using __pop_cat = typename
> - std::iterator_traits<_PopulationIterator>::iterator_category;
> - using __samp_cat = typename
> - std::iterator_traits<_SampleIterator>::iterator_category;
> + using __pop_cat
> + =
> __decltype(std::__iter_concept_or_category<_PopulationIterator>());
> + using __samp_cat
> + = typename iterator_traits<_SampleIterator>::iterator_category;
>
> static_assert(
> __or_<is_convertible<__pop_cat, forward_iterator_tag>,
> diff --git a/libstdc++-v3/include/bits/stl_algobase.h
> b/libstdc++-v3/include/bits/stl_algobase.h
> index 443cbef76dee..701910afd837 100644
> --- a/libstdc++-v3/include/bits/stl_algobase.h
> +++ b/libstdc++-v3/include/bits/stl_algobase.h
> @@ -480,7 +480,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>
> template<bool _IsMove, typename _II, typename _Tp>
> typename __gnu_cxx::__enable_if<
> - __is_random_access_iter<_II>::__value,
> + __is_any_random_access_iter<_II>::__value,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Tp&, _Tp*> >::__type
> __copy_move_a1(_II, _II, _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Tp&,
> _Tp*>);
>
> @@ -769,7 +769,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>
> template<bool _IsMove, typename _II, typename _Tp>
> typename __gnu_cxx::__enable_if<
> - __is_random_access_iter<_II>::__value,
> + __is_any_random_access_iter<_II>::__value,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Tp&, _Tp*> >::__type
> __copy_move_backward_a1(_II, _II,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Tp&,
> _Tp*>);
> @@ -1219,7 +1219,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>
> template<typename _Tp, typename _Ref, typename _Ptr, typename _II>
> typename __gnu_cxx::__enable_if<
> - __is_random_access_iter<_II>::__value, bool>::__type
> + __is_any_random_access_iter<_II>::__value, bool>::__type
> __equal_aux1(_GLIBCXX_STD_C::_Deque_iterator<_Tp, _Ref, _Ptr>,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Ref, _Ptr>,
> _II);
> @@ -1233,7 +1233,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
>
> template<typename _II, typename _Tp, typename _Ref, typename _Ptr>
> typename __gnu_cxx::__enable_if<
> - __is_random_access_iter<_II>::__value, bool>::__type
> + __is_any_random_access_iter<_II>::__value, bool>::__type
> __equal_aux1(_II, _II,
> _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Ref, _Ptr>);
>
> @@ -1332,8 +1332,8 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
> _II2 __first2, _II2 __last2,
> _Compare __comp)
> {
> - typedef typename iterator_traits<_II1>::iterator_category
> _Category1;
> - typedef typename iterator_traits<_II2>::iterator_category
> _Category2;
> + typedef __decltype(std::__iter_concept_or_category<_II1>())
> _Category1;
> + typedef __decltype(std::__iter_concept_or_category<_II2>())
> _Category2;
> typedef std::__lc_rai<_Category1, _Category2> __rai_type;
>
> __last1 = __rai_type::__newlast1(__first1, __last1, __first2,
> __last2);
> @@ -1649,8 +1649,8 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
> __equal4(_II1 __first1, _II1 __last1, _II2 __first2, _II2 __last2)
> {
> using _RATag = random_access_iterator_tag;
> - using _Cat1 = typename iterator_traits<_II1>::iterator_category;
> - using _Cat2 = typename iterator_traits<_II2>::iterator_category;
> + using _Cat1 = __decltype(std::__iter_concept_or_category<_II1>());
> + using _Cat2 = __decltype(std::__iter_concept_or_category<_II2>());
> using _RAIters = __and_<is_same<_Cat1, _RATag>, is_same<_Cat2,
> _RATag>>;
> if constexpr (_RAIters::value)
> {
> @@ -1676,8 +1676,8 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
> _BinaryPredicate __binary_pred)
> {
> using _RATag = random_access_iterator_tag;
> - using _Cat1 = typename iterator_traits<_II1>::iterator_category;
> - using _Cat2 = typename iterator_traits<_II2>::iterator_category;
> + using _Cat1 = __decltype(std::__iter_concept_or_category<_II1>());
> + using _Cat2 = __decltype(std::__iter_concept_or_category<_II2>());
> using _RAIters = __and_<is_same<_Cat1, _RATag>, is_same<_Cat2,
> _RATag>>;
> if constexpr (_RAIters::value)
> {
> diff --git a/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
> b/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
> index 7c80e1423e45..c8b73e8e6c66 100644
> --- a/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
> +++ b/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
> @@ -130,28 +130,6 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
> __distance(_OutputIterator, _OutputIterator, output_iterator_tag) =
> delete;
> #endif
>
> -#ifdef __glibcxx_concepts
> -namespace __detail
> -{
> - // Satisfied if ITER_TRAITS(Iter)::iterator_category is valid and is
> - // at least as strong as ITER_TRAITS(Iter)::iterator_concept.
