[libc++] [ranges] SFINAE-friendly "write it three times" in views::counted.

[• Quuxplusone]

Summary

Before this patch, the new test's CountedInvocable<int*, int*> would hard-error instead of SFINAEing and cleanly returning false.

Notice that views::counted specifically does NOT work with pipes; counted(42) is ill-formed. This is because counted's first argument is supposed to be an iterator, not a range.

Also in this PR, drive-by replace some stray std:: with _VSTD::-or-nothing, as appropriate. These will be committed in a separate commit, but I want to poke buildkite about them just to be on the safe side.

The new free function base(cpp20_input_iterator) is needed for the new counted test; it's just filling out a bit of the test_iterators.h API that was missing originally. It's on my TODO list to move the other test iterators' base(i) functions into hidden friends too; I've just been procrastinating and/or rabbit-holing. (See D111372, which got most of the way down the rabbit hole and then I went off to do other things and haven't come back yet.)

[jloser]

libcxx/include/__ranges/counted.h

43

Should to_address be SFINAE friendly instead? I vaguely remember running into this in a PR a few months ago (https://reviews.llvm.org/D108855).

[ldionne]

libcxx/include/__compare/weak_order.h

45–51 ↗

(On Diff #392152)

Please make those changes in a separate NFC patch. They really don't belong here.

libcxx/include/__ranges/counted.h

36–37

I think this can be removed now.

62

You're adding [[nodiscard]]. Please call it out in the commit message and add a test for it.

libcxx/test/std/ranges/range.adaptors/range.counted/counted.pass.cpp

20–21

#include <memory> for addressof, <cstddef> to size_t.

[• Quuxplusone]

Rationalize the nodiscard tests.
Fix the GCC failure, and add another test inspired by it.
Remove the dead concept (thanks!).

[ldionne]

LGTM. You can ignore the failing CI, it should have been fixed on main.

libcxx/include/__ranges/counted.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // 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___RANGES_COUNTED_H
 #define _LIBCPP___RANGES_COUNTED_H
 
+#include <__concepts/convertible_to.h>
 #include <__config>
 #include <__iterator/concepts.h>
 #include <__iterator/counted_iterator.h>
 #include <__iterator/default_sentinel.h>
 #include <__iterator/incrementable_traits.h>
 #include <__iterator/iterator_traits.h>
 #include <__memory/pointer_traits.h>
-#include <__ranges/concepts.h>
 #include <__ranges/subrange.h>
-#include <__utility/decay_copy.h>
-#include <__utility/declval.h>
 #include <__utility/forward.h>
 #include <__utility/move.h>
 #include <span>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
 namespace ranges::views {
 
 namespace __counted {
-  template<class _From, class _To>
-  concept __explicitly_convertible = requires {
-    _To(_From{});
-  };
 
   struct __fn {
-    template<class _Iter, class _Diff>
-      requires contiguous_iterator<decay_t<_Iter>> &&
-               __explicitly_convertible<_Diff, iter_difference_t<_Iter>>
+    template<contiguous_iterator _It>
     _LIBCPP_HIDE_FROM_ABI
-    constexpr auto operator()(_Iter&& __it, _Diff __c) const
-      noexcept(noexcept(
-        span(_VSTD::to_address(__it), static_cast<iter_difference_t<_Iter>>(__c))
-      ))
-    {
-      return span(_VSTD::to_address(__it), static_cast<iter_difference_t<_Iter>>(__c));
-    }
-
-    template<class _Iter, class _Diff>
-      requires random_access_iterator<decay_t<_Iter>> &&
-               __explicitly_convertible<_Diff, iter_difference_t<_Iter>>
+    static constexpr auto __go(_It __it, iter_difference_t<_It> __count)
+      noexcept(noexcept(span(_VSTD::to_address(__it), static_cast<size_t>(__count))))
+      // Deliberately omit return-type SFINAE, because to_address is not SFINAE-friendly
+      { return          span(_VSTD::to_address(__it), static_cast<size_t>(__count)); }
+
+    template<random_access_iterator _It>
     _LIBCPP_HIDE_FROM_ABI
-    constexpr auto operator()(_Iter&& __it, _Diff __c) const
-      noexcept(
-        noexcept(__it + static_cast<iter_difference_t<_Iter>>(__c)) &&
-        noexcept(ranges::subrange(_VSTD::forward<_Iter>(__it), _VSTD::__decay_copy(__it)))
-      )
-    {
-      auto __last = __it + static_cast<iter_difference_t<_Iter>>(__c);
-      return ranges::subrange(_VSTD::forward<_Iter>(__it), _VSTD::move(__last));
-    }
+    static constexpr auto __go(_It __it, iter_difference_t<_It> __count)
+      noexcept(noexcept(subrange(__it, __it + __count)))
+      -> decltype(      subrange(__it, __it + __count))
+      { return          subrange(__it, __it + __count); }
 
