Implement LWG-4074 compatible-joinable-ranges is underconstrained (#4814)

frederick-vs-ja · StephanTLavavej · web-flow · commit de1eb6ff46be · 2024-07-11T15:07:37.000-07:00
Co-authored-by: Stephan T. Lavavej &lt;stl@nuwen.net&gt;
diff --git a/stl/inc/ranges b/stl/inc/ranges
@@ -3049,15 +3049,40 @@ namespace ranges {
     } // namespace views
 
 #if _HAS_CXX23
-    template <class _Rng, class _Pat>
-    concept _Compatible_joinable_ranges =
-        common_with<range_value_t<_Rng>, range_value_t<_Pat>>
-        && common_reference_with<range_reference_t<_Rng>, range_reference_t<_Pat>>
-        && common_reference_with<range_rvalue_reference_t<_Rng>, range_rvalue_reference_t<_Pat>>;
+    template <class... _Rngs>
+    using _Concat_reference_t = common_reference_t<range_reference_t<_Rngs>...>;
+
+    template <class... _Rngs>
+    using _Concat_value_t = common_type_t<range_value_t<_Rngs>...>;
+
+    template <class... _Rngs>
+    using _Concat_rvalue_reference_t = common_reference_t<range_rvalue_reference_t<_Rngs>...>;
+
+    template <class _Ref, class _RRef, class _It>
+    concept _Concat_indirectly_readable_impl = requires(const _It __i) {
+        { *__i } -> convertible_to<_Ref>;
+        { _RANGES iter_move(__i) } -> convertible_to<_RRef>;
+    };
+
+    template <class... _Rngs>
+    concept _Concat_indirectly_readable =
+        common_reference_with<_Concat_reference_t<_Rngs...>&&, _Concat_value_t<_Rngs...>&>
+        && common_reference_with<_Concat_reference_t<_Rngs...>&&, _Concat_rvalue_reference_t<_Rngs...>&&>
+        && common_reference_with<_Concat_rvalue_reference_t<_Rngs...>&&, const _Concat_value_t<_Rngs...>&>
+        && (_Concat_indirectly_readable_impl<_Concat_reference_t<_Rngs...>, _Concat_rvalue_reference_t<_Rngs...>,
+                iterator_t<_Rngs>>
+            && ...);
+
+    template <class... _Rngs>
+    concept _Concatable = requires {
+        typename _Concat_reference_t<_Rngs...>;
+        typename _Concat_value_t<_Rngs...>;
+        typename _Concat_rvalue_reference_t<_Rngs...>;
+    } && _Concat_indirectly_readable<_Rngs...>;
 
     _EXPORT_STD template <input_range _Vw, forward_range _Pat>
         requires view<_Vw> && input_range<range_reference_t<_Vw>> && view<_Pat>
-              && _Compatible_joinable_ranges<range_reference_t<_Vw>, _Pat>
+              && _Concatable<range_reference_t<_Vw>, _Pat>
     class join_with_view;
 
     template <class _Vw, class _Pat>
@@ -3091,7 +3116,7 @@ namespace ranges {
 
     _EXPORT_STD template <input_range _Vw, forward_range _Pat>
         requires view<_Vw> && input_range<range_reference_t<_Vw>> && view<_Pat>
-              && _Compatible_joinable_ranges<range_reference_t<_Vw>, _Pat>
+              && _Concatable<range_reference_t<_Vw>, _Pat>
     class join_with_view : public _Join_with_view_outer_iter_base<_Vw, _Pat> {
     private:
         template <bool _Const>
@@ -3502,7 +3527,7 @@ namespace ranges {
 
         _NODISCARD constexpr auto begin() const
             requires forward_range<const _Vw> && forward_range<const _Pat> && is_reference_v<_InnerRng<true>>
-                  && input_range<_InnerRng<true>>
+                  && input_range<_InnerRng<true>> && _Concatable<range_reference_t<const _Vw>, const _Pat>
         {
             return _Iterator<true>{*this, _RANGES begin(_Range)};
         }
@@ -3519,7 +3544,7 @@ namespace ranges {
 
