[libc++][ranges] implement `std::ranges::set_symmetric_difference`

[huixie90]

Summary

[libc++][ranges] implement std::ranges::set_symmetric_difference

[huixie90]

fix CI

[var-const]

libcxx/include/__algorithm/ranges_set_symmetric_difference.h

48

Use ranges::less explicitly?

libcxx/include/__algorithm/set_symmetric_difference.h

15

Nit: seems unused?

38

Nit: this space looks weird -- is this done by clang-format?

75

Move?

93

Move?

libcxx/test/std/algorithms/alg.sorting/alg.set.operations/set.symmetric.difference/ranges_set_symmetric_difference.pass.cpp

82

Optional: do you think a short alias like template <class T> using R = UncheckedRange<T> would make these checks a little more readable?

387

Now that https://reviews.llvm.org/D129414 is merged, these tests (for member pointers) are no longer necessary.

445

Ditto -- these tests are no longer necessary.

480

Optional: testing both overloads adds a lot of boilerplate. Consider:
a) make numberOf* members of a struct, move the definitions of comp and proj1/proj2 out of the nested scopes, reset the struct (numberOf = {}) before each test case;
b) make the test a lambda taking a functor, invoke the lambda twice passing another lambda that invokes either overload as the functor.

543

Ditto -- these tests are no longer necessary.

[philnik]

libcxx/include/__algorithm/set_symmetric_difference.h

38

clang-format keeps the spacing as-is because of SpacesInAngles: Leave. I think it could be improved to remove whitespaces anwhere where there aren't two consecutive brackets, but you can just remove the whitespace and clang-format will be happy. This is so clang-format doesn't break C++03 code.

[huixie90]

addressed review feedback

[huixie90]

libcxx/include/__algorithm/set_symmetric_difference.h

27–29

should these names be double underscored? if so, what is the naming convention?

afaik, __out is a nasty macro and __out_iter clashes with the constructor argument name.

[philnik]

libcxx/include/__algorithm/set_symmetric_difference.h

27–29

They should be called __in1_, __in2_ and __out_.

[huixie90]

rename variables

[var-const]

Thanks for addressing the feedback!

libcxx/docs/Status/RangesAlgorithms.csv

 Write,rotate_copy,Nikolas Klauser,`D127211 <https://llvm.org/D127211>`_,✅
 Write,sample,Not assigned,n/a,Not started
 Write,unique_copy,Not assigned,n/a,Not started
 Write,partition_copy,Not assigned,n/a,Not started
 Write,partial_sort_copy,Not assigned,n/a,Not started
 Merge,merge,Hui Xie,`D128611 <https://llvm.org/D128611>`_,✅
 Merge,set_difference,Hui Xie,`D128983 <https://llvm.org/D128983>`,✅
 Merge,set_intersection,Hui Xie,`D129233 <https://llvm.org/D129233>`,✅
-Merge,set_symmetric_difference,Hui Xie,n/a,Not started
+Merge,set_symmetric_difference,Hui Xie,`D129520 <https://llvm.org/D129520>`,✅
 Merge,set_union,Hui Xie,n/a,Not started
 Permutation,remove,Nikolas Klauser,`D128618 <https://llvm.org/D128618>`_,✅
 Permutation,remove_if,Nikolas Klauser,`D128618 <https://llvm.org/D128618>`_,✅
 Permutation,reverse,Nikolas Klauser,`D125752 <https://llvm.org/D125752>`_,✅
 Permutation,rotate,Nikolas Klauser,`D124122 <https://llvm.org/D124122>`_,Under review
 Permutation,shuffle,Not assigned,n/a,Not started
 Permutation,unique,Not assigned,n/a,Not started
 Permutation,partition,Not assigned,n/a,Not started

libcxx/include/CMakeLists.txt

set(files
   __algorithm/ranges_replace_copy.h
   __algorithm/ranges_replace_copy_if.h
   __algorithm/ranges_replace_if.h
   __algorithm/ranges_reverse.h
   __algorithm/ranges_reverse_copy.h
   __algorithm/ranges_rotate_copy.h
   __algorithm/ranges_set_difference.h
   __algorithm/ranges_set_intersection.h
+  __algorithm/ranges_set_symmetric_difference.h
   __algorithm/ranges_set_union.h
   __algorithm/ranges_shuffle.h
   __algorithm/ranges_sort.h
   __algorithm/ranges_sort_heap.h
   __algorithm/ranges_stable_partition.h
   __algorithm/ranges_stable_sort.h
   __algorithm/ranges_swap_ranges.h
   __algorithm/ranges_transform.h

libcxx/include/__algorithm/ranges_set_intersection.h

_LIBCPP_HIDE_FROM_ABI constexpr set_intersection_result<_InIter1, _InIter2, _OutIter> operator()(
       _Proj2 __proj2 = {}) const {
     auto __ret = std::__set_intersection<_RangeAlgPolicy>(
         std::move(__first1),
         std::move(__last1),
         std::move(__first2),
         std::move(__last2),
         std::move(__result),
         ranges::__make_projected_comp(__comp, __proj1, __proj2));
-    return {std::move(__ret.in1), std::move(__ret.in2), std::move(__ret.out)};
+    return {std::move(__ret.__in1_), std::move(__ret.__in2_), std::move(__ret.__out_)};
   }
 
   template <
       input_range _Range1,
       input_range _Range2,
       weakly_incrementable _OutIter,
       class _Comp  = less,
       class _Proj1 = identity,
operator()(
         _Proj2 __proj2 = {}) const {
     auto __ret = std::__set_intersection<_RangeAlgPolicy>(
         ranges::begin(__range1),
         ranges::end(__range1),
         ranges::begin(__range2),
         ranges::end(__range2),
         std::move(__result),
         ranges::__make_projected_comp(__comp, __proj1, __proj2));
-    return {std::move(__ret.in1), std::move(__ret.in2), std::move(__ret.out)};
+    return {std::move(__ret.__in1_), std::move(__ret.__in2_), std::move(__ret.__out_)};
   }
 };
 
 } // namespace __set_intersection
 
 inline namespace __cpo {
   inline constexpr auto set_intersection = __set_intersection::__fn{};
 } // namespace __cpo
 } // namespace ranges
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
 #endif // _LIBCPP___ALGORITHM_RANGES_SET_INTERSECTION_H

libcxx/include/__algorithm/ranges_set_symmetric_difference.h

 //===----------------------------------------------------------------------===//
 //
 // 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_SET_INTERSECTION_H
-#define _LIBCPP___ALGORITHM_RANGES_SET_INTERSECTION_H
+#ifndef _LIBCPP___ALGORITHM_RANGES_SET_SYMMETRIC_DIFFERENCE_H
+#define _LIBCPP___ALGORITHM_RANGES_SET_SYMMETRIC_DIFFERENCE_H
 