-    template<class _Iter, class _Diff>
-      requires __explicitly_convertible<_Diff, iter_difference_t<_Iter>>
+    template<class _It>
     _LIBCPP_HIDE_FROM_ABI
-    constexpr auto operator()(_Iter&& __it, _Diff __c) const
-      noexcept(noexcept(
-        ranges::subrange(counted_iterator(_VSTD::forward<_Iter>(__it), __c), default_sentinel)
-      ))
-    {
-      return ranges::subrange(counted_iterator(_VSTD::forward<_Iter>(__it), __c), default_sentinel);
-    }
+    static constexpr auto __go(_It __it, iter_difference_t<_It> __count)
+      noexcept(noexcept(subrange(counted_iterator(_VSTD::move(__it), __count), default_sentinel)))
+      -> decltype(      subrange(counted_iterator(_VSTD::move(__it), __count), default_sentinel))
+      { return          subrange(counted_iterator(_VSTD::move(__it), __count), default_sentinel); }
+
+    template<class _It, convertible_to<iter_difference_t<_It>> _Diff>
+      requires input_or_output_iterator<decay_t<_It>>
+    [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
+    constexpr auto operator()(_It&& __it, _Diff&& __count) const
+      noexcept(noexcept(__go(_VSTD::forward<_It>(__it), _VSTD::forward<_Diff>(__count))))
+      -> decltype(      __go(_VSTD::forward<_It>(__it), _VSTD::forward<_Diff>(__count)))
+      { return          __go(_VSTD::forward<_It>(__it), _VSTD::forward<_Diff>(__count)); }
   };
-}
+
+} // namespace __counted
 
 inline namespace __cpo {
   inline constexpr auto counted = __counted::__fn{};
 } // namespace __cpo
 
 } // namespace ranges::views
 
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___RANGES_COUNTED_H

libcxx/test/libcxx/ranges/range.adaptors/range.all/all.nodiscard.verify.cpp

 // UNSUPPORTED: libcpp-has-no-incomplete-ranges
 
 // Test the libc++ extension that std::views::all is marked as [[nodiscard]].
 
 #include <ranges>
 
 void test() {
   int range[] = {1, 2, 3};
-  auto f = [](int i) { return i; };
 
   std::views::all(range); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   range | std::views::all; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
-  std::views::transform(f) | std::views::all; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
-  std::views::all | std::views::transform(f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::all | std::views::all; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 }

libcxx/test/libcxx/ranges/range.adaptors/range.common.view/adaptor.nodiscard.verify.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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-ranges
 
-// Test the libc++ extension that std::views::common is marked as [[nodiscard]] to avoid
-// the potential for user mistakenly thinking they're calling an algorithm.
+// Test the libc++ extension that std::views::common is marked as [[nodiscard]].
 
 #include <ranges>
 
 void test() {
   int range[] = {1, 2, 3};
 
   std::views::common(range); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   range | std::views::common; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::all | std::views::common; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 }

libcxx/test/libcxx/ranges/range.adaptors/range.counted/adaptor.nodiscard.verify.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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// Test the libc++ extension that std::views::counted is marked as [[nodiscard]].
+
+#include <ranges>
+
+void test() {
+  int range[] = {1, 2, 3};
+
+  std::views::counted(range, 1); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}

libcxx/test/libcxx/ranges/range.adaptors/range.reverse/adaptor.nodiscard.verify.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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-ranges
-// REQUIRES: libc++
 
-// Test the libc++ extension that std::views::reverse is marked as [[nodiscard]] to avoid
-// the potential for user mistakenly thinking they're calling an algorithm.
+// Test the libc++ extension that std::views::reverse is marked as [[nodiscard]].
 
 #include <ranges>
 
 void test() {
   int range[] = {1, 2, 3};
 
   std::views::reverse(range); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   range | std::views::reverse; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
-  std::views::reverse | std::views::reverse; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::all | std::views::reverse; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 }

libcxx/test/libcxx/ranges/range.adaptors/range.transform/adaptor.nodiscard.verify.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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-ranges
 
-// Test the libc++ extension that std::views::transform is marked as [[nodiscard]] to avoid
-// the potential for user mistakenly thinking they're calling an algorithm.
+// Test the libc++ extension that std::views::transform is marked as [[nodiscard]].
 
