[libc++] Implement ranges::starts_with

[ZijunZhao]

Summary

[libc++] Implement ranges::starts_with

[philnik]

Looks pretty good already. Please make sure the CI is green.

libcxx/include/__algorithm/ranges_starts_with.h

12

You have to #include <__config>.

50

Please update the nodiscard_extension tests!

58

I think we should just forward this for ranges::equal (for random access ranges) or ranges::mismatch (otherwise) to grab any optimizations that are done there.

libcxx/test/std/algorithms/alg.nonmodifying/alg.starts_with/ranges.starts_with.pass.cpp

13

This isn't a relevant flag anymore.

33

Looks like clang-format does weird things again. Please update the formatting.

194–202

This can be simplified now. See ranges.copy.pass.cpp for an example.

204–224

We have a few robust_against_footests for this and other things. Please make sure you update them.

233

Newline!

[ZijunZhao]

libcxx/include/__algorithm/ranges_starts_with.h

58

yes, I used ranges::mismatch directly before but the comparators and projections will be copied, which fail ranges_robust_against_copying_comparators.pass.cpp and ranges_robust_against_copying_projections.pass.cpp

[philnik]

libcxx/include/__algorithm/ranges_starts_with.h

58

You can call the actual implementation instead of the public interface to avoid that.

[ZijunZhao]

libcxx/include/__algorithm/ranges_starts_with.h

58

I see! Thank you!

libcxx/test/std/algorithms/alg.nonmodifying/alg.starts_with/ranges.starts_with.pass.cpp

33

Are there any command instructions to disable clang-format? I fixed part of the code manually 😂

[ZijunZhao]

libcxx/include/__algorithm/ranges_starts_with.h

12

I see. Yes, according to the build error log, I need to include more header files. WIP

[philnik]

LGTM % comments with green CI.

libcxx/include/__algorithm/ranges_starts_with.h

13

53

Would you be interested in optimizing random access iterators? If not, I can also do that in a follow-up patch.

libcxx/test/std/algorithms/alg.nonmodifying/alg.starts_with/ranges.starts_with.pass.cpp

33

You can enable and disable clang-format with // clang-format off/on.

66

Please use two spaces per indentation level.

218–225

254–255

[ldionne]

LGTM, thanks!

[ZijunZhao]

Submit: https://github.com/llvm/llvm-project/commit/205175578e0d73b4cd63d4d124a900fff10da7f8

libcxx/include/CMakeLists.txt

set(files
   __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_starts_with.h
   __algorithm/ranges_swap_ranges.h
   __algorithm/ranges_transform.h
   __algorithm/ranges_unique.h
   __algorithm/ranges_unique_copy.h
   __algorithm/ranges_upper_bound.h
   __algorithm/remove.h
   __algorithm/remove_copy.h
   __algorithm/remove_copy_if.h

libcxx/include/__algorithm/ranges_starts_with.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_STARTS_WITH_H
+#define _LIBCPP___ALGORITHM_RANGES_STARTS_WITH_H
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+namespace __starts_with {
+struct __fn {
+  template <class _Iter1, class _Sent1, class _Iter2, class _Sent2, class _Pred, class _Proj1, class _Proj2>
+  static _LIBCPP_HIDE_FROM_ABI constexpr bool __starts_with_impl(
+      _Iter1 __first1,
+      _Sent1 __last1,
+      _Iter2 __first2,
+      _Sent2 __last2,
+      _Pred& __pred,
+      _Proj1& __proj1,
+      _Proj2& __proj2) {
+    while (__first1 != __last1 && __first2 != __last2) {
+      if (!std::invoke(__pred, std::invoke(__proj1, *__first1), std::invoke(__proj2, *__first2)))
+        return false;
+      ++__first1;
+      ++__first2;
+    }
+    return __first2 == __last2;
+  }
+
+  template <input_iterator _Iter1,
+            sentinel_for<_Iter1> _Sent1,
+            input_iterator _Iter2,
+            sentinel_for<_Iter2> _Sent2,
+            class _Pred  = ranges::equal_to,
+            class _Proj1 = identity,
+            class _Proj2 = identity>
+    requires indirectly_comparable<_Iter1, _Iter2, _Pred, _Proj1, _Proj2>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(
+      _Iter1 __first1,
+      _Sent1 __last1,
+      _Iter2 __first2,
+      _Sent2 __last2,
+      _Pred __pred   = {},
+      _Proj1 __proj1 = {},
+      _Proj2 __proj2 = {}) const {
+    return __starts_with_impl(
+        std::move(__first1), std::move(__last1), std::move(__first2), std::move(__last2), __pred, __proj1, __proj2);
+  }
+
+  template <input_range _Range1,
+            input_range _Range2,
+            class _Pred  = ranges::equal_to,
+            class _Proj1 = identity,
+            class _Proj2 = identity>
+    requires indirectly_comparable<iterator_t<_Range1>, iterator_t<_Range2>, _Pred, _Proj1, _Proj2>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(
+      _Range1&& __range1, _Range2&& __range2, _Pred __pred = {}, _Proj1 __proj1 = {}, _Proj2 __proj2 = {}) const {
+    return __starts_with_impl(
+        ranges::begin(__range1),
+        ranges::end(__range1),
+        ranges::begin(__range2),
+        ranges::end(__range2),
+        __pred,
+        __proj1,
+        __proj2);
+  }
+};
+} // namespace __starts_with
+inline namespace __cpo {
+inline constexpr auto starts_with = __starts_with::__fn{};
+} // namespace __cpo
+} // namespace ranges
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 23
+
+#endif // _LIBCPP___ALGORITHM_RANGES_STARTS_WITH_H