 #include <__algorithm/in_in_out_result.h>
-#include <__algorithm/iterator_operations.h>
 #include <__algorithm/make_projected.h>
-#include <__algorithm/set_intersection.h>
+#include <__algorithm/set_symmetric_difference.h>
 #include <__config>
 #include <__functional/identity.h>
 #include <__functional/invoke.h>
 #include <__functional/ranges_operations.h>
 #include <__iterator/concepts.h>
 #include <__iterator/mergeable.h>
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
 #include <__ranges/dangling.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 {
 
 template <class _InIter1, class _InIter2, class _OutIter>
-using set_intersection_result = in_in_out_result<_InIter1, _InIter2, _OutIter>;
+using set_symmetric_difference_result = in_in_out_result<_InIter1, _InIter2, _OutIter>;
 
-namespace __set_intersection {
+namespace __set_symmetric_difference {
 
 struct __fn {
   template <
       input_iterator _InIter1,
       sentinel_for<_InIter1> _Sent1,
       input_iterator _InIter2,
       sentinel_for<_InIter2> _Sent2,
       weakly_incrementable _OutIter,
-      class _Comp  = less,
+      class _Comp  = ranges::less,
       class _Proj1 = identity,
       class _Proj2 = identity>
     requires mergeable<_InIter1, _InIter2, _OutIter, _Comp, _Proj1, _Proj2>
-  _LIBCPP_HIDE_FROM_ABI constexpr set_intersection_result<_InIter1, _InIter2, _OutIter> operator()(
+  _LIBCPP_HIDE_FROM_ABI constexpr set_symmetric_difference_result<_InIter1, _InIter2, _OutIter> operator()(
       _InIter1 __first1,
       _Sent1 __last1,
       _InIter2 __first2,
       _Sent2 __last2,
       _OutIter __result,
       _Comp __comp   = {},
       _Proj1 __proj1 = {},
       _Proj2 __proj2 = {}) const {
-    auto __ret = std::__set_intersection<_RangeAlgPolicy>(
+    auto __ret = std::__set_symmetric_difference(
         std::move(__first1),
         std::move(__last1),
         std::move(__first2),
         std::move(__last2),
         std::move(__result),
         ranges::__make_projected_comp(__comp, __proj1, __proj2));
-    return {std::move(__ret.in1), std::move(__ret.in2), std::move(__ret.out)};
+    return {std::move(__ret.__in1_), std::move(__ret.__in2_), std::move(__ret.__out_)};
   }
 
   template <
       input_range _Range1,
       input_range _Range2,
       weakly_incrementable _OutIter,
-      class _Comp  = less,
+      class _Comp  = ranges::less,
       class _Proj1 = identity,
       class _Proj2 = identity>
     requires mergeable<
         iterator_t<_Range1>,
         iterator_t<_Range2>,
         _OutIter,
         _Comp,
         _Proj1,
         _Proj2> 
-    _LIBCPP_HIDE_FROM_ABI constexpr set_intersection_result<borrowed_iterator_t<_Range1>,
+  _LIBCPP_HIDE_FROM_ABI constexpr set_symmetric_difference_result<borrowed_iterator_t<_Range1>,
                                                                   borrowed_iterator_t<_Range2>,
                                                                   _OutIter>
     operator()(
         _Range1&& __range1,
         _Range2&& __range2,
         _OutIter __result,
         _Comp __comp   = {},
         _Proj1 __proj1 = {},
         _Proj2 __proj2 = {}) const {
-    auto __ret = std::__set_intersection<_RangeAlgPolicy>(
+    auto __ret = std::__set_symmetric_difference(
         ranges::begin(__range1),
         ranges::end(__range1),
         ranges::begin(__range2),
         ranges::end(__range2),
         std::move(__result),
         ranges::__make_projected_comp(__comp, __proj1, __proj2));
-    return {std::move(__ret.in1), std::move(__ret.in2), std::move(__ret.out)};
+    return {std::move(__ret.__in1_), std::move(__ret.__in2_), std::move(__ret.__out_)};
   }
 };
 
-} // namespace __set_intersection
+} // namespace __set_symmetric_difference
 
 inline namespace __cpo {
-  inline constexpr auto set_intersection = __set_intersection::__fn{};
+  inline constexpr auto set_symmetric_difference = __set_symmetric_difference::__fn{};
 } // namespace __cpo
 } // namespace ranges
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
-#endif // _LIBCPP___ALGORITHM_RANGES_SET_INTERSECTION_H
+#endif // _LIBCPP___ALGORITHM_RANGES_SET_SYMMETRIC_DIFFERENCE_H

libcxx/include/__algorithm/set_intersection.h

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _InIter1, class _InIter2, class _OutIter>
 struct __set_intersection_result {
-  _InIter1 in1;
-  _InIter2 in2;
-  _OutIter out;
+  _InIter1 __in1_;
+  _InIter2 __in2_;
+  _OutIter __out_;
 
   // need a constructor as C++03 aggregate init is hard
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
   __set_intersection_result(_InIter1&& __in_iter1, _InIter2&& __in_iter2, _OutIter&& __out_iter)
-      : in1(std::move(__in_iter1)), in2(std::move(__in_iter2)), out(std::move(__out_iter)) {}
+      : __in1_(std::move(__in_iter1)), __in2_(std::move(__in_iter2)), __out_(std::move(__out_iter)) {}
 };
 
 template <class _AlgPolicy, class _Compare, class _InIter1, class _Sent1, class _InIter2, class _Sent2, class _OutIter>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 __set_intersection_result<_InIter1, _InIter2, _OutIter>
 __set_intersection(
     _InIter1 __first1, _Sent1 __last1, _InIter2 __first2, _Sent2 __last2, _OutIter __result, _Compare&& __comp) {
   while (__first1 != __last1 && __first2 != __last2) {
     if (__comp(*__first1, *__first2))
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 _OutputIterator set_intersection(
   typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
   return std::__set_intersection<_ClassicAlgPolicy, _Comp_ref>(
              std::move(__first1),
              std::move(__last1),
              std::move(__first2),
              std::move(__last2),
              std::move(__result),
              __comp)
-      .out;
+      .__out_;
 }
 
 template <class _InputIterator1, class _InputIterator2, class _OutputIterator>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 _OutputIterator set_intersection(
     _InputIterator1 __first1,
     _InputIterator1 __last1,
     _InputIterator2 __first2,
     _InputIterator2 __last2,
     _OutputIterator __result) {
   return std::__set_intersection<_ClassicAlgPolicy>(
              std::move(__first1),
              std::move(__last1),
              std::move(__first2),
              std::move(__last2),
              std::move(__result),
              __less<typename iterator_traits<_InputIterator1>::value_type,
                     typename iterator_traits<_InputIterator2>::value_type>())
-      .out;
+      .__out_;
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ALGORITHM_SET_INTERSECTION_H

libcxx/include/__algorithm/set_symmetric_difference.h

 //===----------------------------------------------------------------------===//
 //
 // 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_SET_SYMMETRIC_DIFFERENCE_H
 #define _LIBCPP___ALGORITHM_SET_SYMMETRIC_DIFFERENCE_H
 