> - template<typename _Iter>
> - concept __iter_category_converts_to_concept
> - = convertible_to<typename __iter_traits<_Iter>::iterator_category,
> - typename __iter_traits<_Iter>::iterator_concept>;
> -
> - // Satisfied if the type is a C++20 iterator that defines
> iterator_concept,
> - // and its iterator_concept is stronger than its iterator_category (if
> any).
> - // Used by std::distance and std::advance to detect iterators which
> should
> - // dispatch based on their C++20 concept not their C++17 category.
> - template<typename _Iter>
> - concept __promotable_iterator
> - = input_iterator<_Iter>
> - && requires { typename __iter_traits<_Iter>::iterator_concept; }
> - && ! __iter_category_converts_to_concept<_Iter>;
> -} // namespace __detail
> -#endif
> -
> /**
> * @brief A generalization of pointer arithmetic.
> * @param __first An input iterator.
> diff --git a/libstdc++-v3/include/bits/stl_iterator_base_types.h
> b/libstdc++-v3/include/bits/stl_iterator_base_types.h
> index 0c34ad792f7a..1b6b3fae4520 100644
> --- a/libstdc++-v3/include/bits/stl_iterator_base_types.h
> +++ b/libstdc++-v3/include/bits/stl_iterator_base_types.h
> @@ -281,6 +281,80 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> { enum { __value = __is_base_of(random_access_iterator_tag, _Cat) };
> };
> #endif
>
> +#ifdef __glibcxx_concepts
> +namespace __detail
> +{
> + // Satisfied if ITER_TRAITS(Iter)::iterator_category is valid and is
> + // at least as strong as ITER_TRAITS(Iter)::iterator_concept.
> + template<typename _Iter>
> + concept __iter_category_converts_to_concept
> + = convertible_to<typename __iter_traits<_Iter>::iterator_category,
> + typename __iter_traits<_Iter>::iterator_concept>;
> +
> + // Satisfied if the type is a C++20 iterator that defines
> iterator_concept,
> + // and its iterator_concept is stronger than its iterator_category (if
> any).
> + // Used by std::distance and std::advance to detect iterators which
> should
> + // dispatch based on their C++20 concept not their C++17 category.
> + template<typename _Iter>
> + concept __promotable_iterator
> + = input_iterator<_Iter>
> + && requires { typename __iter_traits<_Iter>::iterator_concept; }
> + && ! __iter_category_converts_to_concept<_Iter>;
> +} // namespace __detail
> +#endif
> +
> +#if __glibcxx_algorithm_iterator_requirements // C++ >= 20
> + template<typename _Iter>
> + consteval auto
> + __iter_concept_or_category()
> + {
> + if constexpr (__detail::__promotable_iterator<_Iter>)
> + {
> + using __type = __detail::__iter_traits<_Iter>::iterator_concept;
> + if constexpr (derived_from<__type, random_access_iterator_tag>)
> + return random_access_iterator_tag{};
> + else
> + return __type{};
> + }
> + else
> + return typename iterator_traits<_Iter>::iterator_category();
> + }
> +
> + template<typename _Iter>
> + consteval auto
> + __iter_concept_or_category(const _Iter&)
> + { return std::__iter_concept_or_category<_Iter>(); }
> +#else
> + template<typename _Iter>
> + __attribute__((__always_inline__))
> + inline _GLIBCXX_CONSTEXPR
> + typename iterator_traits<_Iter>::iterator_category
> + __iter_concept_or_category()
> + { return typename iterator_traits<_Iter>::iterator_category(); }
> +
> + template<typename _Iter>
> + __attribute__((__always_inline__))
> + inline _GLIBCXX_CONSTEXPR
> + typename iterator_traits<_Iter>::iterator_category
> + __iter_concept_or_category(const _Iter&)
> + { return typename iterator_traits<_Iter>::iterator_category(); }
> +#endif
> +
> +#if __cplusplus >= 201103L
> + // Like __is_random_access_iter, but based off of
> __iter_concept_or_category
> + // instead of iterator_traits::iterator_category.
