libstdc++: Do not require a time-of-day when parsing sys_days [PR114240]

When parsing a std::chrono::sys_days (or a sys_time with an even longer
period) we should not require a time-of-day to be present in the input,
because we can't represent that in the result type anyway.

Rather than trying to decide which specializations should require a
time-of-date and which should not, follow the direction of Howard
Hinnant's date library, which allows extracting a sys_time of any period
from input that only contains a date, defaulting the time-of-day part to
00:00:00. This seems consistent with the intent of the standard, which
says it's an error "If the parse fails to decode a valid date" (i.e., it
doesn't care about decoding a valid time, only a date).

libstdc++-v3/ChangeLog:

	PR libstdc++/114240
	* include/bits/chrono_io.h (_Parser::operator()): Assume
	hours(0) for a time_point, so that a time is not required
	to be present.
	* testsuite/std/time/parse/114240.cc: New test.

2 files changed, 47 insertions, 1 deletions

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index eaa36b8a074..b9eb3d2be53 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -3157,6 +3157,16 @@ namespace __detail
 	  minutes __tz_offset = __bad_min;
 	  basic_string<_CharT, _Traits> __tz_abbr;
 
+	  if ((_M_need & _ChronoParts::_TimeOfDay)
+		&& (_M_need & _ChronoParts::_Year))
+	    {
+	      // For time_points assume "00:00:00" is implicitly present,
+	      // so we don't fail to parse if it's not (PR libstdc++/114240).
+	      // We will still fail to parse if there's no year+month+day.
+	      __h = hours(0);
+	      __parts = _ChronoParts::_TimeOfDay;
+	    }
+
 	  // bool __is_neg = false; // TODO: how is this handled for parsing?
 
 	  _CharT __mod{}; // One of 'E' or 'O' or nul.
@@ -4098,7 +4108,7 @@ namespace __detail
 	      const bool __need_wday = _M_need & _ChronoParts::_Weekday;
 
 	      // Whether the caller wants _M_sys_days and _M_time.
-	      // Only true for time_points.
+	      // Only true for durations and time_points.
 	      const bool __need_time = _M_need & _ChronoParts::_TimeOfDay;
 
 	      if (__need_wday && __wday != __bad_wday)
diff --git a/libstdc++-v3/testsuite/std/time/parse/114240.cc b/libstdc++-v3/testsuite/std/time/parse/114240.cc
new file mode 100644
index 00000000000..46310efd09a
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/parse/114240.cc
@@ -0,0 +1,36 @@
+// { dg-do run { target c++20 } }
+
+// PR libstdc++/114240 sys_days not being parsed with only a date in the stream
+
+#include <chrono>
+#include <sstream>
+#include <testsuite_hooks.h>
+
+template<class Clock>
+void
+test_parse_date_only()
+{
+  using namespace std::chrono;
+
+  using CDays = time_point<Clock, days>;
+  CDays td;
+  std::istringstream is("2024-03-05");
+  VERIFY( is >> parse("%Y-%m-%d ", td) );
+  if constexpr (std::is_same_v<Clock, std::chrono::local_t>)
+    VERIFY( td == static_cast<local_time<days>>(2024y/March/5) );
+  else
+  {
+    auto tp = clock_cast<Clock>(sys_days(2024y/March/5));
+    VERIFY( td == time_point_cast<days>(tp) );
+  }
+}
+
+int main()
+{
+  test_parse_date_only<std::chrono::system_clock>();
+  test_parse_date_only<std::chrono::utc_clock>();
+  test_parse_date_only<std::chrono::tai_clock>();
+  test_parse_date_only<std::chrono::gps_clock>();
+  test_parse_date_only<std::chrono::file_clock>();
+  test_parse_date_only<std::chrono::local_t>();
+}