Implement LWG-3564: `transform_view::iterator<true>::value_type` and `iterator_category` should use `const F&` (#2961)

stl/inc/ranges

@@ -1903,20 +1903,22 @@ namespace ranges {
         template <bool _Const>
         class _Sentinel;
 
-        template <class _Base>
+        template <bool _Const>
         struct _Category_base {};
 
-        template <forward_range _Base>
-        struct _Category_base<_Base> {
-            using iterator_category =
-                conditional_t<is_lvalue_reference_v<invoke_result_t<_Fn&, range_reference_t<_Base>>>,
-                    conditional_t<derived_from<_Iter_cat_t<iterator_t<_Base>>, contiguous_iterator_tag>,
-                        random_access_iterator_tag, _Iter_cat_t<iterator_t<_Base>>>,
-                    input_iterator_tag>;
+        template <bool _Const>
+            requires forward_range<_Maybe_const<_Const, _Vw>>
+        struct _Category_base<_Const> {
+            using _Base             = _Maybe_const<_Const, _Vw>;
+            using iterator_category = conditional_t<
+                is_lvalue_reference_v<invoke_result_t<_Maybe_const<_Const, _Fn>&, range_reference_t<_Base>>>,
+                conditional_t<derived_from<_Iter_cat_t<iterator_t<_Base>>, contiguous_iterator_tag>,
+                    random_access_iterator_tag, _Iter_cat_t<iterator_t<_Base>>>,
+                input_iterator_tag>;
         };
 
         template <bool _Const>
-        class _Iterator : public _Category_base<_Maybe_const<_Const, _Vw>> {
+        class _Iterator : public _Category_base<_Const> {
         private:
             template <bool>
             friend class _Iterator;
@@ -1946,8 +1948,8 @@ namespace ranges {
             using iterator_concept = conditional_t<random_access_range<_Base>, random_access_iterator_tag,
                 conditional_t<bidirectional_range<_Base>, bidirectional_iterator_tag,
                     conditional_t<forward_range<_Base>, forward_iterator_tag, input_iterator_tag>>>;
-            using value_type       = remove_cvref_t<invoke_result_t<_Fn&, range_reference_t<_Base>>>;
-            using difference_type  = range_difference_t<_Base>;
+            using value_type = remove_cvref_t<invoke_result_t<_Maybe_const<_Const, _Fn>&, range_reference_t<_Base>>>;
+            using difference_type = range_difference_t<_Base>;
 
             // clang-format off
             _Iterator() requires default_initializable<iterator_t<_Base>> = default;

tests/std/tests/P0896R4_views_transform/test.cpp

@@ -321,6 +321,50 @@ constexpr bool test_one(Rng&& rng, Expected&& expected) {
     return true;
 }
 
+// Test a function object whose const and non-const versions behave differently
+struct difference_teller {
+    constexpr auto& operator()(auto&& x) noexcept {
+        auto& ref = x;
+        return ref;
+    }
+
+    constexpr auto operator()(auto&& x) const noexcept {
+        return type_identity<decltype(x)>{};
+    }
+};
+
+template <ranges::input_range Rng>
+constexpr void test_difference_on_const_functor(Rng&& rng) {
+    using ranges::transform_view, ranges::input_range, ranges::forward_range, ranges::bidirectional_range,
+        ranges::random_access_range, ranges::iterator_t, ranges::range_reference_t, ranges::range_value_t;
+
+    using V  = views::all_t<Rng>;
+    using TV = transform_view<V, difference_teller>;
+
+    auto r = forward<Rng>(rng) | views::transform(difference_teller{});
+    STATIC_ASSERT(is_same_v<decltype(r), TV>);
+
+    STATIC_ASSERT(is_lvalue_reference_v<range_reference_t<TV>>);
+    if constexpr (input_range<const TV>) {
+        STATIC_ASSERT(is_object_v<range_reference_t<const TV>>);
+        STATIC_ASSERT(!is_same_v<range_value_t<TV>, range_value_t<const TV>>);
+    }
+
+    if constexpr (forward_range<V>) {
+        using It      = iterator_t<V>;
+        using TVIt    = iterator_t<TV>;
+        using VItCat  = typename iterator_traits<It>::iterator_category;
+        using TVItCat = typename iterator_traits<TVIt>::iterator_category;
+        STATIC_ASSERT(
+            is_same_v<TVItCat, VItCat> //
+            || (is_same_v<TVItCat, random_access_iterator_tag> && is_same_v<VItCat, contiguous_iterator_tag>) );
+    }
+
+    if constexpr (forward_range<const V>) {
+        STATIC_ASSERT(is_same_v<typename iterator_traits<iterator_t<const TV>>::iterator_category, input_iterator_tag>);
+    }
+}
+
 static constexpr int some_ints[]        = {0, 1, 2, 3, 4, 5, 6, 7};
 static constexpr int transformed_ints[] = {8, 9, 10, 11, 12, 13, 14, 15};
 
@@ -329,6 +373,9 @@ struct instantiator {
     static constexpr void call() {
         R r{some_ints};
         test_one(r, transformed_ints);
+
+        R r2{some_ints};
+        test_difference_on_const_functor(r2);
     }
 };