> + template<typename _Iter,
> + typename _Cat = __decltype(__iter_concept_or_category<_Iter>())>
> + struct __is_any_random_access_iter
> + : is_base_of<random_access_iterator_tag, _Cat>
> + { enum { __value = __is_any_random_access_iter::value }; };
> +#else
> + template<typename _Iter,
> + typename _Cat = __decltype(__iter_concept_or_category<_Iter>())>
> + struct __is_any_random_access_iter
> + { enum { __value = __is_base_of(random_access_iterator_tag, _Cat) };
> };
> +#endif
> +
> _GLIBCXX_END_NAMESPACE_VERSION
> } // namespace
>
> diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h
> b/libstdc++-v3/include/bits/stl_uninitialized.h
> index 70a564659814..96f740e86e7b 100644
> --- a/libstdc++-v3/include/bits/stl_uninitialized.h
> +++ b/libstdc++-v3/include/bits/stl_uninitialized.h
> @@ -1177,7 +1177,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> uninitialized_copy_n(_InputIterator __first, _Size __n,
> _ForwardIterator __result)
> { return std::__uninitialized_copy_n(__first, __n, __result,
> -
> std::__iterator_category(__first)); }
> +
> std::__iter_concept_or_category(__first)); }
>
> /// @cond undocumented
> template<typename _InputIterator, typename _Size, typename
> _ForwardIterator>
> @@ -1188,7 +1188,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> {
> return
> std::__uninitialized_copy_n_pair(__first, __n, __result,
> -
> std::__iterator_category(__first));
> +
> std::__iter_concept_or_category(__first));
> }
> /// @endcond
> #endif
> diff --git a/libstdc++-v3/include/bits/version.def
> b/libstdc++-v3/include/bits/version.def
> index 29ecf15c7e39..9177af610b16 100644
> --- a/libstdc++-v3/include/bits/version.def
> +++ b/libstdc++-v3/include/bits/version.def
> @@ -1972,6 +1972,15 @@ ftms = {
> };
> };
>
> +ftms = {
> + name = algorithm_iterator_requirements;
> + values = {
> + v = 202207;
> + // P2408R5 is a C++23 feature, but we support it in C++20.
> + cxxmin = 20;
> + };
> +};
> +
> ftms = {
> name = algorithm_default_value_type;
> values = {
> diff --git a/libstdc++-v3/include/bits/version.h
> b/libstdc++-v3/include/bits/version.h
> index 5901d27113d7..3a930db2239b 100644
> --- a/libstdc++-v3/include/bits/version.h
> +++ b/libstdc++-v3/include/bits/version.h
> @@ -2206,6 +2206,16 @@
> #endif /* !defined(__cpp_lib_observable_checkpoint) */
> #undef __glibcxx_want_observable_checkpoint
>
> +#if !defined(__cpp_lib_algorithm_iterator_requirements)
> +# if (__cplusplus >= 202002L)
> +# define __glibcxx_algorithm_iterator_requirements 202207L
> +# if defined(__glibcxx_want_all) ||
> defined(__glibcxx_want_algorithm_iterator_requirements)
> +# define __cpp_lib_algorithm_iterator_requirements 202207L
> +# endif
> +# endif
> +#endif /* !defined(__cpp_lib_algorithm_iterator_requirements) */
> +#undef __glibcxx_want_algorithm_iterator_requirements
> +
> #if !defined(__cpp_lib_algorithm_default_value_type)
> # if (__cplusplus > 202302L)
> # define __glibcxx_algorithm_default_value_type 202403L
> diff --git a/libstdc++-v3/include/std/algorithm
> b/libstdc++-v3/include/std/algorithm
> index 1563cdf2b17c..a39474df8b1d 100644
> --- a/libstdc++-v3/include/std/algorithm
> +++ b/libstdc++-v3/include/std/algorithm
> @@ -66,6 +66,7 @@
> #endif
>
> #define __glibcxx_want_algorithm_default_value_type
> +#define __glibcxx_want_algorithm_iterator_requirements
> #define __glibcxx_want_clamp