 #include <__algorithm/comp.h>
 #include <__algorithm/comp_ref_type.h>
 #include <__algorithm/copy.h>
 #include <__config>
 #include <__iterator/iterator_traits.h>
+#include <__utility/move.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Compare, class _InputIterator1, class _InputIterator2, class _OutputIterator>
-_LIBCPP_CONSTEXPR_AFTER_CXX17 _OutputIterator
-__set_symmetric_difference(_InputIterator1 __first1, _InputIterator1 __last1,
-                           _InputIterator2 __first2, _InputIterator2 __last2, _OutputIterator __result, _Compare __comp)
-{
-    while (__first1 != __last1)
-    {
-        if (__first2 == __last2)
-            return _VSTD::copy(__first1, __last1, __result);
-        if (__comp(*__first1, *__first2))
-        {
-            *__result = *__first1;
-            ++__result;
-            ++__first1;
-        }
-        else
-        {
-            if (__comp(*__first2, *__first1))
-            {
-                *__result = *__first2;
-                ++__result;
-            }
-            else
-                ++__first1;
-            ++__first2;
-        }
-    }
-    return _VSTD::copy(__first2, __last2, __result);
+template <class _InIter1, class _InIter2, class _OutIter>
+struct __set_symmetric_difference_result {
+  _InIter1 __in1_;
+  _InIter2 __in2_;
+  _OutIter __out_;
+
+  // need a constructor as C++03 aggregate init is hard
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
+  __set_symmetric_difference_result(_InIter1&& __in_iter1, _InIter2&& __in_iter2, _OutIter&& __out_iter)
+      : __in1_(std::move(__in_iter1)), __in2_(std::move(__in_iter2)), __out_(std::move(__out_iter)) {}
+};
+
+template <class _Compare, class _InIter1, class _Sent1, class _InIter2, class _Sent2, class _OutIter>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 __set_symmetric_difference_result<_InIter1, _InIter2, _OutIter>
+__set_symmetric_difference(
+    _InIter1 __first1, _Sent1 __last1, _InIter2 __first2, _Sent2 __last2, _OutIter __result, _Compare&& __comp) {
+  while (__first1 != __last1) {
+    if (__first2 == __last2) {
+      auto __ret1 = std::__copy_impl(std::move(__first1), std::move(__last1), std::move(__result));
+      return __set_symmetric_difference_result<_InIter1, _InIter2, _OutIter>(
+          std::move(__ret1.first), std::move(__first2), std::move((__ret1.second)));
+    }
+    if (__comp(*__first1, *__first2)) {
+    } else {
+      if (__comp(*__first2, *__first1)) {
+      } else {
+      }
+  auto __ret2 = std::__copy_impl(std::move(__first2), std::move(__last2), std::move(__result));
+  return __set_symmetric_difference_result<_InIter1, _InIter2, _OutIter>(
+      std::move(__first1), std::move(__ret2.first), std::move((__ret2.second)));
 }
 
 template <class _InputIterator1, class _InputIterator2, class _OutputIterator, class _Compare>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-_OutputIterator
-set_symmetric_difference(_InputIterator1 __first1, _InputIterator1 __last1,
-                         _InputIterator2 __first2, _InputIterator2 __last2, _OutputIterator __result, _Compare __comp)
-{
-    typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
-    return _VSTD::__set_symmetric_difference<_Comp_ref>(__first1, __last1, __first2, __last2, __result, __comp);
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 _OutputIterator set_symmetric_difference(
+    _InputIterator1 __first1,
+    _InputIterator1 __last1,
+    _InputIterator2 __first2,
+    _InputIterator2 __last2,
+    _OutputIterator __result,
+    _Compare __comp) {
+  return std::__set_symmetric_difference<_Comp_ref>(
+             std::move(__first1),
+             std::move(__last1),
+             std::move(__first2),
+             std::move(__last2),
+             std::move(__result),
+             __comp)
+      .__out_;
 }
 
 template <class _InputIterator1, class _InputIterator2, class _OutputIterator>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-_OutputIterator
-set_symmetric_difference(_InputIterator1 __first1, _InputIterator1 __last1,
-                         _InputIterator2 __first2, _InputIterator2 __last2, _OutputIterator __result)
-{
-    return _VSTD::set_symmetric_difference(__first1, __last1, __first2, __last2, __result,
-                                          __less<typename iterator_traits<_InputIterator1>::value_type,
-                                                 typename iterator_traits<_InputIterator2>::value_type>());
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 _OutputIterator set_symmetric_difference(
+    _InputIterator1 __first1,
+    _InputIterator1 __last1,
+    _InputIterator2 __first2,
+    _InputIterator2 __last2,
+    _OutputIterator __result) {
+  return std::set_symmetric_difference(
+      std::move(__first1),
+      std::move(__last1),
+      std::move(__first2),
+      std::move(__last2),
+      std::move(__result),
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ALGORITHM_SET_SYMMETRIC_DIFFERENCE_H

libcxx/include/algorithm

template<forward_iterator I, sentinel_for<I> S, weakly_incrementable O>
     constexpr ranges::rotate_copy_result<I, O>
       ranges::rotate_copy(I first, I middle, S last, O result);                                     // since C++20
 
   template<forward_range R, weakly_incrementable O>
     requires indirectly_copyable<iterator_t<R>, O>
     constexpr ranges::rotate_copy_result<borrowed_iterator_t<R>, O>
       ranges::rotate_copy(R&& r, iterator_t<R> middle, O result);                                   // since C++20
 