libcxx/include/algorithm

template<input_range R1, input_range R2, class Pred = ranges::equal_to,
   template<input_iterator I, sentinel_for<I> S, class Proj = identity,
            indirect_unary_predicate<projected<I, Proj>> Pred>
     constexpr bool ranges::none_of(I first, S last, Pred pred, Proj proj = {});             // since C++20
 
   template<input_range R, class Proj = identity,
            indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
     constexpr bool ranges::none_of(R&& r, Pred pred, Proj proj = {});                       // since C++20
 
+  template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
+          class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
+    requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
+    constexpr bool ranges::starts_with(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
+                                      Proj1 proj1 = {}, Proj2 proj2 = {});                 // since C++23
+
+  template<input_range R1, input_range R2, class Pred = ranges::equal_to, class Proj1 = identity,
+          class Proj2 = identity>
+    requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
+    constexpr bool ranges::starts_with(R1&& r1, R2&& r2, Pred pred = {},
+                                      Proj1 proj1 = {}, Proj2 proj2 = {});                // since C++23
+
   template<input_iterator I1, sentinel_for<I1> S1,
           random_access_iterator I2, sentinel_for<I2> S2,
           class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
     requires indirectly_copyable<I1, I2> && sortable<I2, Comp, Proj2> &&
             indirect_strict_weak_order<Comp, projected<I1, Proj1>, projected<I2, Proj2>>
     constexpr partial_sort_copy_result<I1, I2>
       partial_sort_copy(I1 first, S1 last, I2 result_first, S2 result_last,
                         Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {});                // since C++20

 #include <__algorithm/ranges_set_intersection.h>
 #include <__algorithm/ranges_set_symmetric_difference.h>
 #include <__algorithm/ranges_set_union.h>
 #include <__algorithm/ranges_shuffle.h>
 #include <__algorithm/ranges_sort.h>
 #include <__algorithm/ranges_sort_heap.h>
 #include <__algorithm/ranges_stable_partition.h>
 #include <__algorithm/ranges_stable_sort.h>
+#include <__algorithm/ranges_starts_with.h>
 #include <__algorithm/ranges_swap_ranges.h>
 #include <__algorithm/ranges_transform.h>
 #include <__algorithm/ranges_unique.h>
 #include <__algorithm/ranges_unique_copy.h>
 #include <__algorithm/ranges_upper_bound.h>
 #include <__algorithm/remove.h>
 #include <__algorithm/remove_copy.h>
 #include <__algorithm/remove_copy_if.h>

libcxx/include/module.modulemap.in

module __algorithm {
         private header "__algorithm/ranges_sort_heap.h"
         export functional.__functional.ranges_operations
       }
       module ranges_stable_partition         { private header "__algorithm/ranges_stable_partition.h" }
       module ranges_stable_sort {
         private header "__algorithm/ranges_stable_sort.h"
         export functional.__functional.ranges_operations
       }
+      module ranges_starts_with         { private header "__algorithm/ranges_starts_with.h" }
       module ranges_swap_ranges {
         private header "__algorithm/ranges_swap_ranges.h"
         export algorithm.__algorithm.in_in_result
       }
       module ranges_transform {
         private header "__algorithm/ranges_transform.h"
         export algorithm.__algorithm.in_in_out_result
         export algorithm.__algorithm.in_out_result

