[PATCH] libstdc++: Fix std::format of chrono::local_days with {} [PR120293]

[Jonathan Wakely]

Formatting of chrono::local_days with an empty chrono-specs should be
equivalent to inserting it into an ostream, which should use the
overload for inserting chrono::sys_days into an ostream. The
implementation of empty chrono-specs in _M_format_to_ostream takes some
short cuts, and that wasn't being done correctly for chrono::local_days.

libstdc++-v3/ChangeLog:

        PR libstdc++/120293
        * include/bits/chrono_io.h (_M_format_to_ostream): Add special
        case for local_time convertible to local_days.
        * testsuite/std/time/clock/local/io.cc: Check formatting of
        chrono::local_days.
---

Tested x86_64-linux.

 libstdc++-v3/include/bits/chrono_io.h             | 3 +++
 libstdc++-v3/testsuite/std/time/clock/local/io.cc | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/libstdc++-v3/include/bits/chrono_io.h 
b/libstdc++-v3/include/bits/chrono_io.h
index ace8b9f26292..92a3098e808c 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -766,6 +766,9 @@ namespace __format
                  // sys_time with period greater or equal to days:
                  if constexpr (is_convertible_v<_Tp, chrono::sys_days>)
                    __os << _S_date(__t);
+                 // Or a local_time with period greater or equal to days:
+                 else if constexpr (is_convertible_v<_Tp, chrono::local_days>)
+                   __os << _S_date(__t);
                  else // Or it's formatted as "{:L%F %T}":
                    {
                      auto __days = chrono::floor<chrono::days>(__t);
diff --git a/libstdc++-v3/testsuite/std/time/clock/local/io.cc 
b/libstdc++-v3/testsuite/std/time/clock/local/io.cc
index b4d562f36d12..67818e876497 100644
--- a/libstdc++-v3/testsuite/std/time/clock/local/io.cc
+++ b/libstdc++-v3/testsuite/std/time/clock/local/io.cc
@@ -89,6 +89,9 @@ test_format()
 
   s = std::format("{}", local_seconds{});
   VERIFY( s == "1970-01-01 00:00:00" );
+
+  s = std::format("{}", local_days{}); // PR libstdc++/120293
+  VERIFY( s == "1970-01-01" );
 }
 
 void
-- 
2.49.0