Adopt LWG-3533 Make base() const & consistent across iterator wrappers that support input_iterators

stl/inc/ranges

@@ -1127,7 +1127,7 @@ namespace ranges {
 #endif // _ITERATOR_DEBUG_LEVEL != 0
             }
 
-            _NODISCARD constexpr iterator_t<_Vw> base() const& noexcept(is_nothrow_copy_constructible_v<
+            _NODISCARD constexpr const iterator_t<_Vw>& base() const& noexcept(is_nothrow_copy_constructible_v<
                 iterator_t<_Vw>>) /* strengthened */ requires copyable<iterator_t<_Vw>> {
                 return _Current;
             }
@@ -1433,7 +1433,7 @@ namespace ranges {
                 : _Current{_STD move(_It._Current)}, _Parent{_It._Parent} {}
             // clang-format on
 
-            _NODISCARD constexpr iterator_t<_Base> base() const& noexcept(is_nothrow_copy_constructible_v<
+            _NODISCARD constexpr const iterator_t<_Base>& base() const& noexcept(is_nothrow_copy_constructible_v<
                 iterator_t<_Base>>) /* strengthened */ requires copyable<iterator_t<_Base>> {
                 return _Current;
             }
@@ -3775,7 +3775,7 @@ namespace ranges {
                 : _Current{_STD move(_It._Current)} {}
             // clang-format on
 
-            _NODISCARD constexpr iterator_t<_Base> base() const& noexcept(
+            _NODISCARD constexpr const iterator_t<_Base>& base() const& noexcept(
                 is_nothrow_copy_constructible_v<iterator_t<_Base>>) /* strengthened */
                 requires copyable<iterator_t<_Base>> {
                 return _Current;

tests/std/test.lst

@@ -385,6 +385,7 @@ tests\P0896R4_views_elements
 tests\P0896R4_views_empty
 tests\P0896R4_views_filter
 tests\P0896R4_views_filter_death
+tests\P0896R4_views_filter_iterator
 tests\P0896R4_views_iota
 tests\P0896R4_views_join
 tests\P0896R4_views_reverse
@@ -395,6 +396,7 @@ tests\P0896R4_views_take_while
 tests\P0896R4_views_take_while_death
 tests\P0896R4_views_transform
 tests\P0896R4_views_transform_death
+tests\P0896R4_views_transform_iterator
 tests\P0898R3_concepts
 tests\P0898R3_identity
 tests\P0912R5_coroutine

tests/std/tests/P0896R4_views_filter_iterator/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\strict_concepts_latest_matrix.lst

