[libc++][ranges] Implement `ranges::nth_element`.

Differential Revision: https://reviews.llvm.org/D128149

libcxx/docs/Status/RangesAlgorithms.csv

@@ -76,7 +76,7 @@ Permutation,stable_partition,Not assigned,n/a,Not started
 Permutation,sort,Konstantin Varlamov,`D127557 <https://llvm.org/D127557>`_,✅
 Permutation,stable_sort,Konstantin Varlamov,`D127834 <https://llvm.org/D127834>`_,✅
 Permutation,partial_sort,Konstantin Varlamov,n/a,In progress
-Permutation,nth_element,Not assigned,n/a,Not started
+Permutation,nth_element,Konstantin Varlamov,`D128149 <https://llvm.org/D128149>`_,✅
 Permutation,inplace_merge,Not assigned,n/a,Not started
 Permutation,make_heap,Not assigned,n/a,Not started
 Permutation,push_heap,Not assigned,n/a,Not started

libcxx/include/CMakeLists.txt

@@ -103,6 +103,7 @@ set(files
   __algorithm/ranges_move.h
   __algorithm/ranges_move_backward.h
   __algorithm/ranges_none_of.h
+  __algorithm/ranges_nth_element.h
   __algorithm/ranges_remove.h
   __algorithm/ranges_remove_if.h
   __algorithm/ranges_replace.h

libcxx/include/__algorithm/nth_element.h

@@ -16,6 +16,7 @@
 #include <__debug>
 #include <__debug_utils/randomize_range.h>
 #include <__iterator/iterator_traits.h>
+#include <__utility/move.h>
 #include <__utility/swap.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -220,25 +221,35 @@ __nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _Rando
 }
 
 template <class _RandomAccessIterator, class _Compare>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-void
-nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
-{
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
+void __nth_element_impl(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last,
+                        _Compare& __comp) {
+  if (__nth == __last)
+    return;
+
   std::__debug_randomize_range(__first, __last);
-  typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
-  _VSTD::__nth_element<_Comp_ref>(__first, __nth, __last, __comp);
+
+  using _Comp_ref = typename __comp_ref_type<_Compare>::type;
+  std::__nth_element<_Comp_ref>(__first, __nth, __last, __comp);
+
   std::__debug_randomize_range(__first, __nth);
   if (__nth != __last) {
     std::__debug_randomize_range(++__nth, __last);
   }
 }
 
+template <class _RandomAccessIterator, class _Compare>
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
+void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last,
+                 _Compare __comp) {
+  std::__nth_element_impl(std::move(__first), std::move(__nth), std::move(__last), __comp);
+}
+
 template <class _RandomAccessIterator>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-void
-nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last)
-{
-    _VSTD::nth_element(__first, __nth, __last, __less<typename iterator_traits<_RandomAccessIterator>::value_type>());
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
+void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last) {
+  std::nth_element(std::move(__first), std::move(__nth), std::move(__last), __less<typename
+      iterator_traits<_RandomAccessIterator>::value_type>());
 }
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/__algorithm/ranges_nth_element.h

@@ -0,0 +1,79 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_RANGES_NTH_ELEMENT_H
+#define _LIBCPP___ALGORITHM_RANGES_NTH_ELEMENT_H
+
+#include <__algorithm/make_projected.h>
+#include <__algorithm/nth_element.h>
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/invoke.h>
+#include <__functional/ranges_operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
+#include <__iterator/projected.h>
+#include <__iterator/sortable.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/dangling.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+namespace __nth_element {
+
+struct __fn {
+  template <class _Iter, class _Sent, class _Comp, class _Proj>
+  _LIBCPP_HIDE_FROM_ABI constexpr static
+  _Iter __nth_element_fn_impl(_Iter __first, _Iter __nth, _Sent __last, _Comp& __comp, _Proj& __proj) {
+    auto __last_iter = ranges::next(__first, __last);
+
+    auto&& __projected_comp = ranges::__make_projected_comp(__comp, __proj);
+    std::__nth_element_impl(std::move(__first), std::move(__nth), __last_iter, __projected_comp);
+
+    return __last_iter;
+  }
+
+  template <random_access_iterator _Iter, sentinel_for<_Iter> _Sent, class _Comp = ranges::less, class _Proj = identity>
+    requires sortable<_Iter, _Comp, _Proj>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  _Iter operator()(_Iter __first, _Iter __nth, _Sent __last, _Comp __comp = {}, _Proj __proj = {}) const {
+    return __nth_element_fn_impl(std::move(__first), std::move(__nth), std::move(__last), __comp, __proj);
+  }
+
+  template <random_access_range _Range, class _Comp = ranges::less, class _Proj = identity>
+    requires sortable<iterator_t<_Range>, _Comp, _Proj>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  borrowed_iterator_t<_Range> operator()(_Range&& __r, iterator_t<_Range> __nth, _Comp __comp = {},
+                                         _Proj __proj = {}) const {
+    return __nth_element_fn_impl(ranges::begin(__r), std::move(__nth), ranges::end(__r), __comp, __proj);
+  }
+};
+
+} // namespace __nth_element
+
+inline namespace __cpo {
+  inline constexpr auto nth_element = __nth_element::__fn{};
+} // namespace __cpo
+} // namespace ranges
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+#endif // _LIBCPP___ALGORITHM_RANGES_NTH_ELEMENT_H

