Conditionally Borrowed Ranges

stl/inc/ranges

@@ -1988,6 +1988,9 @@ namespace ranges {
     template <class _Rng>
     take_view(_Rng&&, range_difference_t<_Rng>) -> take_view<views::all_t<_Rng>>;
 
+    template <class _Rng>
+    inline constexpr bool enable_borrowed_range<take_view<_Rng>> = enable_borrowed_range<_Rng>;
+
     namespace views {
         // VARIABLE views::take
         template <class>
@@ -2394,6 +2397,9 @@ namespace ranges {
     template <class _Rng>
     drop_view(_Rng&&, range_difference_t<_Rng>) -> drop_view<views::all_t<_Rng>>;
 
+    template <class _Rng>
+    inline constexpr bool enable_borrowed_range<drop_view<_Rng>> = enable_borrowed_range<_Rng>;
+
     namespace views {
         // VARIABLE views::drop
         class _Drop_fn {
@@ -2550,6 +2556,9 @@ namespace ranges {
     template <class _Rng, class _Pr>
     drop_while_view(_Rng&&, _Pr) -> drop_while_view<views::all_t<_Rng>, _Pr>;
 
+    template <class _Rng, class _Pr>
+    inline constexpr bool enable_borrowed_range<drop_while_view<_Rng, _Pr>> = enable_borrowed_range<_Rng>;
+
     namespace views {
         // VARIABLE views::drop_while
         class _Drop_while_fn {
@@ -3502,6 +3511,9 @@ namespace ranges {
     template <class _Rng>
     common_view(_Rng&&) -> common_view<views::all_t<_Rng>>;
 
+    template <class _Rng>
+    inline constexpr bool enable_borrowed_range<common_view<_Rng>> = enable_borrowed_range<_Rng>;
+
     namespace views {
         // VARIABLE views::common
         class _Common_fn : public _Pipe::_Base<_Common_fn> {
@@ -3614,6 +3626,9 @@ namespace ranges {
     template <class _Rng>
     reverse_view(_Rng&&) -> reverse_view<views::all_t<_Rng>>;
 
+    template <class _Rng>
+    inline constexpr bool enable_borrowed_range<reverse_view<_Rng>> = enable_borrowed_range<_Rng>;
+
     namespace views {
         // VARIABLE views::reverse
         template <class _Rng>
@@ -4042,6 +4057,9 @@ namespace ranges {
         }
     };
 
+    template <class _Rng, size_t _Index>
+    inline constexpr bool enable_borrowed_range<elements_view<_Rng, _Index>> = enable_borrowed_range<_Rng>;
+
     template <class _Rng>
     using keys_view = elements_view<views::all_t<_Rng>, 0>;
     template <class _Rng>

stl/inc/yvals_core.h

@@ -240,6 +240,7 @@
 // P1976R2 Explicit Constructors For Fixed-Extent span From Dynamic-Extent Ranges
 // P1983R0 Fixing Minor Ranges Issues
 // P1994R1 elements_view Needs Its Own sentinel
+// P2017R1 Conditionally Borrowed Ranges
 // P2091R0 Fixing Issues With Range Access CPOs
 // P2102R0 Making "Implicit Expression Variations" More Explicit
 // P2106R0 Range Algorithm Result Types

tests/std/tests/P0896R4_views_common/test.cpp

@@ -74,7 +74,7 @@ constexpr bool test_one(Rng&& rng, Expected&& expected) {
     using ranges::common_view, ranges::bidirectional_range, ranges::common_range, ranges::contiguous_range,
         ranges::enable_borrowed_range, ranges::forward_range, ranges::input_range, ranges::iterator_t, ranges::prev,
         ranges::random_access_range, ranges::range, ranges::range_reference_t, ranges::size, ranges::sized_range,
-        ranges::range_size_t;
+        ranges::range_size_t, ranges::borrowed_range;
 
     constexpr bool is_view   = ranges::view<remove_cvref_t<Rng>>;
     using V                  = views::all_t<Rng>;
@@ -316,8 +316,8 @@ constexpr bool test_one(Rng&& rng, Expected&& expected) {
         }
 