 #include <ranges>
 
 void test() {
   int range[] = {1, 2, 3};
   auto f = [](int i) { return i; };
 
   std::views::transform(f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   std::views::transform(range, f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   range | std::views::transform(f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
-  std::views::transform(f) | std::views::transform(f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::all | std::views::transform(f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 }

libcxx/test/std/ranges/range.adaptors/range.counted/counted.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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-ranges
 
 // std::views::counted;
 
-#include <concepts>
 #include <ranges>
+#include <cassert>
+#include <concepts>
+#include <cstddef>
+#include <memory>
 #include <span>
+#include <utility>
 
-#include <cassert>
 #include "test_macros.h"
 #include "test_iterators.h"
 
-struct Unrelated {};
+struct RvalueConvertible {
+  RvalueConvertible(const RvalueConvertible&) = delete;
+  operator int() &&;
+};
 
-struct ConvertibleToSize {
-  constexpr operator std::ptrdiff_t() const { return 8; }
+struct LvalueConvertible {
+  LvalueConvertible(const LvalueConvertible&) = delete;
+  operator int() &;
 };
 
-struct ImplicitlyConvertible {
-    operator short();
-    explicit operator std::ptrdiff_t() = delete;
+struct OnlyExplicitlyConvertible {
+  explicit operator int() const;
 };
 
-template<class Iter, class T>
-concept CountedInvocable = requires(Iter& i, T t) { std::views::counted(i, t); };
+template<class... Ts>
+concept CountedInvocable = requires (Ts&&... ts) {
+  std::views::counted(std::forward<Ts>(ts)...);
+};
 
 constexpr bool test() {
   int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
 
   {
-    static_assert( CountedInvocable<contiguous_iterator<int*>, ConvertibleToSize>);
-    static_assert(!CountedInvocable<contiguous_iterator<int*>, ImplicitlyConvertible>);
-    static_assert(!CountedInvocable<contiguous_iterator<int*>, Unrelated>);
+    static_assert(std::addressof(std::views::counted) == std::addressof(std::ranges::views::counted));
 
-    static_assert(std::semiregular<std::remove_const_t<decltype(std::views::counted)>>);
+    auto copy = std::views::counted;
+    static_assert(std::semiregular<decltype(copy)>);
+
+    static_assert( CountedInvocable<int*, size_t>);
+    static_assert(!CountedInvocable<int*, LvalueConvertible>);
+    static_assert( CountedInvocable<int*, LvalueConvertible&>);
+    static_assert( CountedInvocable<int*, RvalueConvertible>);
+    static_assert(!CountedInvocable<int*, RvalueConvertible&>);
+    static_assert(!CountedInvocable<int*, OnlyExplicitlyConvertible>);
+    static_assert(!CountedInvocable<int*, int*>);
+    static_assert(!CountedInvocable<int*>);
+    static_assert(!CountedInvocable<size_t>);
+    static_assert(!CountedInvocable<>);
   }
 
   {
-    {
-      contiguous_iterator<int*> iter(buffer);
-      std::span<int> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.data() == buffer);
+    auto c1 = std::views::counted(buffer, 3);
+    auto c2 = std::views::counted(std::as_const(buffer), 3);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)), std::span<int>);
-    }
-    {
-      const contiguous_iterator<int*> iter(buffer);
-      std::span<int> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.data() == buffer);
+    ASSERT_SAME_TYPE(decltype(c1), std::span<int>);
+    ASSERT_SAME_TYPE(decltype(c2), std::span<const int>);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)), std::span<int>);
-    }
-    {
-      contiguous_iterator<const int*> iter(buffer);
-      std::span<const int> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.data() == buffer);
+    assert(c1.data() == buffer && c1.size() == 3);
+    assert(c2.data() == buffer && c2.size() == 3);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)), std::span<const int>);
-    }
-    {
-      const contiguous_iterator<const int*> iter(buffer);
-      std::span<const int> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.data() == buffer);
+    auto it = contiguous_iterator<int*>(buffer);
+    auto cit = contiguous_iterator<const int*>(buffer);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)), std::span<const int>);
-    }
-  }
+    auto c1 = std::views::counted(it, 3);
+    auto c2 = std::views::counted(std::as_const(it), 3);
+    auto c3 = std::views::counted(std::move(it), 3);
+    auto c4 = std::views::counted(contiguous_iterator<int*>(buffer), 3);
+    auto c5 = std::views::counted(cit, 3);
+    auto c6 = std::views::counted(std::as_const(cit), 3);
+    auto c7 = std::views::counted(std::move(cit), 3);
+    auto c8 = std::views::counted(contiguous_iterator<const int*>(buffer), 3);
 