> #define __glibcxx_want_constexpr_algorithms
> #define __glibcxx_want_freestanding_algorithm
> diff --git a/libstdc++-v3/include/std/memory
> b/libstdc++-v3/include/std/memory
> index 9763760b8d61..52056d2f02a5 100644
> --- a/libstdc++-v3/include/std/memory
> +++ b/libstdc++-v3/include/std/memory
> @@ -102,6 +102,7 @@
> #endif
>
> #define __glibcxx_want_addressof_constexpr
> +#define __glibcxx_want_algorithm_iterator_requirements
> #define __glibcxx_want_allocator_traits_is_always_equal
> #define __glibcxx_want_assume_aligned
> #define __glibcxx_want_atomic_shared_ptr
> diff --git a/libstdc++-v3/include/std/numeric
> b/libstdc++-v3/include/std/numeric
> index cbabf031216e..8809d1db22c9 100644
> --- a/libstdc++-v3/include/std/numeric
> +++ b/libstdc++-v3/include/std/numeric
> @@ -81,6 +81,7 @@
> # include <limits>
> #endif
>
> +#define __glibcxx_want_algorithm_iterator_requirements
> #define __glibcxx_want_constexpr_numeric
> #define __glibcxx_want_gcd
> #define __glibcxx_want_gcd_lcm
> @@ -298,7 +299,7 @@ namespace __detail
> static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, __ref,
> _Tp&>);
> static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, _Tp&, _Tp&>);
> static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, __ref,
> __ref>);
> - if constexpr (__is_random_access_iter<_InputIterator>::value)
> + if constexpr (__is_any_random_access_iter<_InputIterator>::value)
> {
> while ((__last - __first) >= 4)
> {
> @@ -378,8 +379,8 @@ namespace __detail
> _BinaryOperation1 __binary_op1,
> _BinaryOperation2 __binary_op2)
> {
> - if constexpr (__and_v<__is_random_access_iter<_InputIterator1>,
> - __is_random_access_iter<_InputIterator2>>)
> + if constexpr (__and_v<__is_any_random_access_iter<_InputIterator1>,
> + __is_any_random_access_iter<_InputIterator2>>)
> {
> while ((__last1 - __first1) >= 4)
> {
> @@ -445,7 +446,7 @@ namespace __detail
> transform_reduce(_InputIterator __first, _InputIterator __last, _Tp
> __init,
> _BinaryOperation __binary_op, _UnaryOperation
> __unary_op)
> {
> - if constexpr (__is_random_access_iter<_InputIterator>::value)
> + if constexpr (__is_any_random_access_iter<_InputIterator>::value)
> {
> while ((__last - __first) >= 4)
> {
> @@ -582,7 +583,11 @@ namespace __detail
> {
> if (__first != __last)
> {
> +#if __cpp_lib_algorithm_iterator_requirements // C++ >= 20
>
+ auto __init = ranges::iter_move(__first);
>
The auto here does not work if the range is proxy range (zip_view),
in once of the discussion we considered having something like:
template<typename _It>
iter_value_t<_It> __iter_move_val(const _It& __it)
{ return ranges::iter_move(__it); }
We can also have declaration for __iter_move_for C++98 or earlier.
template<typename _It>
iterator_traits<_It>::value_type
iter_value_t<_It> __iter_move_val(const _It& __it)
{ return __GLIBCXX_MOVE(*__it); }
Pelase consider adding test with proxy ranges.
+#else
> auto __init = std::move(*__first);
>
+#endif
> *__result++ = __init;
> ++__first;
> if (__first != __last)
> diff --git a/libstdc++-v3/testsuite/25_algorithms/find_end/p2408r5.cc
> b/libstdc++-v3/testsuite/25_algorithms/find_end/p2408r5.cc
> new file mode 100644
> index 000000000000..687d8021a57a
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/25_algorithms/find_end/p2408r5.cc
> @@ -0,0 +1,23 @@
> +// Verify std::find_end is Ranges iterator aware as per P2408R5.
