Implement LWG-3749 `common_iterator` should handle integer-class difference types (#4190)

stl/inc/iterator

@@ -1103,14 +1103,22 @@ concept _Has_forward_category = requires {
     requires derived_from<_Iter_cat_t<_Iter>, forward_iterator_tag>;
 };
 
-template <input_iterator _Iter, class _Se>
-struct iterator_traits<common_iterator<_Iter, _Se>> {
-    using iterator_concept  = conditional_t<forward_iterator<_Iter>, forward_iterator_tag, input_iterator_tag>;
+template <class _Iter>
+struct _Common_iterator_category_base {};
+
+template <class _Iter>
+    requires is_integral_v<iter_difference_t<_Iter>>
+struct _Common_iterator_category_base<_Iter> {
     using iterator_category = conditional_t<_Has_forward_category<_Iter>, forward_iterator_tag, input_iterator_tag>;
-    using value_type        = iter_value_t<_Iter>;
-    using difference_type   = iter_difference_t<_Iter>;
-    using pointer           = typename _Common_iterator_pointer_type<_Iter, _Se>::pointer;
-    using reference         = iter_reference_t<_Iter>;
+};
+
+template <input_iterator _Iter, class _Se>
+struct iterator_traits<common_iterator<_Iter, _Se>> : _Common_iterator_category_base<_Iter> {
+    using iterator_concept = conditional_t<forward_iterator<_Iter>, forward_iterator_tag, input_iterator_tag>;
+    using value_type       = iter_value_t<_Iter>;
+    using difference_type  = iter_difference_t<_Iter>;
+    using pointer          = typename _Common_iterator_pointer_type<_Iter, _Se>::pointer;
+    using reference        = iter_reference_t<_Iter>;
 };
 
 template <class _Iter>

tests/std/tests/P0896R4_common_iterator/test.cpp

@@ -315,6 +315,24 @@ constexpr bool test_lwg_3574() {
     return true;
 }
 
+template <class It, class Se>
+concept common_iterator_has_iterator_category =
+    requires { typename iterator_traits<common_iterator<It, Se>>::iterator_category; };
+
+// LWG-3749 common_iterator should handle integer-class difference types
+void test_lwg_3749() { // COMPILE-ONLY
+    static_assert(common_iterator_has_iterator_category<int*, const int*>);
+    static_assert(common_iterator_has_iterator_category<int*, unreachable_sentinel_t>);
+
+    using small_unbounded_iota = decltype(views::iota(42));
+    static_assert(common_iterator_has_iterator_category<ranges::iterator_t<small_unbounded_iota>,
+        ranges::sentinel_t<small_unbounded_iota>>);
+
+    using large_unbounded_iota = decltype(views::iota(42ull));
+    static_assert(!common_iterator_has_iterator_category<ranges::iterator_t<large_unbounded_iota>,
+                  ranges::sentinel_t<large_unbounded_iota>>);
+}
+
 // Validate that _Variantish works when fed with a non-trivially-destructible type
 void test_non_trivially_destructible_type() { // COMPILE-ONLY
     struct non_trivially_destructible_input_iterator {