-  {
-    {
-      random_access_iterator<int*> iter(buffer);
-      std::ranges::subrange<random_access_iterator<int*>> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.begin() == iter);
+    ASSERT_SAME_TYPE(decltype(c1), std::span<int>);
+    ASSERT_SAME_TYPE(decltype(c2), std::span<int>);
+    ASSERT_SAME_TYPE(decltype(c3), std::span<int>);
+    ASSERT_SAME_TYPE(decltype(c4), std::span<int>);
+    ASSERT_SAME_TYPE(decltype(c5), std::span<const int>);
+    ASSERT_SAME_TYPE(decltype(c6), std::span<const int>);
+    ASSERT_SAME_TYPE(decltype(c7), std::span<const int>);
+    ASSERT_SAME_TYPE(decltype(c8), std::span<const int>);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)), std::ranges::subrange<random_access_iterator<int*>>);
-    }
-    {
-      const random_access_iterator<int*> iter(buffer);
-      std::ranges::subrange<random_access_iterator<int*>> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.begin() == iter);
+    assert(c1.data() == buffer && c1.size() == 3);
+    assert(c2.data() == buffer && c2.size() == 3);
+    assert(c3.data() == buffer && c3.size() == 3);
+    assert(c4.data() == buffer && c4.size() == 3);
+    assert(c5.data() == buffer && c5.size() == 3);
+    assert(c6.data() == buffer && c6.size() == 3);
+    assert(c7.data() == buffer && c7.size() == 3);
+    assert(c8.data() == buffer && c8.size() == 3);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)), std::ranges::subrange<random_access_iterator<int*>>);
-    }
-    {
-      random_access_iterator<const int*> iter(buffer);
-      std::ranges::subrange<random_access_iterator<const int*>> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.begin() == iter);
+    auto it = random_access_iterator<int*>(buffer);
+    auto cit = random_access_iterator<const int*>(buffer);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)), std::ranges::subrange<random_access_iterator<const int*>>);
-    }
-    {
-      const random_access_iterator<const int*> iter(buffer);
-      std::ranges::subrange<random_access_iterator<const int*>> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.begin() == iter);
+    auto c1 = std::views::counted(it, 3);
+    auto c2 = std::views::counted(std::as_const(it), 3);
+    auto c3 = std::views::counted(std::move(it), 3);
+    auto c4 = std::views::counted(random_access_iterator<int*>(buffer), 3);
+    auto c5 = std::views::counted(cit, 3);
+    auto c6 = std::views::counted(std::as_const(cit), 3);
+    auto c7 = std::views::counted(std::move(cit), 3);
+    auto c8 = std::views::counted(random_access_iterator<const int*>(buffer), 3);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)), std::ranges::subrange<random_access_iterator<const int*>>);
-    }
+    ASSERT_SAME_TYPE(decltype(c1), std::ranges::subrange<random_access_iterator<int*>>);
+    ASSERT_SAME_TYPE(decltype(c2), std::ranges::subrange<random_access_iterator<int*>>);
+    ASSERT_SAME_TYPE(decltype(c3), std::ranges::subrange<random_access_iterator<int*>>);
+    ASSERT_SAME_TYPE(decltype(c4), std::ranges::subrange<random_access_iterator<int*>>);
+    ASSERT_SAME_TYPE(decltype(c5), std::ranges::subrange<random_access_iterator<const int*>>);
+    ASSERT_SAME_TYPE(decltype(c6), std::ranges::subrange<random_access_iterator<const int*>>);
+    ASSERT_SAME_TYPE(decltype(c7), std::ranges::subrange<random_access_iterator<const int*>>);
+    ASSERT_SAME_TYPE(decltype(c8), std::ranges::subrange<random_access_iterator<const int*>>);
+
+    assert(c1.begin() == it && c1.end() == it + 3);
+    assert(c2.begin() == it && c2.end() == it + 3);
+    assert(c3.begin() == it && c3.end() == it + 3);
+    assert(c4.begin() == it && c4.end() == it + 3);
+    assert(c5.begin() == cit && c5.end() == cit + 3);
+    assert(c6.begin() == cit && c6.end() == cit + 3);
+    assert(c7.begin() == cit && c7.end() == cit + 3);
+    assert(c8.begin() == cit && c8.end() == cit + 3);
   }
 