+  template<class I1, class I2, class O>
+    using set_symmetric_difference_result = in_in_out_result<I1, I2, O>;                            // since C++20
+
+  template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
+           weakly_incrementable O, class Comp = ranges::less,
+           class Proj1 = identity, class Proj2 = identity>
+    requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
+    constexpr set_symmetric_difference_result<I1, I2, O>
+      set_symmetric_difference(I1 first1, S1 last1, I2 first2, S2 last2, O result,
+                               Comp comp = {}, Proj1 proj1 = {},
+                               Proj2 proj2 = {});                                                   // since C++20
+  
+  template<input_range R1, input_range R2, weakly_incrementable O,
+           class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
+    requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
+    constexpr set_symmetric_difference_result<borrowed_iterator_t<R1>,
+                                                      borrowed_iterator_t<R2>, O>
+      set_symmetric_difference(R1&& r1, R2&& r2, O result, Comp comp = {},
+                               Proj1 proj1 = {}, Proj2 proj2 = {});                                 // since C++20
+  
 }
 
     constexpr bool     // constexpr in C++20
     all_of(InputIterator first, InputIterator last, Predicate pred);
 
 template <class InputIterator, class Predicate>
     constexpr bool     // constexpr in C++20
     any_of(InputIterator first, InputIterator last, Predicate pred);

 #include <__algorithm/ranges_remove_if.h>
 #include <__algorithm/ranges_replace.h>
 #include <__algorithm/ranges_replace_if.h>
 #include <__algorithm/ranges_reverse.h>
 #include <__algorithm/ranges_reverse_copy.h>
 #include <__algorithm/ranges_rotate_copy.h>
 #include <__algorithm/ranges_set_difference.h>
 #include <__algorithm/ranges_set_intersection.h>
+#include <__algorithm/ranges_set_symmetric_difference.h>
 #include <__algorithm/ranges_sort.h>
 #include <__algorithm/ranges_sort_heap.h>
 #include <__algorithm/ranges_stable_sort.h>
 #include <__algorithm/ranges_swap_ranges.h>
 #include <__algorithm/ranges_transform.h>
 #include <__algorithm/ranges_upper_bound.h>
 #include <__algorithm/remove.h>
 #include <__algorithm/remove_copy.h>

libcxx/include/module.modulemap.in

module __algorithm {
       module ranges_replace_copy             { private header "__algorithm/ranges_replace_copy.h" }
       module ranges_replace_copy_if          { private header "__algorithm/ranges_replace_copy_if.h" }
       module ranges_replace_if               { private header "__algorithm/ranges_replace_if.h" }
       module ranges_reverse                  { private header "__algorithm/ranges_reverse.h" }
       module ranges_reverse_copy             { private header "__algorithm/ranges_reverse_copy.h" }
       module ranges_rotate_copy              { private header "__algorithm/ranges_rotate_copy.h" }
       module ranges_set_difference           { private header "__algorithm/ranges_set_difference.h" }
       module ranges_set_intersection         { private header "__algorithm/ranges_set_intersection.h" }
+      module ranges_set_symmetric_difference { private header "__algorithm/ranges_set_symmetric_difference.h" }
       module ranges_set_union                { private header "__algorithm/ranges_set_union.h" }
       module ranges_shuffle                  { private header "__algorithm/ranges_shuffle.h" }
       module ranges_sort                     { private header "__algorithm/ranges_sort.h" }
       module ranges_sort_heap                { private header "__algorithm/ranges_sort_heap.h" }
       module ranges_stable_partition         { private header "__algorithm/ranges_stable_partition.h" }
       module ranges_stable_sort              { private header "__algorithm/ranges_stable_sort.h" }
       module ranges_swap_ranges              { private header "__algorithm/ranges_swap_ranges.h" }
       module ranges_transform                { private header "__algorithm/ranges_transform.h" }

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

#endif
     //(void)std::ranges::search(first, last, first2, mid2, Equal(&copies)); assert(copies == 0);
     //(void)std::ranges::search(a, b, Equal(&copies)); assert(copies == 0);
     //(void)std::ranges::search_n(first, last, count, value, Equal(&copies)); assert(copies == 0);
     //(void)std::ranges::search_n(a, count, value, Equal(&copies)); assert(copies == 0);
     (void)std::ranges::set_difference(first, mid, mid, last, first2, Less(&copies)); assert(copies == 0);
     (void)std::ranges::set_difference(a, b, first2, Less(&copies)); assert(copies == 0);
     (void)std::ranges::set_intersection(first, mid, mid, last, first2, Less(&copies)); assert(copies == 0);
     (void)std::ranges::set_intersection(a, b, first2, Less(&copies)); assert(copies == 0);
-    //(void)std::ranges::set_symmetric_difference(first, mid, mid, last, first2, Less(&copies)); assert(copies == 0);
-    //(void)std::ranges::set_symmetric_difference(a, b, first2, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::set_symmetric_difference(first, mid, mid, last, first2, Less(&copies)); assert(copies == 0);
+    (void)std::ranges::set_symmetric_difference(a, b, first2, Less(&copies)); assert(copies == 0);
     //(void)std::ranges::set_union(first, mid, mid, last, first2, Less(&copies)); assert(copies == 0);
     //(void)std::ranges::set_union(a, b, first2, Less(&copies)); assert(copies == 0);
     (void)std::ranges::sort(first, last, Less(&copies)); assert(copies == 0);
     (void)std::ranges::sort(a, Less(&copies)); assert(copies == 0);
     (void)std::ranges::sort_heap(first, last, Less(&copies)); assert(copies == 0);
     (void)std::ranges::sort_heap(a, Less(&copies)); assert(copies == 0);
     //if (!std::is_constant_evaluated()) { (void)std::ranges::stable_partition(first, last, UnaryTrue(&copies)); assert(copies == 0); }
     //if (!std::is_constant_evaluated()) { (void)std::ranges::stable_partition(a, UnaryTrue(&copies)); assert(copies == 0); }