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

#endif
     (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); }
     if (!std::is_constant_evaluated()) { (void)std::ranges::stable_sort(first, last, Less(&copies)); assert(copies == 0); }
     if (!std::is_constant_evaluated()) { (void)std::ranges::stable_sort(a, Less(&copies)); assert(copies == 0); }
 #if TEST_STD_VER > 20
-    //(void)std::ranges::starts_with(first, last, first2, last2, Equal(&copies)); assert(copies == 0);
+    (void)std::ranges::starts_with(first, last, first2, last2, Equal(&copies)); assert(copies == 0);
+    (void)std::ranges::starts_with(a, b, Equal(&copies)); assert(copies == 0);
 #endif
     (void)std::ranges::transform(first, last, first2, UnaryTransform(&copies)); assert(copies == 0);
     (void)std::ranges::transform(a, first2, UnaryTransform(&copies)); assert(copies == 0);
     (void)std::ranges::transform(first, mid, mid, last, first2, BinaryTransform(&copies)); assert(copies == 0);
     (void)std::ranges::transform(a, b, first2, BinaryTransform(&copies)); assert(copies == 0);
     (void)std::ranges::unique(first, last, Equal(&copies)); assert(copies == 0);
     (void)std::ranges::unique(a, Equal(&copies)); assert(copies == 0);
     (void)std::ranges::unique_copy(first, last, first2, Equal(&copies)); assert(copies == 0);

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

#endif
     (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); }
     if (!std::is_constant_evaluated()) { (void)std::ranges::stable_sort(first, last, Less(), Proj(&copies)); assert(copies == 0); }
     if (!std::is_constant_evaluated()) { (void)std::ranges::stable_sort(a, Less(), Proj(&copies)); assert(copies == 0); }
 #if TEST_STD_VER > 20
-    //(void)std::ranges::starts_with(first, last, first2, last2, Equal(), Proj(&copies), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::starts_with(first, last, first2, last2, Equal(), Proj(&copies), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::starts_with(a, b, Equal(), Proj(&copies), Proj(&copies)); assert(copies == 0);
 #endif
     (void)std::ranges::transform(first, last, first2, UnaryTransform(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::transform(a, first2, UnaryTransform(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::transform(first, mid, mid, last, first2, BinaryTransform(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::transform(a, b, first2, BinaryTransform(), Proj(&copies), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::unique(first, last, Equal(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::unique(a, Equal(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::unique_copy(first, last, first2, Equal(), Proj(&copies)); assert(copies == 0);

libcxx/test/libcxx/private_headers.verify.cpp

 #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_starts_with.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_starts_with.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'}}
 #include <__algorithm/ranges_unique.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_unique.h'}}
 #include <__algorithm/ranges_unique_copy.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_unique_copy.h'}}
 #include <__algorithm/ranges_upper_bound.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_upper_bound.h'}}
 #include <__algorithm/remove.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/remove.h'}}
 #include <__algorithm/remove_copy.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/remove_copy.h'}}
 #include <__algorithm/remove_copy_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/remove_copy_if.h'}}

