[libcxx-commits] [libcxx] Fix LWG issue4481 disallow chrono::duration<const T, P> (PR #207558)

[via libcxx-commits]

https://github.com/emmett2020 created https://github.com/llvm/llvm-project/pull/207558

None

>From 4e0a007bf696183fbb99d73970ea5fc6b2b415dc Mon Sep 17 00:00:00 2001
From: Emmett <emmettzhang2020 at outlook.com>
Date: Sun, 5 Jul 2026 13:38:58 +0800
Subject: [PATCH] Fix LWG issue4481 disallow chrono::duration<const T, P>

---
 libcxx/include/__chrono/duration.h            |  8 +++++
 .../std/time/time.duration/cv_rep.verify.cpp  | 31 +++++++++++++++++++
 2 files changed, 39 insertions(+)
 create mode 100644 libcxx/test/std/time/time.duration/cv_rep.verify.cpp

diff --git a/libcxx/include/__chrono/duration.h b/libcxx/include/__chrono/duration.h
index 92efd82f72145..16fd4b36690d3 100644
--- a/libcxx/include/__chrono/duration.h
+++ b/libcxx/include/__chrono/duration.h
@@ -22,6 +22,11 @@
 #include <limits>
 #include <ratio>
 
+#if _LIBCPP_STD_VER >= 26
+#  include <__type_traits/is_same.h>
+#  include <__type_traits/remove_cv.h>
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
@@ -164,6 +169,9 @@ template <class _ToDuration, class _Rep, class _Period, enable_if_t<__is_duratio
 
 template <class _Rep, class _Period>
 class duration {
+#if _LIBCPP_STD_VER >= 26
+  static_assert(is_same_v<_Rep, remove_cv_t<_Rep>>, "A duration representation cannot be cv-qualified");
+#endif
   static_assert(!__is_duration_v<_Rep>, "A duration representation can not be a duration");
   static_assert(__is_ratio_v<_Period>, "Second template parameter of duration must be a std::ratio");
   static_assert(_Period::num > 0, "duration period must be positive");
diff --git a/libcxx/test/std/time/time.duration/cv_rep.verify.cpp b/libcxx/test/std/time/time.duration/cv_rep.verify.cpp
new file mode 100644
index 0000000000000..d51181c0c4c6f
--- /dev/null
+++ b/libcxx/test/std/time/time.duration/cv_rep.verify.cpp
@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+// <chrono>
+
+// LWG 4481
+// A program that instantiates duration with a cv-qualified Rep is ill-formed.
+
+#include <chrono>
+
+void test_const() {
+  // expected-error-re@*:* {{static assertion failed {{.*}}A duration representation cannot be cv-qualified}}
+  (void)sizeof(std::chrono::duration<const int>);
+}
+
+void test_volatile() {
+  // expected-error-re@*:* {{static assertion failed {{.*}}A duration representation cannot be cv-qualified}}
+  (void)sizeof(std::chrono::duration<volatile int>);
+}
+
+void test_const_volatile() {
+  // expected-error-re@*:* {{static assertion failed {{.*}}A duration representation cannot be cv-qualified}}
+  (void)sizeof(std::chrono::duration<const volatile int>);
+}