[PATCH] libstdc++: Disallow duration of cv-qualified types and references.

[Tomasz Kamiński]

This implements LWG 4481, "Disallow chrono::duration<const T, P>",
which was approved in Croydon 2026.

libstdc++-v3/ChangeLog:

	* include/bits/chrono.h: Add static_assert requiring cv-unqualified
	non-reference type.
	* testsuite/20_util/duration/io.cc: Remove test for duration
	with const rep.
	* testsuite/20_util/duration/requirements/typedefs_neg4.cc:
	New test.
---
Testing on x86_64-linux. All *duration* tests passed in all standard modes.
OK for trunk when all test passes?

 libstdc++-v3/include/bits/chrono.h                 |  4 ++++
 libstdc++-v3/testsuite/20_util/duration/io.cc      | 14 --------------
 .../20_util/duration/requirements/typedefs_neg4.cc | 11 +++++++++++
 3 files changed, 15 insertions(+), 14 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg4.cc

diff --git a/libstdc++-v3/include/bits/chrono.h b/libstdc++-v3/include/bits/chrono.h
index fd91505c16f..d639cfaa485 100644
--- a/libstdc++-v3/include/bits/chrono.h
+++ b/libstdc++-v3/include/bits/chrono.h
@@ -514,6 +514,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     template<typename _Rep, typename _Period>
       class duration
       {
+	// _GLIBCXX_RESOLVE_LIB_DEFECTS
+	// 4481. Disallow chrono::duration<const T, P>
+	static_assert(is_same<_Rep, __remove_cvref_t<_Rep>>::value,
+		      "rep should be cv-unqualified object type");
 	static_assert(!__is_duration<_Rep>::value,
 		      "rep cannot be a std::chrono::duration");
 	static_assert(__is_ratio<_Period>::value,
diff --git a/libstdc++-v3/testsuite/20_util/duration/io.cc b/libstdc++-v3/testsuite/20_util/duration/io.cc
index 6ada4fcfa84..463d9feb21f 100644
--- a/libstdc++-v3/testsuite/20_util/duration/io.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/io.cc
@@ -38,13 +38,6 @@ test01()
   ss << duration<char16_t>(126) << ' ';
   ss << duration<char32_t>(127) << ' ';
   VERIFY( ss.str() == "121s 122s 123s 124s 125s 126s 127s " );
-
-  ss.str("");
-  ss << std::hex << std::uppercase << duration<const char>(0x1A) << ' ';
-  ss << std::hex << std::uppercase << duration<const wchar_t>(0x2A) << ' ';
-  ss << std::hex << std::uppercase << duration<signed char>(0x3A) << ' ';
-  ss << std::scientific << duration<const double>(4.5) << ' ';
-  VERIFY( ss.str() == "1As 2As 3As 4.500000E+00s " );
 }
 
 void
@@ -77,13 +70,6 @@ test02()
   ss << duration<char16_t>(126) << ' ';
   ss << duration<char32_t>(127) << ' ';
   VERIFY( ss.str() == L"121s 122s 123s 124s 125s 126s 127s " );
-
-  ss.str(L"");
-  ss << std::hex << std::uppercase << duration<const char>(0x1A) << ' ';
-  ss << std::hex << std::uppercase << duration<const wchar_t>(0x2A) << ' ';
-  ss << std::hex << std::uppercase << duration<signed char>(0x3A) << ' ';
-  ss << std::scientific << duration<const double>(4.5) << ' ';
-  VERIFY( ss.str() == L"1As 2As 3As 4.500000E+00s " );
 #endif
 }
 
diff --git a/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg4.cc b/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg4.cc
new file mode 100644
index 00000000000..52a5d92cc8b
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg4.cc
@@ -0,0 +1,11 @@
+// { dg-do compile { target c++11 } }
+
+#include <chrono>
+
+const int i = 10;
+// Check if rep is a cv-unqualifed object type
+std::chrono::duration<const int> d1(i); // { dg-error "required from here" }
+std::chrono::duration<volatile int> d2(i); // { dg-error "required from here" }
+std::chrono::duration<const int&> d3(i); // { dg-error "required from here" }
+
+// { dg-error "rep should be cv-unqualified object type" "" { target *-*-* } 0 }
-- 
2.53.0