libcxx/test/std/algorithms/alg.nonmodifying/alg.starts_with/ranges.starts_with.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, c++20
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
+//          class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
+//   requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
+//   constexpr bool ranges::starts_with(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
+//                                      Proj1 proj1 = {}, Proj2 proj2 = {});
+// template<input_range R1, input_range R2, class Pred = ranges::equal_to, class Proj1 = identity,
+//          class Proj2 = identity>
+//   requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
+//   constexpr bool ranges::starts_with(R1&& r1, R2&& r2, Pred pred = {},
+//                                      Proj1 proj1 = {}, Proj2 proj2 = {});
+
+#include <algorithm>
+#include <array>
+#include <ranges>
+
+#include "almost_satisfies_types.h"
+#include "test_iterators.h"
+
+template <class Iter1, class Sent1, class Iter2, class Sent2 = Iter2>
+constexpr void test_iterators() {
+  {// simply tests
+   {int a[] = {1, 2, 3, 4, 5, 6};
+  int p[] = {1, 2};
+  std::same_as<bool> decltype(auto) ret =
+      std::ranges::starts_with(Iter1(a), Sent1(Iter1(a + 6)), Iter2(p), Sent2(Iter2(p + 2)));
+  assert(ret);
+}
+{
+  int a[]                               = {1, 2, 3, 4, 5, 6};
+  int p[]                               = {1, 2};
+  auto whole                            = std::ranges::subrange(Iter1(a), Sent1(Iter1(a + 6)));
+  auto prefix                           = std::ranges::subrange(Iter2(p), Sent2(Iter2(p + 2)));
+  std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(whole, prefix);
+  assert(ret);
+}
+}
+
+{// prefix doesn't match
+ {int a[] = {1, 2, 3, 4, 5, 6};
+int p[] = {4, 5, 6};
+std::same_as<bool> decltype(auto) ret =
+    std::ranges::starts_with(Iter1(a), Sent1(Iter1(a + 6)), Iter2(p), Sent2(Iter2(p + 3)));
+assert(!ret);
+}
+{
+  int a[]                               = {1, 2, 3, 4, 5, 6};
+  int p[]                               = {4, 5, 6};
+  auto whole                            = std::ranges::subrange(Iter1(a), Sent1(Iter1(a + 6)));
+  auto prefix                           = std::ranges::subrange(Iter2(p), Sent2(Iter2(p + 3)));
+  std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(whole, prefix);
+  assert(!ret);
+}
+}
+
+{// range and prefix are identical
+ {int a[] = {1, 2, 3, 4, 5, 6};
+int p[] = {1, 2, 3, 4, 5, 6};
+std::same_as<bool> decltype(auto) ret =
+    std::ranges::starts_with(Iter1(a), Sent1(Iter1(a + 6)), Iter2(p), Sent2(Iter2(p + 6)));
+assert(ret);
+}
+{
+  int a[]                               = {1, 2, 3, 4, 5, 6};
+  int p[]                               = {1, 2, 3, 4, 5, 6};
+  auto whole                            = std::ranges::subrange(Iter1(a), Sent1(Iter1(a + 6)));
+  auto prefix                           = std::ranges::subrange(Iter2(p), Sent2(Iter2(p + 6)));
+  std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(whole, prefix);
+  assert(ret);
+}
+}
+
+{// prefix is longer than range
+ {int a[] = {1, 2, 3, 4, 5, 6};
+int p[] = {1, 2, 3, 4, 5, 6, 7, 8};
+std::same_as<bool> decltype(auto) ret =
+    std::ranges::starts_with(Iter1(a), Sent1(Iter1(a + 6)), Iter2(p), Sent2(Iter2(p + 8)));
+assert(!ret);
+}
+{
+  int a[]                               = {1, 2, 3, 4, 5, 6};
+  int p[]                               = {1, 2, 3, 4, 5, 6, 7, 8};
+  auto whole                            = std::ranges::subrange(Iter1(a), Sent1(Iter1(a + 6)));
+  auto prefix                           = std::ranges::subrange(Iter2(p), Sent2(Iter2(p + 8)));
+  std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(whole, prefix);
+  assert(!ret);
+}
+}
+
+{// prefix has zero length
+ {int a[] = {1, 2, 3, 4, 5, 6};
+int p[] = {};
+std::same_as<bool> decltype(auto) ret =
+    std::ranges::starts_with(Iter1(a), Sent1(Iter1(a + 6)), Iter2(p), Sent2(Iter2(p)));
+assert(ret);
+}
+{
+  int a[]                               = {1, 2, 3, 4, 5, 6};
+  int p[]                               = {};
+  auto whole                            = std::ranges::subrange(Iter1(a), Sent1(Iter1(a + 6)));
+  auto prefix                           = std::ranges::subrange(Iter2(p), Sent2(Iter2(p)));
+  std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(whole, prefix);
+  assert(ret);
+}
+}
+
+{// range has zero length
+ {int a[] = {};
+int p[] = {1, 2, 3, 4, 5, 6, 7, 8};
+std::same_as<bool> decltype(auto) ret =
+    std::ranges::starts_with(Iter1(a), Sent1(Iter1(a)), Iter2(p), Sent2(Iter2(p + 8)));
+assert(!ret);
+}
+{
+  int a[]                               = {};
