Implement LWG-4027 possibly-const-range should prefer returning `co…

…nst R&` (microsoft#5221) Co-authored-by: Stephan T. Lavavej <[email protected]>
frederick-vs-ja · Jan 14, 2025 · 22b5573 · 22b5573
1 parent ba284eb
commit 22b5573
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Showing 5 changed files with 19 additions and 4 deletions.
diff --git a/stl/inc/xutility b/stl/inc/xutility
@@ -2893,7 +2893,7 @@ namespace ranges {
 
     template <input_range _Rng>
     _NODISCARD _MSVC_INTRINSIC constexpr auto& _Possibly_const_range(_Rng& _Range) noexcept {
-        if constexpr (constant_range<const _Rng> && !constant_range<_Rng>) {
+        if constexpr (input_range<const _Rng>) {
             return _STD as_const(_Range);
         } else {
             return _Range;

diff --git a/tests/std/tests/P0896R4_views_join/test.cpp b/tests/std/tests/P0896R4_views_join/test.cpp
@@ -269,7 +269,7 @@ constexpr bool test_one(Outer&& rng, Expected&& expected) {
         static_assert(CanMemberCEnd<const R>
                       == (forward_range<const V> && is_reference_v<range_reference_t<const V>>
                           && input_range<range_reference_t<const V>>) );
-        const same_as<const_sentinel_t<R>> auto cs = r.end();
+        const auto cs = r.end();
         if (!is_empty) {
             if constexpr (bidirectional_range<R> && common_range<R>) {
                 assert(*prev(cs) == *prev(end(expected)));

diff --git a/tests/std/tests/P0896R4_views_lazy_split/test.cpp b/tests/std/tests/P0896R4_views_lazy_split/test.cpp
@@ -359,6 +359,14 @@ constexpr bool test_lwg_3904() {
     return j != r.end();
 }
 
+void test_lwg_4027() { // COMPILE-ONLY
+    auto r   = views::single(0) | views::lazy_split(0);
+    using R1 = decltype((*ranges::cbegin(r)).front());
+    using R2 = decltype((*cbegin(r)).front());
+    static_assert(same_as<R1, R2>);
+    static_assert(is_const_v<remove_reference_t<R1>>);
+}
+
 int main() {
     static_assert(instantiation_test());
     instantiation_test();

diff --git a/tests/std/tests/P0896R4_views_transform/test.cpp b/tests/std/tests/P0896R4_views_transform/test.cpp
@@ -879,6 +879,13 @@ void test_gh_3014() { // COMPILE-ONLY
     [[maybe_unused]] decltype(as_const(r).begin()) i = r.begin(); // Check 'iterator(iterator<!Const> i)'
 }
 
+void test_lwg_4027() { // COMPILE-ONLY
+    auto r     = views::single(0) | views::transform([](int) { return 0; });
+    using CIt1 = decltype(ranges::cbegin(r));
+    using CIt2 = decltype(cbegin(r));
+    static_assert(same_as<CIt1, CIt2>);
+}
+
 int main() {
     { // Validate copyable views
         constexpr span<const int> s{some_ints};

diff --git a/tests/std/tests/P2278R4_ranges_const_range_machinery/test.cpp b/tests/std/tests/P2278R4_ranges_const_range_machinery/test.cpp
@@ -127,8 +127,8 @@ namespace test_prvalue_range {
     static_assert(CanRangeConstIterator<Rng>);
     static_assert(CanRangeConstSentinel<Rng>);
     static_assert(CanRangeConstReference<Rng>);
-    static_assert(same_as<ranges::const_iterator_t<Rng>, const_iterator<ranges::iterator_t<Rng>>>);
-    static_assert(same_as<ranges::const_sentinel_t<Rng>, const_iterator<ranges::iterator_t<Rng>>>);
+    static_assert(!same_as<ranges::const_iterator_t<Rng>, const_iterator<ranges::iterator_t<Rng>>>);
+    static_assert(!same_as<ranges::const_sentinel_t<Rng>, const_iterator<ranges::iterator_t<Rng>>>);
     static_assert(same_as<ranges::range_const_reference_t<Rng>, int>);
     static_assert(ranges::constant_range<Rng>);