         // Validate view_interface::data
-        static_assert(!CanData<R>);
-        static_assert(!CanData<const R>);
+        static_assert(CanData<R> == contiguous_range<R>);
+        static_assert(CanData<const R> == contiguous_range<const R>);
         if (!is_constant_evaluated() && !is_empty) {
             // Validate view_interface::operator[]
             if constexpr (CanIndex<R>) {
@@ -362,6 +362,9 @@ constexpr bool test_one(Rng&& rng, Expected&& expected) {
         if (!is_empty) {
             assert(*b2.begin() == *begin(expected));
         }
+
+        // Validate borrowed_range
+        static_assert(borrowed_range<R> == borrowed_range<V>);
     }
 
     return true;

tests/std/tests/P0896R4_views_drop/test.cpp

@@ -81,7 +81,7 @@ concept CanViewDrop = requires(Rng&& r) {
 template <ranges::input_range Rng, ranges::random_access_range Expected>
 constexpr bool test_one(Rng&& rng, Expected&& expected) {
     using ranges::drop_view, ranges::common_range, ranges::enable_borrowed_range, ranges::iterator_t, ranges::prev,
-        ranges::range, ranges::sentinel_t, ranges::sized_range;
+        ranges::range, ranges::sentinel_t, ranges::sized_range, ranges::borrowed_range;
     using ranges::input_range, ranges::forward_range, ranges::bidirectional_range, ranges::random_access_range,
         ranges::contiguous_range;
 
@@ -209,6 +209,7 @@ constexpr bool test_one(Rng&& rng, Expected&& expected) {
     STATIC_ASSERT(bidirectional_range<R> == bidirectional_range<Rng>);
     STATIC_ASSERT(random_access_range<R> == random_access_range<Rng>);
     STATIC_ASSERT(contiguous_range<R> == contiguous_range<Rng>);
+    STATIC_ASSERT(borrowed_range<R> == borrowed_range<V>);
 
     // Validate drop_view::size
     STATIC_ASSERT(CanMemberSize<R> == CanSize<Rng>);
@@ -516,4 +517,17 @@ int main() {
 
     STATIC_ASSERT((instantiation_test(), true));
     instantiation_test();
+
+    {
+        // Validate a non-view borrowed range
+        constexpr span s{some_ints};
+        STATIC_ASSERT(test_one(s, only_four_ints));
+        test_one(s, only_four_ints);
+
+        // Validate a view borrowed range
+        constexpr auto v =
+            views::iota(0ull, ranges::size(some_ints)) | views::transform([](auto i) { return some_ints[i]; });
+        STATIC_ASSERT(test_one(v, only_four_ints));
+        test_one(v, only_four_ints);
+    }
 }

tests/std/tests/P0896R4_views_drop_while/test.cpp

@@ -36,7 +36,7 @@ template <ranges::input_range Rng, ranges::random_access_range Expected>
 constexpr bool test_one(Rng&& rng, Expected&& expected) {
     using ranges::drop_while_view, ranges::bidirectional_range, ranges::common_range, ranges::contiguous_range,
         ranges::enable_borrowed_range, ranges::forward_range, ranges::iterator_t, ranges::prev,
-        ranges::random_access_range;
+        ranges::random_access_range, ranges::borrowed_range;
 
     constexpr bool is_view = ranges::view<remove_cvref_t<Rng>>;
 
@@ -48,6 +48,7 @@ constexpr bool test_one(Rng&& rng, Expected&& expected) {
     STATIC_ASSERT(bidirectional_range<R> == bidirectional_range<Rng>);
     STATIC_ASSERT(random_access_range<R> == random_access_range<Rng>);
     STATIC_ASSERT(contiguous_range<R> == contiguous_range<Rng>);
+    STATIC_ASSERT(borrowed_range<R> == borrowed_range<Rng>);
 