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

#endif
     //(void)std::ranges::search(first, last, first2, mid2, Equal(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::search(a, b, Equal(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::search_n(first, last, count, value, Equal(), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::search_n(a, count, value, Equal(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::set_difference(first, mid, mid, last, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::set_difference(a, b, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::set_intersection(first, mid, mid, last, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::set_intersection(a, b, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::set_symmetric_difference(first, mid, mid, last, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::set_symmetric_difference(a, b, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::set_symmetric_difference(first, mid, mid, last, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::set_symmetric_difference(a, b, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::set_union(first, mid, mid, last, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     //(void)std::ranges::set_union(a, b, first2, Less(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::sort(first, last, Less(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::sort(a, Less(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::sort_heap(first, last, Less(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::sort_heap(a, Less(), Proj(&copies)); assert(copies == 0);
     //if (!std::is_constant_evaluated()) { (void)std::ranges::stable_partition(first, last, UnaryTrue(), Proj(&copies)); assert(copies == 0); }
     //if (!std::is_constant_evaluated()) { (void)std::ranges::stable_partition(a, UnaryTrue(), Proj(&copies)); assert(copies == 0); }

libcxx/test/libcxx/private_headers.verify.cpp

 #include <__algorithm/ranges_replace_copy.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_replace_copy.h'}}
 #include <__algorithm/ranges_replace_copy_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_replace_copy_if.h'}}
 #include <__algorithm/ranges_replace_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_replace_if.h'}}
 #include <__algorithm/ranges_reverse.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_reverse.h'}}
 #include <__algorithm/ranges_reverse_copy.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_reverse_copy.h'}}
 #include <__algorithm/ranges_rotate_copy.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_rotate_copy.h'}}
 #include <__algorithm/ranges_set_difference.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_set_difference.h'}}
 #include <__algorithm/ranges_set_intersection.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_set_intersection.h'}}
+#include <__algorithm/ranges_set_symmetric_difference.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_set_symmetric_difference.h'}}
 #include <__algorithm/ranges_set_union.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_set_union.h'}}
 #include <__algorithm/ranges_shuffle.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_shuffle.h'}}
 #include <__algorithm/ranges_sort.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_sort.h'}}
 #include <__algorithm/ranges_sort_heap.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_sort_heap.h'}}
 #include <__algorithm/ranges_stable_partition.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_stable_partition.h'}}
 #include <__algorithm/ranges_stable_sort.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_stable_sort.h'}}
 #include <__algorithm/ranges_swap_ranges.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_swap_ranges.h'}}
 #include <__algorithm/ranges_transform.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_transform.h'}}