libcxx/include/algorithm

@@ -379,6 +379,17 @@ namespace ranges {
     constexpr borrowed_iterator_t<R>
       ranges::is_sorted_until(R&& r, Comp comp = {}, Proj proj = {});                       // since C++20
 
+  template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less,
+          class Proj = identity>
+    requires sortable<I, Comp, Proj>
+    constexpr I
+      ranges::nth_element(I first, I nth, S last, Comp comp = {}, Proj proj = {});            // since C++20
+
+  template<random_access_range R, class Comp = ranges::less, class Proj = identity>
+    requires sortable<iterator_t<R>, Comp, Proj>
+    constexpr borrowed_iterator_t<R>
+      ranges::nth_element(R&& r, iterator_t<R> nth, Comp comp = {}, Proj proj = {});          // since C++20
+
   template<forward_iterator I, sentinel_for<I> S, class T, class Proj = identity,
            indirect_strict_weak_order<const T*, projected<I, Proj>> Comp = ranges::less>
     constexpr I upper_bound(I first, S last, const T& value, Comp comp = {}, Proj proj = {}); // since C++20
@@ -1313,6 +1324,7 @@ template <class BidirectionalIterator, class Compare>
 #include <__algorithm/ranges_move.h>
 #include <__algorithm/ranges_move_backward.h>
 #include <__algorithm/ranges_none_of.h>
+#include <__algorithm/ranges_nth_element.h>
 #include <__algorithm/ranges_remove.h>
 #include <__algorithm/ranges_remove_if.h>
 #include <__algorithm/ranges_replace.h>

libcxx/include/module.modulemap.in

@@ -342,6 +342,7 @@ module std [system] {
       module ranges_move                     { private header "__algorithm/ranges_move.h" }
       module ranges_move_backward            { private header "__algorithm/ranges_move_backward.h" }
       module ranges_none_of                  { private header "__algorithm/ranges_none_of.h" }
+      module ranges_nth_element              { private header "__algorithm/ranges_nth_element.h" }
       module ranges_remove                   { private header "__algorithm/ranges_remove.h" }
       module ranges_remove_if                { private header "__algorithm/ranges_remove_if.h" }
       module ranges_replace                  { private header "__algorithm/ranges_replace.h" }

libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp

@@ -169,8 +169,8 @@ constexpr bool all_the_algorithms()
     //(void)std::ranges::next_permutation(a, Less(&copies)); assert(copies == 0);
     (void)std::ranges::none_of(first, last, UnaryTrue(&copies)); assert(copies == 0);
     (void)std::ranges::none_of(a, UnaryTrue(&copies)); assert(copies == 0);
-    //(void)std::ranges::nth_element(first, mid, last, Less(&copies)); assert(copies == 0);
-    //(void)std::ranges::nth_element(a, mid, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::nth_element(first, mid, last, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::nth_element(a, mid, Less(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort(first, mid, last, Less(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort(a, mid, Less(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort_copy(first, last, first2, mid2, Less(&copies)); assert(copies == 0);

libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp

@@ -152,8 +152,8 @@ constexpr bool all_the_algorithms()
     //(void)std::ranges::next_permutation(a, Less(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::none_of(first, last, UnaryTrue(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::none_of(a, UnaryTrue(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::nth_element(first, mid, last, Less(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::nth_element(a, mid, Less(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::nth_element(first, mid, last, Less(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::nth_element(a, mid, Less(), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort(first, mid, last, Less(), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort(a, mid, Less(), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::partial_sort_copy(first, last, first2, mid2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);

libcxx/test/libcxx/private_headers.verify.cpp

@@ -140,6 +140,7 @@ END-SCRIPT
 #include <__algorithm/ranges_move.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_move.h'}}
 #include <__algorithm/ranges_move_backward.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_move_backward.h'}}
 #include <__algorithm/ranges_none_of.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_none_of.h'}}
+#include <__algorithm/ranges_nth_element.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_nth_element.h'}}
 #include <__algorithm/ranges_remove.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_remove.h'}}
 #include <__algorithm/ranges_remove_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_remove_if.h'}}
 #include <__algorithm/ranges_replace.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_replace.h'}}

libcxx/test/std/algorithms/alg.sorting/alg.nth.element/ranges_nth_element.pass.cpp

@@ -0,0 +1,294 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// <algorithm>
+
+// template<random_­access_­iterator I, sentinel_­for<I> S, class Comp = ranges::less,
+//         class Proj = identity>
+//   requires sortable<I, Comp, Proj>
+//   constexpr I
+//     ranges::nth_element(I first, I nth, S last, Comp comp = {}, Proj proj = {});            // since C++20
+//
+// template<random_­access_­range R, class Comp = ranges::less, class Proj = identity>
+//   requires sortable<iterator_t<R>, Comp, Proj>
+//   constexpr borrowed_iterator_t<R>
+//     ranges::nth_element(R&& r, iterator_t<R> nth, Comp comp = {}, Proj proj = {});          // since C++20
+
+#include <algorithm>
+#include <array>
+#include <concepts>
+#include <functional>
+#include <iterator>
+#include <optional>
+#include <ranges>
+
+#include "almost_satisfies_types.h"
+#include "boolean_testable.h"
+#include "test_iterators.h"
+
+// SFINAE tests.
+
+using BadComparator = ComparatorNotCopyable<int*>;
+static_assert(!std::sortable<int*, BadComparator>);
+
+template <class Iter, class Sent = sentinel_wrapper<Iter>, class Comp = std::ranges::less>
+concept HasNthElementIt = requires(Iter first, Iter nth, Sent last, Comp comp) {
+  std::ranges::nth_element(first, nth, last, comp);
+};
+
+static_assert(HasNthElementIt<int*>);
+static_assert(!HasNthElementIt<RandomAccessIteratorNotDerivedFrom>);
+static_assert(!HasNthElementIt<RandomAccessIteratorBadIndex>);
+static_assert(!HasNthElementIt<int*, SentinelForNotSemiregular>);
+static_assert(!HasNthElementIt<int*, SentinelForNotWeaklyEqualityComparableWith>);
+static_assert(!HasNthElementIt<int*, int*, BadComparator>);
+static_assert(!HasNthElementIt<const int*>); // Doesn't satisfy `sortable`.
+
+template <class Range, class Comp = std::ranges::less>
+concept HasNthElementR = requires(Range range, std::ranges::iterator_t<Range> nth, Comp comp) {
+  std::ranges::nth_element(range, nth, comp);
+};
+
+static_assert(HasNthElementR<UncheckedRange<int*>>);
