P2836R1: basic_const_iterator Should Follow Its Underlying Type's Convertibility

stl/inc/xutility

@@ -2155,6 +2155,19 @@ public:
         return _Current == _Se;
     }
 
+    template <_Not_a_const_iterator _Other>
+        requires _Constant_iterator<_Other> && convertible_to<const _Iter&, _Other>
+    _NODISCARD constexpr operator _Other() const& noexcept(
+        is_nothrow_convertible_v<const _Iter&, _Other>) /* strengthened */ {
+        return _Current;
+    }
+
+    template <_Not_a_const_iterator _Other>
+        requires _Constant_iterator<_Other> && convertible_to<_Iter, _Other>
+    _NODISCARD constexpr operator _Other() && noexcept(is_nothrow_convertible_v<_Iter, _Other>) /* strengthened */ {
+        return _STD move(_Current);
+    }
+
     _NODISCARD constexpr bool operator<(const basic_const_iterator& _Right) const
         noexcept(noexcept(_Fake_copy_init<bool>(_Current < _Right._Current))) // strengthened
         requires random_access_iterator<_Iter>

stl/inc/yvals_core.h

@@ -383,6 +383,7 @@
 // P2693R1 Formatting thread::id And stacktrace
 // P2713R1 Escaping Improvements In std::format
 // P2763R1 Fixing layout_stride's Default Constructor For Fully Static Extents
+// P2836R1 basic_const_iterator Should Follow Its Underlying Type's Convertibility
 
 // _HAS_CXX23 and _SILENCE_ALL_CXX23_DEPRECATION_WARNINGS control:
 // P1413R3 Deprecate aligned_storage And aligned_union
@@ -1831,7 +1832,7 @@ _EMIT_STL_ERROR(STL1004, "C++98 unexpected() is incompatible with C++23 unexpect
 #ifdef __cpp_lib_concepts
 #define __cpp_lib_out_ptr                  202106L
 #define __cpp_lib_print                    202207L
-#define __cpp_lib_ranges_as_const          202207L
+#define __cpp_lib_ranges_as_const          202311L
 #define __cpp_lib_ranges_as_rvalue         202207L
 #define __cpp_lib_ranges_cartesian_product 202207L
 #define __cpp_lib_ranges_chunk             202202L

tests/std/tests/P2278R4_basic_const_iterator/test.cpp

@@ -298,7 +298,89 @@ constexpr void instantiation_test() {
     instantiator::call<test_iterator<contiguous_iterator_tag, CanDifference::yes>>();
 }
 
-int main() {
-    static_assert((instantiation_test(), true));
+template <class T>
+class tracking_input_iterator {
+public:
+    using value_type      = remove_const_t<T>;
+    using difference_type = ptrdiff_t;
+
+    constexpr tracking_input_iterator() = default;
+
+    constexpr tracking_input_iterator(const tracking_input_iterator&) : copied{true} {}
+
+    constexpr tracking_input_iterator(tracking_input_iterator&&) : moved{true} {}
+
+    template <class U>
+        requires convertible_to<const U&, T>
+    constexpr tracking_input_iterator(const tracking_input_iterator<U>&) : copied{true} {}
+
+    template <class U>
+        requires convertible_to<U, T>
+    constexpr tracking_input_iterator(tracking_input_iterator<U>&&) : moved{true} {}
+
+    tracking_input_iterator& operator=(const tracking_input_iterator&) = default;
+    tracking_input_iterator& operator=(tracking_input_iterator&&)      = default;
+
+    constexpr tracking_input_iterator& operator++(); // not defined
+    constexpr void operator++(int); // not defined
+    T& operator*() const; // not defined
+
+    constexpr bool is_copied() const {
+        return copied;
+    }
+
+    constexpr bool is_moved() const {
+        return moved;
+    }
+
+private:
+    bool copied = false;
+    bool moved  = false;
+};
+
+static_assert(input_iterator<tracking_input_iterator<int>>);
+static_assert(!_Constant_iterator<tracking_input_iterator<int>>);
+static_assert(_Constant_iterator<tracking_input_iterator<const int>>);
+
+// P2836R1 basic_const_iterator Should Follow Its Underlying Type's Convertibility
+constexpr void test_p2836r1() {
+    { // Code from P2836R1
+        vector<int> v;
+        auto t  = v | views::take_while([](const int x) { return x < 100; });
+        auto f  = [](vector<int>::const_iterator) {};
+        auto i2 = ranges::cbegin(t);
+        f(i2); // Error before P2836R1
+    }
+
+    {
+        tracking_input_iterator<int> i;
+        basic_const_iterator ci{i};
+
+        tracking_input_iterator<const float> j1 = ci;
+        assert(j1.is_copied());
+        assert(!j1.is_moved());
+
+        tracking_input_iterator<const long> j2 = move(ci);
+        assert(!j2.is_copied());
+        assert(j2.is_moved());
+
+        // Incorrect conversions
+        static_assert(!constructible_from<tracking_input_iterator<void* const>, const decltype(ci)&>);
+        static_assert(!constructible_from<tracking_input_iterator<void* const>, decltype(ci)>);
+
+        // Non-const target iterator
+        static_assert(!constructible_from<tracking_input_iterator<float>, const decltype(ci)&>);
+        static_assert(!constructible_from<tracking_input_iterator<long>, decltype(ci)>);
+    }
+}
+
+constexpr bool all_tests() {
     instantiation_test();
+    test_p2836r1();
+    return true;
+}
+
+int main() {
+    static_assert(all_tests());
+    all_tests();
 }

tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp

@@ -674,7 +674,7 @@ STATIC_ASSERT(__cpp_lib_ranges == 202110L);
 #endif
 
 #if _HAS_CXX23 && defined(__cpp_lib_concepts) // TRANSITION, GH-395
-STATIC_ASSERT(__cpp_lib_ranges_as_const == 202207L);
+STATIC_ASSERT(__cpp_lib_ranges_as_const == 202311L);
 #elif defined(__cpp_lib_ranges_as_const)
 #error __cpp_lib_ranges_as_const is defined
 #endif