tests/std/tests/P0896R4_views_filter_iterator/test.cpp

@@ -0,0 +1,187 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <ranges>
+#include <type_traits>
+#include <utility>
+
+#include <range_algorithm_support.hpp>
+using namespace std;
+
+#if _ITERATOR_DEBUG_LEVEL == 0
+#define NOEXCEPT_IDL0(...) noexcept(__VA_ARGS__)
+#else
+#define NOEXCEPT_IDL0(...) true
+#endif // _ITERATOR_DEBUG_LEVEL == 0
+
+constexpr auto is_even = [](const auto& x) { return x % 2 == 0; };
+using Pred             = remove_const_t<decltype(is_even)>;
+
+struct iterator_instantiator {
+    template <input_iterator Iter>
+    static constexpr void call() {
+        // Pre: Iter is a specialization of test::iterator whose element type is const int
+        int mutable_ints[]   = {0, 1, 2, 3, 4, 5, 6, 7};
+        const auto make_view = [&] {
+            return views::filter(
+                ranges::subrange{Iter{mutable_ints}, test::sentinel<int>{ranges::end(mutable_ints)}}, is_even);
+        };
+        using R = decltype(make_view());
+        using I = ranges::iterator_t<R>;
+        using S = ranges::sentinel_t<R>;
+
+        // Validate nested types
+        static_assert(is_same_v<typename I::iterator_concept,
+            conditional_t<bidirectional_iterator<Iter>, bidirectional_iterator_tag,
+                conditional_t<forward_iterator<Iter>, forward_iterator_tag, input_iterator_tag>>>);
+
+        using C = typename iterator_traits<Iter>::iterator_category;
+        static_assert(is_same_v<typename I::iterator_category,
+            conditional_t<derived_from<C, bidirectional_iterator_tag>, bidirectional_iterator_tag,
+                conditional_t<derived_from<C, forward_iterator_tag>, forward_iterator_tag, input_iterator_tag>>>);
+
+        { // Validate iterator special member functions and base
+            I defaultConstructed{};
+            assert(std::move(defaultConstructed).base().peek() == nullptr);
+            static_assert(is_nothrow_default_constructible_v<I>);
+
+            auto r0 = make_view();
+            I valueConstructed{r0, Iter{mutable_ints}};
+            static_assert(is_nothrow_constructible_v<I, R&, Iter>);
+
+            if constexpr (copyable<Iter>) {
+                I copyConstructed{valueConstructed};
+                assert(copyConstructed == valueConstructed);
+                static_assert(is_nothrow_copy_constructible_v<I>);
+
+                defaultConstructed = copyConstructed;
+                assert(defaultConstructed == valueConstructed);
+                static_assert(is_nothrow_copy_assignable_v<I>);
+                static_assert(same_as<const Iter&, decltype(as_const(copyConstructed).base())>);
+            }
+            assert(std::move(valueConstructed).base().peek() == mutable_ints);
+            static_assert(same_as<Iter, decltype(move(valueConstructed).base())>);
+        }
+
+        { // Validate sentinel constructors and base
+            S defaultConstructed{};
+            assert(defaultConstructed.base().peek() == nullptr);
+            static_assert(is_nothrow_default_constructible_v<S>);
+
+            auto r0 = make_view();
+            S valueConstructed{r0};
+            assert(valueConstructed.base().peek() == end(mutable_ints));
+
+            S copyConstructed{valueConstructed};
+            assert(copyConstructed.base().peek() == valueConstructed.base().peek());
+            static_assert(is_nothrow_copy_constructible_v<S>);
+
+            defaultConstructed = copyConstructed;
+            assert(defaultConstructed.base().peek() == valueConstructed.base().peek());
+            static_assert(is_nothrow_copy_assignable_v<S>);
+        }
+
+        { // Validate dereference ops
+            auto r0 = make_view();
+            auto i0 = r0.begin();
+            assert(*i0 == 0);
+            static_assert(NOEXCEPT_IDL0(*i0));
+
+            assert(ranges::iter_move(i0) == 0); // NB: moving from int leaves it unchanged
+            // static_assert(NOEXCEPT_IDL0(ranges::iter_move(i0)));
+
+            if constexpr (forward_iterator<Iter>) {
+                auto i1 = ranges::next(i0);
+                ranges::iter_swap(i0, i1);
+                assert(mutable_ints[0] == 2);
+                assert(mutable_ints[2] == 0);
+                ranges::iter_swap(i1, i0);
+                assert(mutable_ints[0] == 0);
+                assert(mutable_ints[2] == 2);
+                static_assert(NOEXCEPT_IDL0(ranges::iter_swap(i0, i1)));
+            }
+        }
+
+        { // Validate increments
+            auto r0 = make_view();
+            auto i0 = r0.begin();
+            assert(&++i0 == &i0);
+            assert(std::move(i0).base().peek() == mutable_ints + 2);
+
+            auto r1 = make_view();
+            auto i1 = r1.begin();
+            if constexpr (forward_iterator<Iter>) {
+                assert(i1++ == r1.begin());
+            } else {
+                i1++;
+            }
+            assert(std::move(i1).base().peek() == mutable_ints + 2);
+        }
+
+        if constexpr (bidirectional_iterator<Iter>) { // Validate decrements
+            auto r            = make_view();
+            const auto second = ranges::next(r.begin());
+            auto i            = second;
+            assert(&--i == &i);
+            assert(i.base().peek() == mutable_ints);
+
+            i = second;
+            assert(i-- == second);
+            assert(i.base().peek() == mutable_ints);
+        }
+
+        if constexpr (equality_comparable<Iter>) {
+            // Validate == and !=
+            auto r           = make_view();
+            const auto first = r.begin();
+            const auto last  = r.end();
+
+            assert(first == first);
+            assert(I{} == I{});
+
+            assert(!(first == last));
+            assert(!(last == first));
+
+            assert(!(first != first));
+            assert(!(I{} != I{}));
+
+            if constexpr (forward_iterator<Iter>) {
+                const auto final = ranges::next(first, last);
+                assert(!(first == final));
+                assert(first != final);
+
+                assert(last == final);
+                assert(final == last);
+
+                assert(!(last != final));
+                assert(!(final != last));
+            }
+        }
+    }
+};
+
+template <class Category, test::CanDifference Diff>
+using test_iterator =
+    test::iterator<Category, int, Diff, test::CanCompare{derived_from<Category, forward_iterator_tag>},
+        test::ProxyRef{!derived_from<Category, contiguous_iterator_tag>}>;
+
+constexpr void iterator_instantiation_test() {
+    using test::CanDifference;
+
+    iterator_instantiator::call<test_iterator<input_iterator_tag, CanDifference::no>>();
+
+    iterator_instantiator::call<test_iterator<forward_iterator_tag, CanDifference::no>>();
+    iterator_instantiator::call<test_iterator<forward_iterator_tag, CanDifference::yes>>();
+
+    iterator_instantiator::call<test_iterator<bidirectional_iterator_tag, CanDifference::no>>();
+    iterator_instantiator::call<test_iterator<bidirectional_iterator_tag, CanDifference::yes>>();
+
+    iterator_instantiator::call<test_iterator<random_access_iterator_tag, CanDifference::yes>>();
+    iterator_instantiator::call<test_iterator<contiguous_iterator_tag, CanDifference::yes>>();
+}
+
+int main() {
+    static_assert((iterator_instantiation_test(), true));
+    iterator_instantiation_test();
+}