         _NODISCARD constexpr auto end() const
             requires forward_range<const _Vw> && forward_range<const _Pat> && is_reference_v<_InnerRng<true>>
-                  && input_range<_InnerRng<true>>
+                  && input_range<_InnerRng<true>> && _Concatable<range_reference_t<const _Vw>, const _Pat>
         {
             if constexpr (forward_range<_InnerRng<true>> && common_range<_Vw> && common_range<_InnerRng<true>>) {
                 return _Iterator<true>{*this, _RANGES end(_Range)};
diff --git a/tests/std/tests/P2441R2_views_join_with/test.cpp b/tests/std/tests/P2441R2_views_join_with/test.cpp
@@ -3,6 +3,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <cstddef>
 #include <ranges>
 #include <span>
 #include <string_view>
@@ -674,6 +675,106 @@ void test_lwg3700() { // COMPILE-ONLY
     static_assert(!CanMemberEnd<const J>);
 }
 
+// LWG-4074 "compatible-joinable-ranges is underconstrained"
+
+template <bool CanCommonRead>
+struct ValCommon;
+
+template <bool CanCommonRead>
+struct RefCommon;
+
+template <bool CanCommonRead>
+struct ValX {
+    operator ValCommon<CanCommonRead>() const;
+};
+
+template <bool CanCommonRead>
+struct RefX {
+    operator ValX<CanCommonRead>() const;
+    operator RefCommon<CanCommonRead>() const;
+};
+
+template <bool CanCommonRead>
+struct IterX {
+    using value_type      = ValX<CanCommonRead>;
+    using difference_type = ptrdiff_t;
+
+    RefX<CanCommonRead> operator*() const;
+    IterX& operator++();
+    IterX operator++(int);
+
+    friend bool operator==(const IterX&, const IterX&);
+};
+
+template <bool CanCommonRead>
+struct ValY {
+    operator ValCommon<CanCommonRead>() const;
+};
+
+template <bool CanCommonRead>
+struct RefY {
+    operator ValY<CanCommonRead>() const;
+    operator RefCommon<CanCommonRead>() const;
+};
+
+template <bool CanCommonRead>
+struct IterY {
+    using value_type      = ValY<CanCommonRead>;
+    using difference_type = ptrdiff_t;
+
+    RefY<CanCommonRead> operator*() const;
+    IterY& operator++();
+    IterY operator++(int);
+
+    friend bool operator==(const IterY&, const IterY&);
+};
+
+template <bool CanCommonRead>
+struct ValCommon {};
+
+template <bool CanCommonRead>
+struct std::common_type<ValX<CanCommonRead>, ValY<CanCommonRead>> {
+    using type = ValCommon<CanCommonRead>;
+};
+template <bool CanCommonRead>
+struct std::common_type<ValY<CanCommonRead>, ValX<CanCommonRead>> {
+    using type = ValCommon<CanCommonRead>;
+};
+
+template <bool CanCommonRead>
+struct RefCommon {
+    operator ValCommon<CanCommonRead>() const
+        requires CanCommonRead;
+};
+
+template <bool CanCommonRead, template <class> class XQual, template <class> class YQual>
+    requires convertible_to<XQual<RefX<CanCommonRead>>, RefCommon<CanCommonRead>>
+          && convertible_to<YQual<RefY<CanCommonRead>>, RefCommon<CanCommonRead>>
+struct std::basic_common_reference<RefX<CanCommonRead>, RefY<CanCommonRead>, XQual, YQual> {
+    using type = RefCommon<CanCommonRead>;
+};
+
+template <bool CanCommonRead, template <class> class YQual, template <class> class XQual>
+    requires convertible_to<YQual<RefY<CanCommonRead>>, RefCommon<CanCommonRead>>
+          && convertible_to<XQual<RefX<CanCommonRead>>, RefCommon<CanCommonRead>>
+struct std::basic_common_reference<RefY<CanCommonRead>, RefX<CanCommonRead>, YQual, XQual> {
+    using type = RefCommon<CanCommonRead>;
+};
+
+static_assert(!CanViewJoinWith<span<ranges::subrange<IterX<false>>>, ranges::subrange<IterY<false>>>);
+static_assert(CanViewJoinWith<span<ranges::subrange<IterX<true>>>, ranges::subrange<IterY<true>>>);
+
+struct NonConstReadableRange {
+    const ranges::subrange<IterX<true>>* begin();
+    const ranges::subrange<IterX<true>>* end();
+
+    const ranges::subrange<IterX<false>>* begin() const;
+    const ranges::subrange<IterX<false>>* end() const;
+};
+
+static_assert(CanViewJoinWith<NonConstReadableRange&, ranges::subrange<IterY<true>>>);
+static_assert(!CanViewJoinWith<const NonConstReadableRange&, ranges::subrange<IterY<false>>>);
+
 int main() {
     {
         auto filtered_and_joined =
