From 16589dc46c45cc6d66369425c158e2f3fe3c14fe Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vojt=C4=9Bch=20Michal?= <vojta.michall@email.cz>
Date: Fri, 21 Nov 2025 13:11:57 +0100
Subject: [PATCH 1/2] Constrain operator<< for chrono::local_time.

---
 stl/inc/chrono                                |  4 ++-
 .../test.cpp                                  | 34 +++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/stl/inc/chrono b/stl/inc/chrono
index deed0e8b51f..f6ad7aa9cd2 100644
--- a/stl/inc/chrono
+++ b/stl/inc/chrono
@@ -5294,7 +5294,9 @@ namespace chrono {
     }
 
     _EXPORT_STD template <class _CharT, class _Traits, class _Duration>
-    basic_ostream<_CharT, _Traits>& operator<<(basic_ostream<_CharT, _Traits>& _Os, const local_time<_Duration>& _Val) {
+    basic_ostream<_CharT, _Traits>& operator<<(basic_ostream<_CharT, _Traits>& _Os, const local_time<_Duration>& _Val)
+        requires requires { _Os << sys_time<_Duration>{_Val.time_since_epoch()}; }
+    {
         return _Os << sys_time<_Duration>{_Val.time_since_epoch()};
     }
 
diff --git a/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp b/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp
index ad371a4d257..1c3c8a51ebc 100644
--- a/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp
+++ b/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp
@@ -20,6 +20,8 @@
 
 #include <timezone_data.hpp>
 
+// Extended to test LWG-4257 "Stream insertion for chrono::local_time should be constrained"
+
 using namespace std;
 using namespace chrono;
 
@@ -1061,7 +1063,39 @@ void test_locale() {
     assert(stream(year_month_weekday_last{2021y / May / Tuesday[last]}) == STR("2021/Mai/Di[last]"));
 }
 
+template <typename T>
+concept ostream_insertable = requires(std::ostream& o, const T& t) { o << t; };
+
+template <typename Dur>
+void check_stream_insertion_operator_for_duration() {
+    if constexpr (ostream_insertable<sys_time<Dur>>) {
+        std::cout << sys_time<Dur>{};
+    }
+    if constexpr (ostream_insertable<local_time<Dur>>) {
+        std::cout << local_time<Dur>{};
+    }
+}
+
+// Test based on example in LWG-4257 (https://cplusplus.github.io/LWG/issue4257)
+void check_stream_insertion_operator() {
+
+    // operator<< is constrained such that it does not participate when underlying duration has floating point rep
+    using ok_dur  = duration<long long>;
+    using bad_dur = duration<double>;
+
+    static_assert(ostream_insertable<sys_time<ok_dur>>);
+    static_assert(ostream_insertable<local_time<ok_dur>>);
+    check_stream_insertion_operator_for_duration<ok_dur>();
+
+    static_assert(!ostream_insertable<sys_time<bad_dur>>);
+    static_assert(!ostream_insertable<local_time<bad_dur>>);
+    check_stream_insertion_operator_for_duration<bad_dur>();
+}
+
 void test() {
+
+    check_stream_insertion_operator();
+
     test_parse_conversion_spec<char>();
     test_parse_conversion_spec<wchar_t>();
 

From dbb77b9ffede36c7ce83ca17145ec471f10d58f1 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@microsoft.com>
Date: Mon, 24 Nov 2025 11:51:26 -0800
Subject: [PATCH 2/2] Code review feedback.

---
 .../test.cpp                                    | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp b/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp
index 1c3c8a51ebc..65a8bc383cf 100644
--- a/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp
+++ b/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp
@@ -1064,22 +1064,26 @@ void test_locale() {
 }
 
 template <typename T>
-concept ostream_insertable = requires(std::ostream& o, const T& t) { o << t; };
+concept ostream_insertable = requires(ostream& o, const T& t) { o << t; };
 
 template <typename Dur>
 void check_stream_insertion_operator_for_duration() {
     if constexpr (ostream_insertable<sys_time<Dur>>) {
-        std::cout << sys_time<Dur>{};
+        ostringstream oss;
+        oss << sys_time<Dur>{};
+        assert(oss.str() == "1970-01-01 00:00:00");
     }
+
     if constexpr (ostream_insertable<local_time<Dur>>) {
-        std::cout << local_time<Dur>{};
+        ostringstream oss;
+        oss << local_time<Dur>{};
+        assert(oss.str() == "1970-01-01 00:00:00");
     }
 }
 
-// Test based on example in LWG-4257 (https://cplusplus.github.io/LWG/issue4257)
+// Test based on example in LWG-4257
 void check_stream_insertion_operator() {
-
-    // operator<< is constrained such that it does not participate when underlying duration has floating point rep
+    // operator<< is constrained such that it does not participate when underlying duration has floating-point rep
     using ok_dur  = duration<long long>;
     using bad_dur = duration<double>;
 
@@ -1093,7 +1097,6 @@ void check_stream_insertion_operator() {
 }
 
 void test() {
-
     check_stream_insertion_operator();
 
     test_parse_conversion_spec<char>();