libcxx/test/std/algorithms/alg.sorting/alg.set.operations/set.symmetric.difference/ranges_set_symmetric_difference.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
+//          weakly_incrementable O, class Comp = ranges::less,
+//          class Proj1 = identity, class Proj2 = identity>
+//   requires mergeable<I1, I2, O, Comp, Proj1, Proj2>
+//   constexpr set_symmetric_difference_result<I1, I2, O>
+//     set_symmetric_difference(I1 first1, S1 last1, I2 first2, S2 last2, O result,
+//                      Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});                         // since C++20
+//
+// template<input_range R1, input_range R2, weakly_incrementable O,
+//          class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
+//   requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
+//   constexpr set_symmetric_difference_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, O>
+//     set_symmetric_difference(R1&& r1, R2&& r2, O result,
+//                      Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});                         // since C++20
+
+#include <algorithm>
+#include <array>
+#include <concepts>
+
+#include "almost_satisfies_types.h"
+#include "MoveOnly.h"
+#include "test_iterators.h"
+#include "../../sortable_helpers.h"
+
+// Test iterator overload's constraints:
+// =====================================
+template <class InIter1 = int*, class Sent1 = int*, class InIter2 = int*, class Sent2 = int*, class OutIter = int*>
+concept HasSetSymmetricDifferenceIter =
+    requires(InIter1&& inIter1, InIter2&& inIter2, OutIter&& outIter, Sent1&& sent1, Sent2&& sent2) {
+      std::ranges::set_symmetric_difference(
+          std::forward<InIter1>(inIter1),
+          std::forward<Sent1>(sent1),
+          std::forward<InIter2>(inIter2),
+          std::forward<Sent2>(sent2),
+          std::forward<OutIter>(outIter));
+    };
+
+static_assert(HasSetSymmetricDifferenceIter<int*, int*, int*, int*, int*>);
+
+// !std::input_iterator<I1>
+static_assert(!HasSetSymmetricDifferenceIter<InputIteratorNotDerivedFrom>);
+
+// !std::sentinel_for<S1, I1>
+static_assert(!HasSetSymmetricDifferenceIter<int*, SentinelForNotSemiregular>);
+
+// !std::input_iterator<I2>
+static_assert(!HasSetSymmetricDifferenceIter<int*, int*, InputIteratorNotDerivedFrom>);
+
+// !std::sentinel_for<S2, I2>
+static_assert(!HasSetSymmetricDifferenceIter<int*, int*, int*, SentinelForNotSemiregular>);
+
+// !std::weakly_incrementable<O>
+static_assert(!HasSetSymmetricDifferenceIter<int*, int*, int*, int*, WeaklyIncrementableNotMovable>);
+
+// !std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
+static_assert(!HasSetSymmetricDifferenceIter<MoveOnly*, MoveOnly*, MoveOnly*, MoveOnly*, MoveOnly*>);
+
+// Test range overload's constraints:
+// =====================================
+
+template <class Range1, class Range2, class OutIter>
+concept HasSetSymmetricDifferenceRange =
+    requires(Range1&& range1, Range2&& range2, OutIter&& outIter) {
+      std::ranges::set_symmetric_difference(
+          std::forward<Range1>(range1), std::forward<Range2>(range2), std::forward<OutIter>(outIter));
+    };
+
+template <class T>
+using R = UncheckedRange<T>;
+
+static_assert(HasSetSymmetricDifferenceRange<R<int*>, R<int*>, int*>);
+
+// !std::input_range<R2>
+static_assert(!HasSetSymmetricDifferenceRange<R<InputIteratorNotDerivedFrom>, R<int*>, int*>);
+
+// !std::input_range<R2>
+static_assert(!HasSetSymmetricDifferenceRange<R<int*>, R<InputIteratorNotDerivedFrom>, int*>);
+
+// !std::weakly_incrementable<O>
+static_assert(!HasSetSymmetricDifferenceRange<R<int*>, R<int*>, WeaklyIncrementableNotMovable >);
+
+// !std::mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
+static_assert(!HasSetSymmetricDifferenceRange<R<MoveOnly*>, R<MoveOnly*>, MoveOnly*>);
+
+using std::ranges::set_symmetric_difference_result;
+
+template <class In1, class In2, class Out, std::size_t N1, std::size_t N2, std::size_t N3>
+constexpr void
+testSetSymmetricDifferenceImpl(std::array<int, N1> in1, std::array<int, N2> in2, std::array<int, N3> expected) {
+  // TODO: std::ranges::set_symmetric_difference calls std::ranges::copy
+  // std::ranges::copy(contiguous_iterator<int*>, sentinel_wrapper<contiguous_iterator<int*>>, contiguous_iterator<int*>) doesn't seem to work.
+  // It seems that std::ranges::copy calls std::copy, which unwraps contiguous_iterator<int*> into int*,
+  // and then it failed because there is no == between int* and sentinel_wrapper<contiguous_iterator<int*>>
+  using Sent1 = std::conditional_t<std::contiguous_iterator<In1>, In1, sentinel_wrapper<In1>>;
+  using Sent2 = std::conditional_t<std::contiguous_iterator<In2>, In2, sentinel_wrapper<In2>>;
+
+  // iterator overload
+  {
+    std::array<int, N3> out;
+    std::same_as<set_symmetric_difference_result<In1, In2, Out>> decltype(auto) result =
+        std::ranges::set_symmetric_difference(
+            In1{in1.data()},
+            Sent1{In1{in1.data() + in1.size()}},
+            In2{in2.data()},
+            Sent2{In2{in2.data() + in2.size()}},
+            Out{out.data()});
+    assert(std::ranges::equal(out, expected));
+
+    assert(base(result.in1) == in1.data() + in1.size());
+    assert(base(result.in2) == in2.data() + in2.size());
+    assert(base(result.out) == out.data() + out.size());
+  }
+
+  // range overload
+  {
+    std::array<int, N3> out;
+    std::ranges::subrange r1{In1{in1.data()}, Sent1{In1{in1.data() + in1.size()}}};
+    std::ranges::subrange r2{In2{in2.data()}, Sent2{In2{in2.data() + in2.size()}}};
+    std::same_as<set_symmetric_difference_result<In1, In2, Out>> decltype(auto) result =
+        std::ranges::set_symmetric_difference(r1, r2, Out{out.data()});
+    assert(std::ranges::equal(out, expected));
+
+    assert(base(result.in1) == in1.data() + in1.size());
+    assert(base(result.in2) == in2.data() + in2.size());
+    assert(base(result.out) == out.data() + out.size());
+  }
+}
+
+template <class In1, class In2, class Out>
+constexpr void testImpl() {
+  // range 1 shorter than range2
+  {
+    std::array in1{0, 1, 5, 6, 9, 10};
+    std::array in2{3, 6, 7, 9, 13, 15, 100};
+    std::array expected{0, 1, 3, 5, 7, 10, 13, 15, 100};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // range 2 shorter than range 1
+  {
+    std::array in1{2, 6, 8, 12, 15, 16};
+    std::array in2{0, 2, 8};
+    std::array expected{0, 6, 12, 15, 16};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // range 1 and range 2 has the same length but different elements
+  {
+    std::array in1{2, 6, 8, 12, 15, 16};
+    std::array in2{0, 2, 8, 15, 17, 19};
+    std::array expected{0, 6, 12, 16, 17, 19};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // range 1 == range 2
+  {
+    std::array in1{0, 1, 2};
+    std::array in2{0, 1, 2};
+    std::array<int, 0> expected{};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // range 1 is super set of range 2
+  {
+    std::array in1{8, 8, 10, 12, 13};
+    std::array in2{8, 10};
+    std::array expected{8, 12, 13};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // range 2 is super set of range 1
+  {
+    std::array in1{0, 1, 1};
+    std::array in2{0, 1, 1, 2, 5};
+    std::array expected{2, 5};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // range 1 and range 2 have no elements in common
+  {
+    std::array in1{7, 7, 9, 12};
+    std::array in2{1, 5, 5, 8, 10};
+    std::array expected{1, 5, 5, 7, 7, 8, 9, 10, 12};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // range 1 and range 2 have duplicated equal elements
+  {
+    std::array in1{7, 7, 9, 12};
+    std::array in2{7, 7, 7, 13};
+    std::array expected{7, 9, 12, 13};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // range 1 is empty
+  {
+    std::array<int, 0> in1{};
+    std::array in2{3, 4, 5};
+    std::array expected{3, 4, 5};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // range 2 is empty
+  {
+    std::array in1{3, 4, 5};
+    std::array<int, 0> in2{};
+    std::array expected{3, 4, 5};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // both ranges are empty
+  {
+    std::array<int, 0> in1{};
+    std::array<int, 0> in2{};
+    std::array<int, 0> expected{};
+    testSetSymmetricDifferenceImpl<In1, In2, Out>(in1, in2, expected);
+  }
+
+  // check that ranges::dangling is returned for non-borrowed_range
+  {
+    std::array r1{3, 6, 7, 9};
+    int r2[] = {2, 3, 4, 5, 6};
+    std::array<int, 5> out;
+    std::same_as<set_symmetric_difference_result<std::ranges::dangling, int*, int*>> decltype(auto) result =
+        std::ranges::set_symmetric_difference(NonBorrowedRange<In1>{r1.data(), r1.size()}, r2, out.data());
+    assert(base(result.in2) == r2 + 5);
+    assert(base(result.out) == out.data() + out.size());