     // Validate range adaptor object and range adaptor closure
     constexpr auto closure = views::drop_while(is_less_than<3>);

tests/std/tests/P0896R4_views_elements/test.cpp

@@ -36,7 +36,8 @@ template <ranges::input_range Rng>
 constexpr bool test_one(Rng&& rng) {
     using ranges::elements_view, ranges::bidirectional_range, ranges::common_range, ranges::contiguous_range,
         ranges::enable_borrowed_range, ranges::forward_range, ranges::input_range, ranges::iterator_t, ranges::prev,
-        ranges::random_access_range, ranges::range, ranges::range_reference_t, ranges::sentinel_t;
+        ranges::random_access_range, ranges::range, ranges::range_reference_t, ranges::sentinel_t,
+        ranges::borrowed_range;
 
     using V = views::all_t<Rng>;
     using R = elements_view<V, 0>;
@@ -46,6 +47,7 @@ constexpr bool test_one(Rng&& rng) {
     STATIC_ASSERT(bidirectional_range<R> == bidirectional_range<Rng>);
     STATIC_ASSERT(random_access_range<R> == random_access_range<Rng>);
     STATIC_ASSERT(!contiguous_range<R>);
+    STATIC_ASSERT(borrowed_range<R> == borrowed_range<Rng>);
 
     // ... with lvalue argument
     STATIC_ASSERT(CanViewElements<Rng&>);

tests/std/tests/P0896R4_views_reverse/test.cpp

@@ -26,7 +26,7 @@ template <ranges::bidirectional_range Rng, class Expected>
 constexpr bool test_one(Rng&& rng, Expected&& expected) {
     using ranges::common_range, ranges::reverse_view, ranges::sized_range, ranges::begin, ranges::end, ranges::size,
         ranges::iterator_t, ranges::range_size_t, ranges::random_access_range, ranges::prev,
-        ranges::enable_borrowed_range;
+        ranges::enable_borrowed_range, ranges::borrowed_range;
     constexpr bool is_view = ranges::view<remove_cvref_t<Rng>>;
 
     using V = views::all_t<Rng>;
@@ -35,6 +35,7 @@ constexpr bool test_one(Rng&& rng, Expected&& expected) {
     static_assert(ranges::bidirectional_range<R>);
     static_assert(random_access_range<R> == random_access_range<Rng>);
     static_assert(!ranges::contiguous_range<R>);
+    static_assert(borrowed_range<R> == borrowed_range<Rng>);
 
     // Validate range adapter object
     // ...with lvalue argument

tests/std/tests/P0896R4_views_take/test.cpp

@@ -81,7 +81,7 @@ concept CanViewTake = requires(Rng&& r) {
 template <ranges::input_range Rng, ranges::random_access_range Expected>
 constexpr bool test_one(Rng&& rng, Expected&& expected) {
     using ranges::input_range, ranges::forward_range, ranges::bidirectional_range, ranges::random_access_range,
-        ranges::contiguous_range;
+        ranges::contiguous_range, ranges::borrowed_range;
     using ranges::take_view, ranges::common_range, ranges::enable_borrowed_range, ranges::iterator_t, ranges::prev,
         ranges::range, ranges::sentinel_t, ranges::sized_range;
 
@@ -208,6 +208,7 @@ constexpr bool test_one(Rng&& rng, Expected&& expected) {
     STATIC_ASSERT(bidirectional_range<R> == bidirectional_range<Rng>);
     STATIC_ASSERT(random_access_range<R> == random_access_range<Rng>);
     STATIC_ASSERT(contiguous_range<R> == contiguous_range<Rng>);
+    STATIC_ASSERT(borrowed_range<R> == borrowed_range<V>);
 
     // Validate take_view::size
     STATIC_ASSERT(CanMemberSize<R> == CanSize<Rng>);
@@ -558,4 +559,17 @@ int main() {
 
     STATIC_ASSERT((instantiation_test(), true));
     instantiation_test();
+
+    {
+        // Validate a non-view borrowed range
+        constexpr span s{some_ints};
+        STATIC_ASSERT(test_one(s, only_four_ints));
+        test_one(s, only_four_ints);
+
+        // Validate a view borrowed range
+        constexpr auto v =
+            views::iota(0ull, ranges::size(some_ints)) | views::transform([](auto i) { return some_ints[i]; });
+        STATIC_ASSERT(test_one(v, only_four_ints));
+        test_one(v, only_four_ints);
+    }
 }