[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.

256

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

12

52

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.

65

Please use two spaces per indentation level.

217–224

253–254

[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
+
+#include "ranges_mismatch.h"
+#include <__algorithm/in_in_result.h>
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/ranges_operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/indirectly_comparable.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__utility/move.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 <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 __mismatch::__fn::__go(
+               std::move(__first1),
+               std::move(__last1),
+               std::move(__first2),
+               std::move(__last2),
+               __pred,
+               __proj1,
+               __proj2)
+               .in2 == __last2;
+  }
+
+  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 __mismatch::__fn::__go(
+               ranges::begin(__range1),
+               ranges::end(__range1),
+               ranges::begin(__range2),
+               ranges::end(__range2),
+               __pred,
+               __proj1,
+               __proj2)
+               .in2 == ranges::end(__range2);
+  }
+};
+} // 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
+
+// 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 = Iter1, class Iter2 = int*, class Sent2 = Iter2>
+concept HasStartsWithIt = requires (Iter1 first1, Sent1 last1, Iter2 first2, Sent2 last2) {
+  std::ranges::starts_with(first1, last1, first2, last2);
+};
+
+static_assert(HasStartsWithIt<int*>);
+static_assert(HasStartsWithIt<ForwardIteratorNotDerivedFrom>);
+static_assert(HasStartsWithIt<ForwardIteratorNotIncrementable>);
+static_assert(!HasStartsWithIt<int*, SentinelForNotSemiregular>);
+static_assert(!HasStartsWithIt<int*, int*, int**>); // not indirectly comparable
+static_assert(!HasStartsWithIt<int*, SentinelForNotWeaklyEqualityComparableWith>);
+static_assert(HasStartsWithIt<int*, int*, ForwardIteratorNotDerivedFrom>);
+static_assert(HasStartsWithIt<int*, int*, ForwardIteratorNotIncrementable>);
+static_assert(!HasStartsWithIt<int*, int*, int*, SentinelForNotSemiregular>);
+static_assert(!HasStartsWithIt<int*, int*, int*, SentinelForNotWeaklyEqualityComparableWith>);
+
+template <class Range1, class Range2 = UncheckedRange<int*>>
+concept HasStartsWithR = requires (Range1 range1, Range2 range2) {
+  std::ranges::starts_with(range1, range2);
+};
+
+static_assert(HasStartsWithR<UncheckedRange<int*>>);
+static_assert(HasStartsWithR<ForwardRangeNotDerivedFrom>);
+static_assert(!HasStartsWithR<ForwardIteratorNotIncrementable>);
+static_assert(!HasStartsWithR<ForwardRangeNotSentinelSemiregular>);
+static_assert(!HasStartsWithR<ForwardRangeNotSentinelEqualityComparableWith>);
+static_assert(!HasStartsWithR<UncheckedRange<int*>, UncheckedRange<int**>>); // not indirectly comparable
+static_assert(HasStartsWithR<UncheckedRange<int*>, ForwardRangeNotDerivedFrom>);
+static_assert(HasStartsWithR<UncheckedRange<int*>, ForwardRangeNotIncrementable>);
+static_assert(!HasStartsWithR<UncheckedRange<int*>, ForwardRangeNotSentinelSemiregular>);
+static_assert(!HasStartsWithR<UncheckedRange<int*>, ForwardRangeNotSentinelEqualityComparableWith>);
+
+template <class Iter1, class Sent1 = Iter1, 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);
+  }
+ }
+}
+
+constexpr bool test() {
+  types::for_each(types::forward_iterator_list<int*>{}, []<class I2>() {
+    types::for_each(types::forward_iterator_list<int*>{}, []<class I1>() {
+      test_iterators<I1, I1, I2, I2>();
+      test_iterators<I1, sized_sentinel<I1>, I2, I2>();
+      test_iterators<I1, I1, I2, sized_sentinel<I2>>();
+      test_iterators<I1, sized_sentinel<I1>, I2, sized_sentinel<I2>>();
+    });
+});
+
+  { // 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;
+}

libcxx/test/std/algorithms/ranges_robust_against_differing_projections.pass.cpp

constexpr bool test_all() {
   test(std::ranges::set_intersection, in, in2, out, less, proj1, proj2);
   test(std::ranges::set_intersection, in, in2, out2, less, proj1, proj2);
   test(std::ranges::set_difference, in, in2, out, less, proj1, proj2);
   test(std::ranges::set_difference, in, in2, out2, less, proj1, proj2);
   test(std::ranges::set_symmetric_difference, in, in2, out, less, proj1, proj2);
   test(std::ranges::set_symmetric_difference, in, in2, out2, less, proj1, proj2);
   test(std::ranges::set_union, in, in2, out, less, proj1, proj2);
   test(std::ranges::set_union, in, in2, out2, less, proj1, proj2);
-  //test(std::ranges::starts_with, in, in2, eq, proj1, proj2);
+  test(std::ranges::starts_with, in, in2, eq, proj1, proj2);
   //test(std::ranges::ends_with, in, in2, eq, proj1, proj2);
 
   return true;
 }
 
 int main(int, char**) {
   test_all();
   static_assert(test_all());
 
   return 0;
 }

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]