+    assert(std::ranges::equal(out, std::array{2, 4, 5, 7, 9}));
+  }
+}
+
+template <class InIter2, class OutIter>
+constexpr void withAllPermutationsOfInIter1() {
+  // C++17 InputIterator may or may not satisfy std::input_iterator
+  testImpl<cpp20_input_iterator<int*>, InIter2, OutIter>();
+  testImpl<forward_iterator<int*>, InIter2, OutIter>();
+  testImpl<bidirectional_iterator<int*>, InIter2, OutIter>();
+  testImpl<random_access_iterator<int*>, InIter2, OutIter>();
+  testImpl<contiguous_iterator<int*>, InIter2, OutIter>();
+}
+
+template <class OutIter>
+constexpr bool withAllPermutationsOfInIter1AndInIter2() {
+  withAllPermutationsOfInIter1<cpp20_input_iterator<int*>, OutIter>();
+  withAllPermutationsOfInIter1<forward_iterator<int*>, OutIter>();
+  withAllPermutationsOfInIter1<bidirectional_iterator<int*>, OutIter>();
+  withAllPermutationsOfInIter1<random_access_iterator<int*>, OutIter>();
+  withAllPermutationsOfInIter1<contiguous_iterator<int*>, OutIter>();
+  return true;
+}
+
+constexpr void runAllIteratorPermutationsTests() {
+  withAllPermutationsOfInIter1AndInIter2<cpp20_output_iterator<int*>>();
+  withAllPermutationsOfInIter1AndInIter2<cpp20_input_iterator<int*>>();
+  withAllPermutationsOfInIter1AndInIter2<forward_iterator<int*>>();
+  withAllPermutationsOfInIter1AndInIter2<bidirectional_iterator<int*>>();
+  withAllPermutationsOfInIter1AndInIter2<random_access_iterator<int*>>();
+  withAllPermutationsOfInIter1AndInIter2<contiguous_iterator<int*>>();
+
+  static_assert(withAllPermutationsOfInIter1AndInIter2<cpp20_output_iterator<int*>>());
+  static_assert(withAllPermutationsOfInIter1AndInIter2<cpp20_input_iterator<int*>>());
+  static_assert(withAllPermutationsOfInIter1AndInIter2<forward_iterator<int*>>());
+  static_assert(withAllPermutationsOfInIter1AndInIter2<bidirectional_iterator<int*>>());
+  static_assert(withAllPermutationsOfInIter1AndInIter2<random_access_iterator<int*>>());
+  static_assert(withAllPermutationsOfInIter1AndInIter2<contiguous_iterator<int*>>());
+}
+
+constexpr bool test() {
+  // check that every element is copied exactly once
+  {
+    std::array<TracedCopy, 5> r1{3, 5, 8, 15, 16};
+    std::array<TracedCopy, 3> r2{1, 3, 8};
+
+    // iterator overload
+    {
+      std::array<TracedCopy, 4> out;
+      auto result = std::ranges::set_symmetric_difference(r1.begin(), r1.end(), r2.begin(), r2.end(), out.data());
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+      assert(std::ranges::equal(out, std::array<TracedCopy, 4>{1, 5, 15, 16}));
+
+      assert(std::ranges::all_of(out, &TracedCopy::copiedOnce));
+    }
+
+    // range overload
+    {
+      std::array<TracedCopy, 4> out;
+      auto result = std::ranges::set_symmetric_difference(r1, r2, out.data());
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+      assert(std::ranges::equal(out, std::array<TracedCopy, 4>{1, 5, 15, 16}));
+
+      assert(std::ranges::all_of(out, &TracedCopy::copiedOnce));
+    }
+  }
+
+  struct IntAndOrder {
+    int data;
+    int order;
+
+    constexpr auto operator==(const IntAndOrder& o) const { return data == o.data; }
+    constexpr auto operator<=>(const IntAndOrder& o) const { return data <=> o.data; }
+  };
+
+  // Stable. If [first1, last1) contains m elements that are equivalent to each other and [first2, last2)
+  // contains n elements that are equivalent to them, the first min(m, n) elements are copied from the first
+  // range to the output range, in order.
+  {
+    std::array<IntAndOrder, 5> r1{{{0, 0}, {0, 1}, {0, 2}, {0, 3}, {0, 4}}};
+    std::array<IntAndOrder, 3> r2{{{0, 5}, {0, 6}, {0, 7}}};
+
+    // iterator overload
+    {
+      std::array<IntAndOrder, 2> out;
+      std::ranges::set_symmetric_difference(r1.begin(), r1.end(), r2.begin(), r2.end(), out.data());
+
+      assert(std::ranges::equal(out, std::array{0, 0}, {}, &IntAndOrder::data));
+      assert(std::ranges::equal(out, std::array{3, 4}, {}, &IntAndOrder::order));
+    }
+
+    // range overload
+    {
+      std::array<IntAndOrder, 2> out;
+      std::ranges::set_symmetric_difference(r1, r2, out.data());
+
+      assert(std::ranges::equal(out, std::array{0, 0}, {}, &IntAndOrder::data));
+      assert(std::ranges::equal(out, std::array{3, 4}, {}, &IntAndOrder::order));
+    }
+  }
+
+  struct Data {
+    int data;
+
+    constexpr bool smallerThan(const Data& o) const { return data < o.data; }
+  };
+
+  // Test custom comparator
+  {
+    std::array r1{Data{4}, Data{8}, Data{12}};
+    std::array r2{Data{8}, Data{9}};
+
+    // iterator overload
+    {
+      std::array<Data, 3> out;
+      auto result = std::ranges::set_symmetric_difference(
+          r1.begin(), r1.end(), r2.begin(), r2.end(), out.data(), [](const Data& x, const Data& y) {
+            return x.data < y.data;
+          });
+
+      assert(std::ranges::equal(out, std::array{4, 9, 12}, {}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+
+    // range overload
+    {
+      std::array<Data, 3> out;
+      auto result = std::ranges::set_symmetric_difference(r1, r2, out.data(), [](const Data& x, const Data& y) {
+        return x.data < y.data;
+      });
+
+      assert(std::ranges::equal(out, std::array{4, 9, 12}, {}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+  }
+
+  // Test Projection
+  {
+    std::array r1{Data{1}, Data{3}, Data{5}};
+    std::array r2{Data{2}, Data{3}, Data{5}};
+
+    const auto proj = [](const Data& d) { return d.data; };
+
+    // iterator overload
+    {
+      std::array<Data, 2> out;
+      auto result = std::ranges::set_symmetric_difference(
+          r1.begin(), r1.end(), r2.begin(), r2.end(), out.data(), std::ranges::less{}, proj, proj);
+
+      assert(std::ranges::equal(out, std::array{1, 2}, {}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+
+    // range overload
+    {
+      std::array<Data, 2> out;
+      auto result = std::ranges::set_symmetric_difference(r1, r2, out.data(), std::ranges::less{}, proj, proj);
+
+      assert(std::ranges::equal(out, std::array{1, 2}, {}, &Data::data));
+
+      assert(result.in1 == r1.end());
+      assert(result.in2 == r2.end());
+      assert(result.out == out.end());
+    }
+  }
+
+  // Complexity: At most 2 * ((last1 - first1) + (last2 - first2)) - 1 comparisons and applications of each projection.
+  {
+    struct CompProjs {
+      std::size_t numberOfComp  = 0;
+      std::size_t numberOfProj1 = 0;
+      std::size_t numberOfProj2 = 0;
+
+      constexpr auto comp() {
+        return [this](int x, int y) {
+          ++numberOfComp;
+          return x < y;
+        };
+      }
+
+      constexpr auto proj1() {
+        return [this](const Data& d) {
+          ++numberOfProj1;
+          return d.data;
+        };
+      }
+
+      constexpr auto proj2() {
+        return [this](const Data& d) {
+          ++numberOfProj2;
+          return d.data;
+        };
+      }
+    };
+
+    std::array<Data, 1> r1{{{7}}};
+    std::array<Data, 6> r2{{{1}, {2}, {3}, {4}, {5}, {6}}};
+    std::array expected{1, 2, 3, 4, 5, 6, 7};
+
+    const std::size_t maxOperation = 2 * (r1.size() + r2.size()) - 1;
+
+    // iterator overload
+    {
+      std::array<Data, 7> out;
+      CompProjs compProjs{};
+
+      std::ranges::set_symmetric_difference(
+          r1.begin(),
+          r1.end(),
+          r2.begin(),
+          r2.end(),
+          out.data(),
+          compProjs.comp(),
+          compProjs.proj1(),
+          compProjs.proj2());
+
+      assert(std::ranges::equal(out, expected, {}, &Data::data));
+      assert(compProjs.numberOfComp < maxOperation);
+      assert(compProjs.numberOfProj1 < maxOperation);
+      assert(compProjs.numberOfProj2 < maxOperation);
+    }
+
+    // range overload
+    {
+      std::array<Data, 7> out;
+      CompProjs compProjs{};
+
+      std::ranges::set_symmetric_difference(r1, r2, out.data(), compProjs.comp(), compProjs.proj1(), compProjs.proj2());
+
+      assert(std::ranges::equal(out, expected, {}, &Data::data));
+      assert(compProjs.numberOfComp < maxOperation);
+      assert(compProjs.numberOfProj1 < maxOperation);
+      assert(compProjs.numberOfProj2 < maxOperation);
+    }
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  // Cannot static_assert on the entire permutation test because it exceeds the constexpr execution step limit
+  // due to the large number of combination of types of iterators (it is a 3-dimensional cartesian product)
+  // Instead of having one single static_assert that tests all the combinations, in the runAllIteratorPermutationsTests
+  // function, it has lots of smaller static_assert and each of them test 2-dimensional cartesian product which is less
+  // than the step limit.
+  runAllIteratorPermutationsTests();
+
+  return 0;
+}

