[libcxx-commits] [libcxx] [libc++][chrono] Remove non-standard relational operators for `std::chrono::weekday` (PR #98730)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: A. Jiang (frederick-vs-ja)

<details>
<summary>Changes</summary>

These operators are absent in [[time.syn]](https://eel.is/c++draft/time.syn), and a note [[time.cal.wd.overview]/1](https://eel.is/c++draft/time.cal.wd.overview#<!-- -->1) indicates that the absence is intended.

This patch removes the undocumented extension, while providing a migration path for vendors by providing the `_LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS` macro. This macro will be honored for the LLVM 19 release and will be removed after that, at which point allocator will be removed unconditionally.

---
Full diff: https://github.com/llvm/llvm-project/pull/98730.diff


3 Files Affected:

- (modified) libcxx/docs/ReleaseNotes/19.rst (+6) 
- (modified) libcxx/include/__chrono/weekday.h (+3) 
- (modified) libcxx/test/std/time/time.cal/time.cal.weekday/time.cal.weekday.nonmembers/comparisons.pass.cpp (+3) 


``````````diff
diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index a4cf411d2a6c1..623aab42a8f16 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -134,6 +134,12 @@ Deprecations and Removals
   `std-allocator-const <https://clang.llvm.org/extra/clang-tidy/checks/portability/std-allocator-const.html>`
   enabled.
 
+- libc++ no longer supports relational comparison for ``std::chrono::weekday``. The relational comparison operators were
+  provided as an undocumented extension. If you were using relational comparison on ``std::chrono::weekday``, compare
+  the results of ``c_encoding()`` or ``iso_encoding()`` instead. The
+  ``_LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS`` macro can be defined to temporarily re-enable this extension
+  as folks transition their code. This macro will be honored for one released and ignored starting in LLVM 20.
+
 
 Upcoming Deprecations and Removals
 ----------------------------------
diff --git a/libcxx/include/__chrono/weekday.h b/libcxx/include/__chrono/weekday.h
index 5a7dedc6e3a16..86c780cc71825 100644
--- a/libcxx/include/__chrono/weekday.h
+++ b/libcxx/include/__chrono/weekday.h
@@ -79,6 +79,8 @@ _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator==(const weekday& __lhs, con
   return __lhs.c_encoding() == __rhs.c_encoding();
 }
 
+// TODO(LLVM 20): Remove the escape hatch
+#  ifdef _LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS
 _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<(const weekday& __lhs, const weekday& __rhs) noexcept {
   return __lhs.c_encoding() < __rhs.c_encoding();
 }
@@ -94,6 +96,7 @@ _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<=(const weekday& __lhs, con
 _LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>=(const weekday& __lhs, const weekday& __rhs) noexcept {
   return !(__lhs < __rhs);
 }
+#  endif // _LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS
 
 _LIBCPP_HIDE_FROM_ABI inline constexpr weekday operator+(const weekday& __lhs, const days& __rhs) noexcept {
   auto const __mu = static_cast<long long>(__lhs.c_encoding()) + __rhs.count();
diff --git a/libcxx/test/std/time/time.cal/time.cal.weekday/time.cal.weekday.nonmembers/comparisons.pass.cpp b/libcxx/test/std/time/time.cal/time.cal.weekday/time.cal.weekday.nonmembers/comparisons.pass.cpp
index 33d8cd9f776d2..0101d205b5555 100644
--- a/libcxx/test/std/time/time.cal/time.cal.weekday/time.cal.weekday.nonmembers/comparisons.pass.cpp
+++ b/libcxx/test/std/time/time.cal/time.cal.weekday/time.cal.weekday.nonmembers/comparisons.pass.cpp
@@ -17,10 +17,13 @@
 #include <chrono>
 #include <type_traits>
 #include <cassert>
+#include <concepts>
 
 #include "test_macros.h"
 #include "test_comparisons.h"
 
+static_assert(!std::totally_ordered<std::chrono::weekday>);
+
 int main(int, char**)
 {
     using weekday = std::chrono::weekday;

``````````

</details>


https://github.com/llvm/llvm-project/pull/98730