+static_assert(!HasNthElementR<RandomAccessRangeNotDerivedFrom>);
+static_assert(!HasNthElementR<RandomAccessRangeBadIndex>);
+static_assert(!HasNthElementR<UncheckedRange<int*, SentinelForNotSemiregular>>);
+static_assert(!HasNthElementR<UncheckedRange<int*, SentinelForNotWeaklyEqualityComparableWith>>);
+static_assert(!HasNthElementR<UncheckedRange<int*>, BadComparator>);
+static_assert(!HasNthElementR<UncheckedRange<const int*>>); // Doesn't satisfy `sortable`.
+
+template <size_t N, class T, class Iter>
+constexpr void verify_nth(const std::array<T, N>& partially_sorted, size_t nth_index, Iter last, T expected_nth) {
+  // Note that the exact output of `nth_element` is unspecified and may vary between implementations.
+
+  assert(base(last) == partially_sorted.end());
+
+  auto b = partially_sorted.begin();
+  auto nth = b + nth_index;
+  auto e = partially_sorted.end();
+  if (nth == e)
+    return;
+
+  assert(*nth == expected_nth);
+
+  // All elements on the left are <= nth.
+  assert(std::all_of(b, nth, [&](const auto& v) { return v <= *nth; }));
+  // All elements on the right are >= nth.
+  assert(std::all_of(nth, e, [&](const auto& v) { return v >= *nth; }));
+
+  {
+    auto sorted = partially_sorted;
+    std::ranges::sort(sorted);
+
+    // The element at index `n` is the same as if the range were fully sorted.
+    assert(sorted[nth_index] == *nth);
+  }
+}
+
+template <class Iter, class Sent, size_t N>
+constexpr void test_one(std::array<int, N> input, size_t nth_index, std::optional<int> expected_nth = {}) {
+  assert(expected_nth || nth_index == N);
+
+  { // (iterator, sentinel) overload.
+    auto partially_sorted = input;
+    auto b = Iter(partially_sorted.data());
+    auto nth = b + nth_index;
+    auto e = Sent(Iter(partially_sorted.data() + partially_sorted.size()));
+
+    std::same_as<Iter> decltype(auto) last = std::ranges::nth_element(b, nth, e);
+    if (nth_index != N) {
+      verify_nth(partially_sorted, nth_index, last, *expected_nth);
+    } else {
+      assert(partially_sorted == input);
+    }
+  }
+
+  { // (range) overload.
+    auto partially_sorted = input;
+    auto b = Iter(partially_sorted.data());
+    auto nth = b + nth_index;
+    auto e = Sent(Iter(partially_sorted.data() + partially_sorted.size()));
+    auto range = std::ranges::subrange(b, e);
+
+    std::same_as<Iter> decltype(auto) last = std::ranges::nth_element(range, nth);
+    if (nth_index != N) {
+      verify_nth(partially_sorted, nth_index, last, *expected_nth);
+    } else {
+      assert(partially_sorted == input);
+    }
+  }
+}
+
+template <class Iter, class Sent, size_t N>
+constexpr void test_all_cases(std::array<int, N> input) {
+  auto sorted = input;
+  std::sort(sorted.begin(), sorted.end());
+
+  for (int n = 0; n != N; ++n) {
+    test_one<Iter, Sent, N>(input, n, sorted[n]);
+  }
+  test_one<Iter, Sent, N>(input, N);
+}
+
+constexpr void test_iterators() {
+  auto check = []<class Iter, class Sent> {
+    // Empty sequence.
+    test_one<Iter, Sent, 0>({}, 0);
+
+    // 1-element sequence.
+    test_all_cases<Iter, Sent>(std::array{1});
+
+    // 2-element sequence.
+    test_all_cases<Iter, Sent>(std::array{2, 1});
+
+    // 3-element sequence.
+    test_all_cases<Iter, Sent>(std::array{2, 1, 3});
+
+    // Longer sequence.
+    test_all_cases<Iter, Sent>(std::array{2, 1, 3, 6, 8, 4, 11, 5});
+
+    // Longer sequence with duplicates.
+    test_all_cases<Iter, Sent>(std::array{2, 1, 3, 6, 2, 8, 6});
+
+    // All elements are the same.
+    test_all_cases<Iter, Sent>(std::array{1, 1, 1, 1});
+
+    { // nth element is in the right place.
+      std::array input = {6, 5, 3, 1, 4, 2};
+      constexpr size_t N = input.size();
+      test_one<Iter, Sent, N>(input, 2, /*expected_nth=*/3);
+    }
+
+    // Already sorted.
+    test_all_cases<Iter, Sent>(std::array{1, 2, 3, 4, 5, 6});
+
+    // Descending.
+    test_all_cases<Iter, Sent>(std::array{6, 5, 4, 3, 2, 1});