libcxx/test/std/algorithms/ranges_robust_against_nonbool_predicates.compile.pass.cpp

void test_all() {
 
   //in_out_pred(std::ranges::unique_copy, in, out, binary_pred);
   // partition_copy
   //std::ranges::partition_copy(in.begin(), in.end(), out, out2, unary_pred);
   //std::ranges::partition_copy(in, out, out2, unary_pred);
   //in2_pred(std::ranges::partial_sort_copy, in, in2, binary_pred);
   in2_out_pred(std::ranges::merge, in, in2, out, binary_pred);
   in2_out_pred(std::ranges::set_difference, in, in2, out, binary_pred);
-  //in2_out_pred(std::ranges::set_intersection, in, in2, out, binary_pred);
-  //in2_out_pred(std::ranges::set_symmetric_difference, in, in2, out, binary_pred);
+  in2_out_pred(std::ranges::set_intersection, in, in2, out, binary_pred);
+  in2_out_pred(std::ranges::set_symmetric_difference, in, in2, out, binary_pred);
   //in2_out_pred(std::ranges::set_union, in, in2, out, binary_pred);
   in_pred(std::ranges::remove_if, in, unary_pred);
   //in_pred(std::ranges::unique, in, binary_pred);
   //in_pred(std::ranges::partition, in, binary_pred);
   //in_pred(std::ranges::stable_partition, in, binary_pred);
   in_pred(std::ranges::sort, in, binary_pred);
   in_pred(std::ranges::stable_sort, in, binary_pred);
   //in_mid_pred(std::ranges::partial_sort, in, mid, binary_pred);

libcxx/test/std/algorithms/ranges_robust_against_omitting_invoke.compile.pass.cpp

void test_all() {
   // `sample` has no requirement that the given generator be invoked via `std::invoke`.
   //in_out_pred(std::ranges::unique_copy, in, out, &Foo::binary_pred, &Bar::val);
   // partition_copy
   //std::ranges::partition_copy(in.begin(), in.end(), out, out2, &Foo::unary_pred, &Bar::val);
   //std::ranges::partition_copy(in, out, out2, &Foo::unary_pred, &Bar::val);
   //in2_pred(std::ranges::partial_sort_copy, in, in2, &Foo::binary_pred, &Bar::val);
   in2_out_pred(std::ranges::merge, in, in2, out, &Foo::binary_pred, &Bar::val, &Bar::val);
   in2_out_pred(std::ranges::set_difference, in, in2, out, &Foo::binary_pred, &Bar::val, &Bar::val);
-  //in2_out_pred(std::ranges::set_intersection, in, in2, out, &Foo::binary_pred, &Bar::val, &Bar::val);
-  //in2_out_pred(std::ranges::set_symmetric_difference, in, in2, out, &Foo::binary_pred, &Bar::val, &Bar::val);
+  in2_out_pred(std::ranges::set_intersection, in, in2, out, &Foo::binary_pred, &Bar::val, &Bar::val);
+  in2_out_pred(std::ranges::set_symmetric_difference, in, in2, out, &Foo::binary_pred, &Bar::val, &Bar::val);
   //in2_out_pred(std::ranges::set_union, in, in2, out, &Foo::binary_pred, &Bar::val, &Bar::val);
   in_val(std::ranges::remove, in, x, &Bar::val);
   in_pred(std::ranges::remove_if, in, &Foo::unary_pred, &Bar::val);
   // `reverse` has neither a projection nor a predicate.
   // `rotate` has neither a projection nor a predicate.
   // `shuffle` has neither a projection nor a predicate.
   //in_pred(std::ranges::unique, in, &Foo::binary_pred, &Bar::val);
   //in_pred(std::ranges::partition, in, &Foo::binary_pred, &Bar::val);

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

 static_assert(test(std::ranges::reverse_copy, a, a));
 //static_assert(test(std::ranges::rotate, a, a+5));
 static_assert(test(std::ranges::rotate_copy, a, a+5, a));
 //static_assert(test(std::ranges::sample, a, a, 5));
 //static_assert(test(std::ranges::search, a, a));
 //static_assert(test(std::ranges::search_n, a, 10, 42));
 static_assert(test(std::ranges::set_difference, a, a, a));
 static_assert(test(std::ranges::set_intersection, a, a, a));
-//static_assert(test(std::ranges::set_symmetric_difference, a, a, a));
+static_assert(test(std::ranges::set_symmetric_difference, a, a, a));
 //static_assert(test(std::ranges::set_union, a, a, a));
 //static_assert(test(std::ranges::shuffle, a, g));
 static_assert(test(std::ranges::sort, a));
 static_assert(test(std::ranges::sort_heap, a));
 //static_assert(test(std::ranges::stable_partition, a, odd));
 static_assert(test(std::ranges::stable_sort, a));
 //static_assert(test(std::ranges::starts_with, a, a));
 static_assert(test(std::ranges::swap_ranges, a, a));