   {
-    {
-      bidirectional_iterator<int*> iter(buffer);
-      std::ranges::subrange<
-        std::counted_iterator<bidirectional_iterator<int*>>,
-        std::default_sentinel_t> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.begin() == std::counted_iterator(iter, 8));
+    auto it = bidirectional_iterator<int*>(buffer);
+    auto cit = bidirectional_iterator<const int*>(buffer);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)),
-                       std::ranges::subrange<
-                         std::counted_iterator<bidirectional_iterator<int*>>,
-                         std::default_sentinel_t>);
-    }
-    {
-      const bidirectional_iterator<int*> iter(buffer);
-      std::ranges::subrange<
-        std::counted_iterator<bidirectional_iterator<int*>>,
-        std::default_sentinel_t> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.begin() == std::counted_iterator(iter, 8));
+    auto c1 = std::views::counted(it, 3);
+    auto c2 = std::views::counted(std::as_const(it), 3);
+    auto c3 = std::views::counted(std::move(it), 3);
+    auto c4 = std::views::counted(bidirectional_iterator<int*>(buffer), 3);
+    auto c5 = std::views::counted(cit, 3);
+    auto c6 = std::views::counted(std::as_const(cit), 3);
+    auto c7 = std::views::counted(std::move(cit), 3);
+    auto c8 = std::views::counted(bidirectional_iterator<const int*>(buffer), 3);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)),
-                       std::ranges::subrange<
-                         std::counted_iterator<bidirectional_iterator<int*>>,
-                         std::default_sentinel_t>);
-    }
-    {
-      output_iterator<const int*> iter(buffer);
-      std::ranges::subrange<
-        std::counted_iterator<output_iterator<const int*>>,
-        std::default_sentinel_t> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.begin() == std::counted_iterator(iter, 8));
+    using Expected = std::ranges::subrange<std::counted_iterator<decltype(it)>, std::default_sentinel_t>;
+    using ConstExpected = std::ranges::subrange<std::counted_iterator<decltype(cit)>, std::default_sentinel_t>;
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)),
-                       std::ranges::subrange<
-                         std::counted_iterator<output_iterator<const int*>>,
-                         std::default_sentinel_t>);
-    }
-    {
-      const output_iterator<const int*> iter(buffer);
-      std::ranges::subrange<
-        std::counted_iterator<output_iterator<const int*>>,
-        std::default_sentinel_t> s = std::views::counted(iter, 8);
-      assert(s.size() == 8);
-      assert(s.begin() == std::counted_iterator(iter, 8));
+    ASSERT_SAME_TYPE(decltype(c1), Expected);
+    ASSERT_SAME_TYPE(decltype(c2), Expected);
+    ASSERT_SAME_TYPE(decltype(c3), Expected);
+    ASSERT_SAME_TYPE(decltype(c4), Expected);
+    ASSERT_SAME_TYPE(decltype(c5), ConstExpected);
+    ASSERT_SAME_TYPE(decltype(c6), ConstExpected);
+    ASSERT_SAME_TYPE(decltype(c7), ConstExpected);
+    ASSERT_SAME_TYPE(decltype(c8), ConstExpected);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(iter, 8)),
-                       std::ranges::subrange<
-                         std::counted_iterator<output_iterator<const int*>>,
-                         std::default_sentinel_t>);
-    }
-    {
-      cpp20_input_iterator<int*> iter(buffer);
-      std::ranges::subrange<
-        std::counted_iterator<cpp20_input_iterator<int*>>,
-        std::default_sentinel_t> s = std::views::counted(std::move(iter), 8);
-      assert(s.size() == 8);
-      assert(s.begin().base().base() == buffer);
+    assert(c1.begin().base() == it && c1.size() == 3);
+    assert(c2.begin().base() == it && c2.size() == 3);
+    assert(c3.begin().base() == it && c3.size() == 3);
+    assert(c4.begin().base() == it && c4.size() == 3);
+    assert(c5.begin().base() == cit && c5.size() == 3);
+    assert(c6.begin().base() == cit && c6.size() == 3);
+    assert(c7.begin().base() == cit && c7.size() == 3);
+    assert(c8.begin().base() == cit && c8.size() == 3);
+
+    auto it = output_iterator<int*>(buffer);
+
+    auto c1 = std::views::counted(it, 3);
+    auto c2 = std::views::counted(std::as_const(it), 3);
+    auto c3 = std::views::counted(std::move(it), 3);
+    auto c4 = std::views::counted(output_iterator<int*>(buffer), 3);
+
+    using Expected = std::ranges::subrange<std::counted_iterator<decltype(it)>, std::default_sentinel_t>;
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(std::move(iter), 8)),
-                       std::ranges::subrange<
-                         std::counted_iterator<cpp20_input_iterator<int*>>,
-                         std::default_sentinel_t>);
-    }
-    {
-      std::ranges::subrange<
-        std::counted_iterator<cpp20_input_iterator<int*>>,
-        std::default_sentinel_t> s = std::views::counted(cpp20_input_iterator<int*>(buffer), 8);
-      assert(s.size() == 8);
-      assert(s.begin().base().base() == buffer);
+    ASSERT_SAME_TYPE(decltype(c1), Expected);
+    ASSERT_SAME_TYPE(decltype(c2), Expected);
+    ASSERT_SAME_TYPE(decltype(c3), Expected);
+    ASSERT_SAME_TYPE(decltype(c4), Expected);
+
+    assert(base(c1.begin().base()) == buffer && c1.size() == 3);
+    assert(base(c2.begin().base()) == buffer && c2.size() == 3);
+    assert(base(c3.begin().base()) == buffer && c3.size() == 3);
+    assert(base(c4.begin().base()) == buffer && c4.size() == 3);
+
+    auto it = cpp17_input_iterator<int*>(buffer);
 