> +// { dg-do compile { target c++20 } }
> +
> +#include <algorithm>
> +#include <ranges>
> +#include <testsuite_hooks.h>
> +
> +constexpr bool
> +test01()
> +{
> + auto r = std::views::iota(0, 10);
> + auto s = std::views::iota(5, 10);
> + auto it = r.begin();
> + auto jt = s.begin();
> + static_assert( std::random_access_iterator<decltype(it)>);
> + static_assert(
> std::same_as<std::iterator_traits<decltype(it)>::iterator_category,
> + std::input_iterator_tag> );
> + it = std::find_end(it, it+10, jt, jt+5);
> + VERIFY( it == r.begin() + 5 );
> + return true;
> +}
> +
> +static_assert(test01());
> diff --git a/libstdc++-v3/testsuite/25_algorithms/sample/p2408r5.cc
> b/libstdc++-v3/testsuite/25_algorithms/sample/p2408r5.cc
> new file mode 100644
> index 000000000000..7f8a798fef96
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/25_algorithms/sample/p2408r5.cc
> @@ -0,0 +1,23 @@
> +// Verify std::sample is Ranges iterator aware as per P2408R5.
> +// { dg-do compile { target c++20 } }
> +
> +#include <algorithm>
> +#include <random>
> +#include <ranges>
> +#include <testsuite_hooks.h>
> +#include <testsuite_iterators.h>
> +
> +std::mt19937 rng;
> +
> +void
> +test01()
> +{
> + auto r = std::views::iota(10);
> + auto it = r.begin();
> + static_assert( std::random_access_iterator<decltype(it)>);
> + static_assert(
> std::same_as<std::iterator_traits<decltype(it)>::iterator_category,
> + std::input_iterator_tag> );
> + int buf[10];
> + __gnu_test::output_container<int> s(buf);
> + std::sample(it, it+10, s.begin(), 10, rng);
> +}
> diff --git a/libstdc++-v3/testsuite/25_algorithms/search_n/p2408r5.cc
> b/libstdc++-v3/testsuite/25_algorithms/search_n/p2408r5.cc
> new file mode 100644
> index 000000000000..9787358e6674
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/25_algorithms/search_n/p2408r5.cc
> @@ -0,0 +1,21 @@
> +// Verify std::search_n is Ranges iterator aware as per P2408R5.
> +// { dg-do compile { target c++20 } }
> +
> +#include <algorithm>
> +#include <ranges>
> +#include <testsuite_hooks.h>
> +
> +constexpr bool
> +test01()
> +{
> + auto r = std::views::iota(0, 10);
> + auto it = r.begin();
> + static_assert( std::random_access_iterator<decltype(it)>);
> + static_assert(
> std::same_as<std::iterator_traits<decltype(it)>::iterator_category,
> + std::input_iterator_tag> );
> + it = std::search_n(it, it+10, 1, 5);
> + VERIFY( it == r.begin() + 5 );
> + return true;
> +}
> +
> +static_assert(test01());
> diff --git a/libstdc++-v3/testsuite/25_algorithms/unique_copy/p2408r5.cc
> b/libstdc++-v3/testsuite/25_algorithms/unique_copy/p2408r5.cc
> new file mode 100644
> index 000000000000..9ecd5ff7c7e2
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/25_algorithms/unique_copy/p2408r5.cc
> @@ -0,0 +1,33 @@
> +// Verify std::unique_copy is Ranges iterator aware as per P2408R5.
> +// { dg-do compile { target c++20 } }
> +
> +#include <algorithm>
> +#include <ranges>
> +#include <testsuite_hooks.h>
> +#include <testsuite_iterators.h>
> +
> +struct noncopyable
> +{
> + constexpr operator int() { return 42; }
> + noncopyable() = default;
> + noncopyable(const noncopyable&) = delete;
> + noncopyable& operator=(const noncopyable&) = delete;
> + friend auto operator<=>(const noncopyable&, const noncopyable&) =
> default;
> +};
> +
> +constexpr bool
> +test01()
> +{
> + auto r = std::views::iota(10)
> + | std::views::transform([](int) { return noncopyable{}; });
> + auto it = r.begin();
> + static_assert( std::random_access_iterator<decltype(it)>);
> + static_assert(
> std::same_as<std::iterator_traits<decltype(it)>::iterator_category,
> + std::input_iterator_tag> );
> + int buf[10];
> + __gnu_test::input_container s(buf);
> + auto jt = std::unique_copy(it, it+10, s.begin());
> + return true;
> +}
> +
> +static_assert(test01());
> --
> 2.52.0.51.gdebbc87557
>
>
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