add tests for interop with C++23 const_iterator adaptor

Author: m-fila

doc/collections_as_container.md

@@ -167,9 +167,9 @@ In addition to the *LegacyForwardIterator* the C++ standard specifies also the *
 | `std::back_insert_iterator` | ❗ attention | Compatible only with SubsetCollections, otherwise throws `std::invalid_argument` |
 | `std::front_insert_iterator` | ❌ no | `push_front` not defined |
 | `std::insert_iterator` | ❌ no | `insert` not defined |
-| `std::const_iterator` (C++23) | ❌ no | C++23 not supported yet |
+| `std::const_iterator` (C++23) | ❗ attention | `operator->` not defined as `iterator`'s and `const_iterator`'s `operator->` are non-`const`. |
 | `std::move_iterator` | ✔️ yes | Limited usefulness since dereference returns `reference` type not rvalue reference (`&&`) |
-| `std::counted_iterator` | ✔️ yes | `operator->` not defined as it requires `std::contiguous_iterator` |
+| `std::counted_iterator` | ❗ attention | `operator->` not defined as it requires `std::contiguous_iterator` |
 
 
 ## Collection as a *range*

tests/unittests/std_interoperability.cpp

@@ -993,10 +993,40 @@ TEST_CASE("Collection and std iterator adaptors", "[collection][container][adapt
     // TODO add runtime checks here
   }
 
+#if (__cplusplus >= 202302L)
   SECTION("Const iterator") {
-    // C++23 required
-    DOCUMENTED_STATIC_FAILURE((__cplusplus >= 202302L));
+    // iterator
+    STATIC_REQUIRE(std::is_base_of_v<std::input_iterator_tag, std::iterator_traits<iterator>::iterator_category>);
+    STATIC_REQUIRE(std::input_iterator<iterator>);
+    // make_const_iterator(iterator) gives basic_const_iterator not our iterator or our const_iterator
+    STATIC_REQUIRE(std::is_same_v<std::const_iterator<iterator>, std::basic_const_iterator<iterator>>);
+    {
+      auto coll = CollectionType();
+      coll.create().cellID(42);
+      auto cit = std::make_const_iterator(std::begin(coll));
+      REQUIRE((*cit).cellID() == 42);
+      // can't -> because iterators' -> is non-const
+      DOCUMENTED_STATIC_FAILURE(traits::has_member_of_pointer_v<std::const_iterator<iterator>>);
+      // REQUIRE(cit->cellID() == 42)
+      // REQUIRE(counted.base()->cellID() == 42);
+    }
+    // const_iterator
+    STATIC_REQUIRE(std::is_base_of_v<std::input_iterator_tag, std::iterator_traits<const_iterator>::iterator_category>);
+    STATIC_REQUIRE(std::input_iterator<const_iterator>);
+    // make_const_iterator(iterator) gives basic_const_iterator not our iterator or our const_iterator
+    STATIC_REQUIRE(std::is_same_v<std::const_iterator<const_iterator>, std::basic_const_iterator<const_iterator>>);
+    {
+      auto coll = CollectionType();
+      coll.create().cellID(42);
+      auto cit = std::make_const_iterator(std::cbegin(coll));
+      REQUIRE((*cit).cellID() == 42);
+      // can't -> because const_iterators' -> is non-const
+      DOCUMENTED_STATIC_FAILURE(traits::has_member_of_pointer_v<std::const_iterator<const_iterator>>);
+      // REQUIRE(cit->cellID() == 42)
+      // REQUIRE(counted.base()->cellID() == 42);
+    }
   }
+#endif
 
   SECTION("Move iterator") {
     // iterator