-      ASSERT_SAME_TYPE(decltype(std::views::counted(cpp20_input_iterator<int*>(buffer), 8)),
-                       std::ranges::subrange<
-                         std::counted_iterator<cpp20_input_iterator<int*>>,
-                         std::default_sentinel_t>);
-    }
+    auto c1 = std::views::counted(it, 3);
+    auto c2 = std::views::counted(std::as_const(it), 3);
+    auto c3 = std::views::counted(std::move(it), 3);
+    auto c4 = std::views::counted(cpp17_input_iterator<int*>(buffer), 3);
+
+    using Expected = std::ranges::subrange<std::counted_iterator<decltype(it)>, std::default_sentinel_t>;
+
+    ASSERT_SAME_TYPE(decltype(c1), Expected);
+    ASSERT_SAME_TYPE(decltype(c2), Expected);
+    ASSERT_SAME_TYPE(decltype(c3), Expected);
+    ASSERT_SAME_TYPE(decltype(c4), Expected);
+
+    assert(base(c1.begin().base()) == buffer && c1.size() == 3);
+    assert(base(c2.begin().base()) == buffer && c2.size() == 3);
+    assert(base(c3.begin().base()) == buffer && c3.size() == 3);
+    assert(base(c4.begin().base()) == buffer && c4.size() == 3);
   }
 
   {
-    static_assert(std::same_as<decltype(std::views::counted), decltype(std::ranges::views::counted)>);
+    auto it = cpp20_input_iterator<int*>(buffer);
+
+    static_assert(!std::copyable<cpp20_input_iterator<int*>>);
+    static_assert(!CountedInvocable<cpp20_input_iterator<int*>&, int>);
+    static_assert(!CountedInvocable<const cpp20_input_iterator<int*>&, int>);
+    auto c3 = std::views::counted(std::move(it), 3);
+    auto c4 = std::views::counted(cpp20_input_iterator<int*>(buffer), 3);
+
+    using Expected = std::ranges::subrange<std::counted_iterator<decltype(it)>, std::default_sentinel_t>;
+
+    ASSERT_SAME_TYPE(decltype(c3), Expected);
+    ASSERT_SAME_TYPE(decltype(c4), Expected);
+
+    assert(base(c3.begin().base()) == buffer && c3.size() == 3);
+    assert(base(c4.begin().base()) == buffer && c4.size() == 3);
   }
 
   return true;
 }
 
 int main(int, char**) {
   test();
   static_assert(test());
 
   return 0;
 }

libcxx/test/support/test_iterators.h

struct cpp20_input_iterator {
     }
 
     constexpr void operator++(int) { ++base_; }
 
     constexpr const I& base() const& { return base_; }
 
     constexpr I base() && { return std::move(base_); }
 
+    friend constexpr I base(const cpp20_input_iterator& i) { return i.base_; }
+
     template <class T>
     void operator,(T const &) = delete;
 
 private:
     I base_ = I();
 };
 
 template <std::input_or_output_iterator I>