+
+    // Repeating pattern.
+    test_all_cases<Iter, Sent>(std::array{2, 1, 2, 1, 2, 1});
+  };
+
+  check.operator()<random_access_iterator<int*>, random_access_iterator<int*>>();
+  check.operator()<random_access_iterator<int*>, sentinel_wrapper<random_access_iterator<int*>>>();
+  check.operator()<contiguous_iterator<int*>, contiguous_iterator<int*>>();
+  check.operator()<contiguous_iterator<int*>, sentinel_wrapper<contiguous_iterator<int*>>>();
+  check.operator()<int*, int*>();
+  check.operator()<int*, sentinel_wrapper<int*>>();
+}
+
+constexpr bool test() {
+  test_iterators();
+
+  { // A custom comparator works.
+    const std::array input = {1, 2, 3, 4, 5};
+    std::ranges::greater comp;
+
+    {
+      auto in = input;
+      auto last = std::ranges::nth_element(in.begin(), in.begin() + 1, in.end(), comp);
+      assert(in[1] == 4);
+      assert(last == in.end());
+    }
+
+    {
+      auto in = input;
+      auto last = std::ranges::nth_element(in, in.begin() + 1, comp);
+      assert(in[1] == 4);
+      assert(last == in.end());
+    }
+  }
+
+  { // A custom projection works.
+    struct A {
+      int a;
+      constexpr bool operator==(const A&) const = default;
+    };
+
+    const std::array input = {A{2}, A{1}, A{3}};
+
+    {
+      auto in = input;
+      auto last = std::ranges::nth_element(in.begin(), in.begin() + 1, in.end(), {}, &A::a);
+      assert(in[1] == A{2});
+      assert(last == in.end());
+    }
+
+    {
+      auto in = input;
+      auto last = std::ranges::nth_element(in, in.begin() + 1, {}, &A::a);
+      assert(in[1] == A{2});
+      assert(last == in.end());
+    }
+  }
+
+  { // `std::invoke` is used in the implementation.
+    struct S {
+      int i;
+      constexpr S(int i_) : i(i_) {}
+
+      constexpr bool comparator(const S& rhs) const { return i < rhs.i; }
+      constexpr const S& projection() const { return *this; }
+
+      constexpr bool operator==(const S&) const = default;
+    };
+
+    const std::array input = {S{2}, S{1}, S{3}};
+
+    {
+      auto in = input;
+      auto last = std::ranges::nth_element(in.begin(), in.begin() + 1, in.end(), &S::comparator, &S::projection);
+      assert(in[1] == S{2});
+      assert(last == in.end());
+    }
+
+    {
+      auto in = input;
+      auto last = std::ranges::nth_element(in, in.begin() + 1, &S::comparator, &S::projection);
+      assert(in[1] == S{2});
+      assert(last == in.end());
+    }
+  }
+
+  { // The comparator can return any type that's convertible to `bool`.
+    const std::array input = {2, 1, 3};
+    auto pred = [](int i, int j) { return BooleanTestable{i < j}; };
+
+    {
+      std::array in = input;
+      auto last = std::ranges::nth_element(in.begin(), in.begin() + 1, in.end(), pred);
+      assert(in[1] == 2);
+      assert(last == in.end());
+    }
+
+    {
+      std::array in = input;
+      auto last = std::ranges::nth_element(in, in.begin() + 1, pred);
+      assert(in[1] == 2);
+      assert(last == in.end());
+    }
+  }
+
+  { // `std::ranges::dangling` is returned.
+    std::array in{1, 2, 3};
+    [[maybe_unused]] std::same_as<std::ranges::dangling> decltype(auto) result =
+        std::ranges::nth_element(std::move(in), in.begin());
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}

libcxx/test/std/library/description/conventions/customization.point.object/niebloid.compile.pass.cpp

@@ -108,7 +108,7 @@ static_assert(test(std::ranges::move, a, a));
 static_assert(test(std::ranges::move_backward, a, a));
 //static_assert(test(std::ranges::next_permutation, a));
 static_assert(test(std::ranges::none_of, a, odd));
-//static_assert(test(std::ranges::nth_element, a, a+5));
+static_assert(test(std::ranges::nth_element, a, a+5));
 //static_assert(test(std::ranges::partial_sort, a, a+5));
 //static_assert(test(std::ranges::partial_sort_copy, a, a));
 //static_assert(test(std::ranges::partition, a, odd));