[PATCH] libstdc++: Constrain operator<< for chrono::local_type

[Jonathan Wakely]

This was reported as LWG 4257 and moved to Tentatively Ready status.

libstdc++-v3/ChangeLog:

        * include/bits/chrono_io.h: Remove unused <iomanip> header.
        (operator<<): Constrain overload for chrono::local_time.
        * testsuite/std/time/clock/local/io.cc: Check constraints.
---

Tested powerpc64le-linux.

 libstdc++-v3/include/bits/chrono_io.h             | 4 +++-
 libstdc++-v3/testsuite/std/time/clock/local/io.cc | 8 ++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/chrono_io.h 
b/libstdc++-v3/include/bits/chrono_io.h
index 809d795cbf2b..593a927811a6 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -37,7 +37,6 @@
 #if __cplusplus >= 202002L
 
 #include <sstream> // ostringstream
-#include <iomanip> // setw, setfill
 #include <format>
 #include <charconv> // from_chars
 #include <stdexcept> // __sso_string
@@ -4014,6 +4013,9 @@ namespace __detail
     inline basic_ostream<_CharT, _Traits>&
     operator<<(basic_ostream<_CharT, _Traits>& __os,
               const local_time<_Duration>& __lt)
+    // _GLIBCXX_RESOLVE_LIB_DEFECTS
+    // 4257. Stream insertion for chrono::local_time should be constrained
+    requires requires(const sys_time<_Duration>& __st) { __os << __st; }
     {
       __os << sys_time<_Duration>{__lt.time_since_epoch()};
       return __os;
diff --git a/libstdc++-v3/testsuite/std/time/clock/local/io.cc 
b/libstdc++-v3/testsuite/std/time/clock/local/io.cc
index 67818e876497..6efd81a87c2a 100644
--- a/libstdc++-v3/testsuite/std/time/clock/local/io.cc
+++ b/libstdc++-v3/testsuite/std/time/clock/local/io.cc
@@ -125,6 +125,14 @@ test_parse()
   VERIFY( tp.time_since_epoch() == 0s );
 }
 
+// LWG 4257. Stream insertion for chrono::local_time should be constrained
+template<typename T>
+concept ostream_insertable = requires (std::ostream& o, const T& t) { o << t; 
};
+using D = std::chrono::duration<double>;
+static_assert( ostream_insertable<std::chrono::local_days> );
+static_assert( ostream_insertable<std::chrono::local_seconds> );
+static_assert( ! ostream_insertable<std::chrono::local_time<D>> );
+
 int main()
 {
   test_ostream();
-- 
2.51.0