+  int p[]                               = {1, 2, 3, 4, 5, 6, 7, 8};
+  auto whole                            = std::ranges::subrange(Iter1(a), Sent1(Iter1(a)));
+  auto prefix                           = std::ranges::subrange(Iter2(p), Sent2(Iter2(p + 8)));
+  std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(whole, prefix);
+  assert(!ret);
+}
+}
+
+{// check that the predicate is used
+ {int a[] = {11, 8, 3, 4, 0, 6};
+int p[]                               = {1, 12};
+std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(
+    Iter1(a), Sent1(Iter1(a + 6)), Iter2(p), Sent2(Iter2(p + 2)), [](int l, int r) { return l > r; });
+assert(!ret);
+}
+{
+  int a[]                               = {11, 8, 3, 4, 0, 6};
+  int p[]                               = {1, 12};
+  auto whole                            = std::ranges::subrange(Iter1(a), Sent1(Iter1(a + 6)));
+  auto prefix                           = std::ranges::subrange(Iter2(p), Sent2(Iter2(p + 2)));
+  std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(whole, prefix, [](int l, int r) { return l > r; });
+  assert(!ret);
+}
+}
+
+{ // check that the projections are used
+  {
+    int a[]                               = {1, 3, 5, 1, 5, 6};
+    int p[]                               = {2, 3, 4};
+    std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(
+        Iter1(a),
+        Sent1(Iter1(a + 6)),
+        Iter2(p),
+        Sent2(Iter2(p + 3)),
+        {},
+        [](int i) { return i + 3; },
+        [](int i) { return i * 2; });
+    assert(ret);
+  }
+  {
+    int a[]                               = {1, 3, 5, 1, 5, 6};
+    int p[]                               = {2, 3, 4};
+    auto whole                            = std::ranges::subrange(Iter1(a), Sent1(Iter1(a + 6)));
+    auto prefix                           = std::ranges::subrange(Iter2(p), Sent2(Iter2(p + 3)));
+    std::same_as<bool> decltype(auto) ret = std::ranges::starts_with(
+        whole, prefix, {}, [](int i) { return i + 3; }, [](int i) { return i * 2; });
+    assert(ret);
+  }
+}
+}
+
+template <class Iter1, class Sent1 = Iter1>
+constexpr void test_iterators2() {
+  test_iterators<Iter1, Sent1, forward_iterator<int*>>();
+  test_iterators<Iter1, Sent1, forward_iterator<int*>, sized_sentinel<forward_iterator<int*>>>();
+  test_iterators<Iter1, Sent1, bidirectional_iterator<int*>>();
+  test_iterators<Iter1, Sent1, bidirectional_iterator<int*>, sized_sentinel<bidirectional_iterator<int*>>>();
+  test_iterators<Iter1, Sent1, random_access_iterator<int*>>();
+  test_iterators<Iter1, Sent1, random_access_iterator<int*>, sized_sentinel<random_access_iterator<int*>>>();
+  test_iterators<Iter1, Sent1, contiguous_iterator<int*>>();
+  test_iterators<Iter1, Sent1, contiguous_iterator<int*>, sized_sentinel<contiguous_iterator<int*>>>();
+  test_iterators<Iter1, Sent1, int*>();
+}
+
+constexpr bool test() {
+  test_iterators2<forward_iterator<int*>>();
+  test_iterators2<forward_iterator<int*>, sized_sentinel<forward_iterator<int*>>>();
+  test_iterators2<bidirectional_iterator<int*>>();
+  test_iterators2<bidirectional_iterator<int*>, sized_sentinel<bidirectional_iterator<int*>>>();
+  test_iterators2<random_access_iterator<int*>>();
+  test_iterators2<random_access_iterator<int*>, sized_sentinel<random_access_iterator<int*>>>();
+  test_iterators2<contiguous_iterator<int*>>();
+  test_iterators2<contiguous_iterator<int*>, sized_sentinel<contiguous_iterator<int*>>>();
+  test_iterators2<int*>();
+
+  { // check that std::invoke is used
+    struct S {
+      int i;
+
+      constexpr S identity() { return *this; }
+
+      constexpr bool compare(const S& s) { return i == s.i; }
+    };
+    {
+      S a[]    = {{1}, {2}, {3}, {4}};
+      S p[]    = {{1}, {2}};
+      auto ret = std::ranges::starts_with(a, a + 4, p, p + 2, &S::compare, &S::identity, &S::identity);
+      assert(ret);
+    }
+    {
+      S a[]    = {{1}, {2}, {3}, {4}};
+      S p[]    = {{1}, {2}};
+      auto ret = std::ranges::starts_with(a, p, &S::compare, &S::identity, &S::identity);
+      assert(ret);
+    }
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}
 No newline at end of file

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

 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_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::starts_with, a, a));
 static_assert(test(std::ranges::swap_ranges, a, a));
 static_assert(test(std::ranges::transform, a, a, triple));
 static_assert(test(std::ranges::unique, a));
 static_assert(test(std::ranges::unique_copy, a, a));
 static_assert(test(std::ranges::upper_bound, a, 42));
 
 // [memory.syn]