Repairing counted_iterator (#1939)

Co-authored-by: Casey Carter <[email protected]>
Co-authored-by: Stephan T. Lavavej <[email protected]>

Author: 3 people

stl/inc/iterator

@@ -1038,10 +1038,35 @@ struct iterator_traits<common_iterator<_Iter, _Se>> {
 };
 
 // CLASS TEMPLATE counted_iterator
+template <class _Iter>
+struct _Counted_iterator_value_type_base {};
+
+template <indirectly_readable _Iter>
+struct _Counted_iterator_value_type_base<_Iter> {
+    using value_type = iter_value_t<_Iter>;
+};
+
+template <class _Iter>
+struct _Counted_iterator_category_base : _Counted_iterator_value_type_base<_Iter> {};
+
+template <_Has_member_iterator_category _Iter>
+struct _Counted_iterator_category_base<_Iter> : _Counted_iterator_value_type_base<_Iter> {
+    using iterator_category = typename _Iter::iterator_category;
+};
+
+template <class _Iter>
+struct _Counted_iterator_concept_base : _Counted_iterator_category_base<_Iter> {};
+
+template <_Has_member_iterator_concept _Iter>
+struct _Counted_iterator_concept_base<_Iter> : _Counted_iterator_category_base<_Iter> {
+    using iterator_concept = typename _Iter::iterator_concept;
+};
+
 template <input_or_output_iterator _Iter>
-class counted_iterator {
+class counted_iterator : public _Counted_iterator_concept_base<_Iter> {
 public:
-    using iterator_type = _Iter;
+    using iterator_type   = _Iter;
+    using difference_type = iter_difference_t<_Iter>;
 
     // [counted.iter.const]
     constexpr counted_iterator() requires default_initializable<_Iter> = default;
@@ -1099,6 +1124,10 @@ public:
         return *_Current;
     }
 
+    _NODISCARD constexpr auto operator->() const noexcept requires contiguous_iterator<_Iter> {
+        return _STD to_address(_Current);
+    }
+
     _NODISCARD constexpr decltype(auto) operator[](const iter_difference_t<_Iter> _Diff) const
         requires random_access_iterator<_Iter> {
 #if _ITERATOR_DEBUG_LEVEL != 0
@@ -1323,25 +1352,12 @@ private:
     iter_difference_t<_Iter> _Length = 0;
 };
 
-template <class _Iter>
-struct incrementable_traits<counted_iterator<_Iter>> {
-    using difference_type = iter_difference_t<_Iter>;
-};
-
+// clang-format off
 template <input_iterator _Iter>
+    requires (!_Is_from_primary<iterator_traits<_Iter>>)
 struct iterator_traits<counted_iterator<_Iter>> : iterator_traits<_Iter> {
-    using pointer = void;
-};
-
-template <contiguous_iterator _Iter>
-struct pointer_traits<counted_iterator<_Iter>> { // TRANSITION, address LWG-3408 and include this
-    using pointer         = counted_iterator<_Iter>;
-    using element_type    = remove_reference_t<iter_reference_t<_Iter>>;
-    using difference_type = iter_difference_t<_Iter>;
-
-    _NODISCARD static constexpr element_type* to_address(const pointer _It) noexcept {
-        return _STD to_address(_It.base());
-    }
+    // clang-format on
+    using pointer = conditional_t<contiguous_iterator<_Iter>, add_pointer_t<iter_reference_t<_Iter>>, void>;
 };
 #endif // __cpp_lib_concepts
 

stl/inc/yvals_core.h

@@ -245,6 +245,8 @@
 // P2102R0 Making "Implicit Expression Variations" More Explicit
 // P2106R0 Range Algorithm Result Types
 // P2116R0 Removing tuple-Like Protocol Support From Fixed-Extent span
+// P2259R1 Repairing Input Range Adaptors And counted_iterator
+//     (partially implemented)
 // P2325R3 Views Should Not Be Required To Be Default Constructible
 // P????R? directory_entry::clear_cache()
 

tests/std/tests/P0896R4_common_iterator/test.cpp

@@ -189,7 +189,7 @@ bool test_operator_arrow() {
     assert(*countedIter == P(0, 1));
     assert(countedIter->first == 0);
     assert(countedIter->second == 1);
-    static_assert(is_same_v<decltype(countedIter.operator->()), P*>);
+    static_assert(is_same_v<decltype(countedIter.operator->()), counted_iterator<P*> const&>);
 
     return true;
 }

tests/std/tests/P0896R4_counted_iterator/test.cpp

@@ -305,6 +305,43 @@ struct instantiator {
     }
 };
 
+// Also test P2259R1 Repairing input range adaptors and counted_iterator
+struct simple_forward_iter {
+    using value_type        = double;
+    using difference_type   = long;
+    using iterator_category = input_iterator_tag;
+    using iterator_concept  = forward_iterator_tag;
+
+    value_type operator*() const;
+    simple_forward_iter& operator++();
+    simple_forward_iter operator++(int);
+
+    bool operator==(const simple_forward_iter&) const;
+};
+
+using CI = counted_iterator<simple_forward_iter>;
+
+static_assert(same_as<iterator_traits<simple_forward_iter>::iterator_category, input_iterator_tag>);
+static_assert(forward_iterator<simple_forward_iter>);
+static_assert(forward_iterator<CI>);
+static_assert(!contiguous_iterator<CI>);
+static_assert(same_as<CI::value_type, double>);
+static_assert(same_as<CI::difference_type, long>);
+static_assert(same_as<CI::iterator_category, input_iterator_tag>);
+static_assert(same_as<CI::iterator_concept, forward_iterator_tag>);
+
+void test_P2259() {
+    struct A {
+        int m;
+    };
+    A a[2] = {{1}, {2}};
+    counted_iterator ci{a, 2};
+    reverse_iterator ri{ci + 1};
+    static_assert(contiguous_iterator<decltype(ci)>);
+    assert(ci->m == 1);
+    assert(ri->m == 1);
+}
+
 int main() {
     STATIC_ASSERT((with_writable_iterators<instantiator, int>::call(), true));
     with_writable_iterators<instantiator, int>::call();
@@ -317,4 +354,6 @@ int main() {
         _Seek_wrapped(ci, uci);
         assert((ci == counted_iterator{ranges::next(lst.begin()), 1}));
     }
+
+    test_P2259();
 }