[libcxx-commits] [libcxx] [libc++][ranges] Implement LWG4053 'Unary call to std::views::repeat does not decay the argument' (PR #88612)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: None (yronglin)

<details>
<summary>Changes</summary>

Implement [LWG4053](https://cplusplus.github.io/LWG/issue4053)


---
Full diff: https://github.com/llvm/llvm-project/pull/88612.diff


3 Files Affected:

- (modified) libcxx/docs/Status/Cxx2cIssues.csv (+1-1) 
- (modified) libcxx/include/__ranges/repeat_view.h (+2-2) 
- (modified) libcxx/test/std/ranges/range.factories/range.repeat.view/views_repeat.pass.cpp (+8) 


``````````diff
diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index 008f7418ab9c05..0c4034a1b412c7 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -59,7 +59,7 @@
 "`4038 <https://wg21.link/LWG4038>`__","``std::text_encoding::aliases_view`` should have constexpr iterators","Tokyo March 2024","","",""
 "`4043 <https://wg21.link/LWG4043>`__","""ASCII"" is not a registered character encoding","Tokyo March 2024","|Nothing To Do|","",""
 "`4045 <https://wg21.link/LWG4045>`__","``tuple`` can create dangling references from ``tuple-like``","Tokyo March 2024","","",""
-"`4053 <https://wg21.link/LWG4053>`__","Unary call to ``std::views::repeat`` does not decay the argument","Tokyo March 2024","","","|ranges|"
+"`4053 <https://wg21.link/LWG4053>`__","Unary call to ``std::views::repeat`` does not decay the argument","Tokyo March 2024","|Complete|","19.0","|ranges|"
 "`4054 <https://wg21.link/LWG4054>`__","Repeating a ``repeat_view`` should repeat the view","Tokyo March 2024","","","|ranges|"
 "","","","","",""
 "`3343 <https://wg21.link/LWG3343>`__","Ordering of calls to ``unlock()`` and ``notify_all()`` in Effects element of ``notify_all_at_thread_exit()`` should be reversed","Not Yet Adopted","|Complete|","16.0",""
diff --git a/libcxx/include/__ranges/repeat_view.h b/libcxx/include/__ranges/repeat_view.h
index 5caea757a39314..2544cf496c838a 100644
--- a/libcxx/include/__ranges/repeat_view.h
+++ b/libcxx/include/__ranges/repeat_view.h
@@ -127,8 +127,8 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS repeat_view : public view_interface<r
   _LIBCPP_NO_UNIQUE_ADDRESS _Bound __bound_ = _Bound();
 };
 
-template <class _Tp, class _Bound>
-repeat_view(_Tp, _Bound) -> repeat_view<_Tp, _Bound>;
+template <class _Tp, class _Bound = unreachable_sentinel_t>
+repeat_view(_Tp, _Bound = _Bound()) -> repeat_view<_Tp, _Bound>;
 
 // [range.repeat.iterator]
 template <move_constructible _Tp, semiregular _Bound>
diff --git a/libcxx/test/std/ranges/range.factories/range.repeat.view/views_repeat.pass.cpp b/libcxx/test/std/ranges/range.factories/range.repeat.view/views_repeat.pass.cpp
index 9cbe505621b989..9077f9a0c607cd 100644
--- a/libcxx/test/std/ranges/range.factories/range.repeat.view/views_repeat.pass.cpp
+++ b/libcxx/test/std/ranges/range.factories/range.repeat.view/views_repeat.pass.cpp
@@ -60,6 +60,14 @@ static_assert(!std::is_invocable_v<decltype(std::views::repeat), NonCopyable>);
 // Tp is move_constructible
 static_assert(std::is_invocable_v<decltype(std::views::repeat), MoveOnly>);
 
+// LWG4053: Unary call to std::views::repeat does not decay the argument
+using RPV = std::ranges::repeat_view<const char*>;
+
+static_assert(std::same_as<decltype(std::views::repeat("foo", std::unreachable_sentinel)), RPV>); // OK
+static_assert(std::same_as<decltype(std::views::repeat(+"foo", std::unreachable_sentinel)), RPV>); // OK
+static_assert(std::same_as<decltype(std::views::repeat("foo")), RPV>); // OK since LWG4053
+static_assert(std::same_as<decltype(std::views::repeat(+"foo")), RPV>); // OK
+
 constexpr bool test() {
   assert(*std::views::repeat(33).begin() == 33);
   assert(*std::views::repeat(33, 10).begin() == 33);

``````````

</details>


https://github.com/llvm/llvm-project/pull/88612