libstdc++: Fix parsing of leap seconds as chrono::utc_time [PR114279]

Implementing all chrono::from_stream overloads in terms of
chrono::sys_time meant that a leap second time like 23:59:60.001 cannot
be parsed, because that cannot be represented in a sys_time.

The fix to support parsing leap seconds as utc_time is to convert the
parsed date to utc_time<days> and then add the parsed time to that,
which allows the result to land in a leap second, rather than doing all
the arithmetic with sys_time which doesn't have leap seconds.

For local_time we also allow %S to parse a 60s value, because doing
otherwise might disallow some valid uses. We can't know all use cases
users have for treating times as local_time.

For all other clocks, we can reject times that have 60 or 60.nnn as the
seconds part, because that cannot occur in a valid UNIX, GPS, or TAI
time. Since our chrono::file_clock uses sys_time, it can't occur for
that clock either.

In order to support this a new _M_is_leap_second member is needed in the
_Parser type. This can be added at the end, where most targets currently
have padding bytes. Similar to what I did recently for formatter _Spec
structs, we can also reserve additional padding bits for future
expansion.

This also fixes bugs in the from_stream overloads for utc_time,
tai_time, gps_time, and file_time, which were not using time_point_cast
to explicitly convert to the result type. That's needed because the
result type might have lower precision than the value returned from
from_sys or from_utc, which has a precision no lower than seconds.

libstdc++-v3/ChangeLog:

	PR libstdc++/114279
	* include/bits/chrono_io.h (_Parser::_M_is_leap_second): New
	data member.
	(_Parser::_M_reserved): Reserve padding bits for future use.
	(_Parser::operator()): Set _M_is_leap_second if %S reads 60s.
	(from_stream): Only allow _M_is_leap_second for utc_time and
	local_time. Adjust arithmetic for utc_time so that leap seconds
	are preserved. Use time_point_cast to convert to a possibly
	lower-precision result type.
	* testsuite/std/time/parse.cc: Move to ...
	* testsuite/std/time/parse/parse.cc: ... here.
	* testsuite/std/time/parse/114279.cc: New test.

3 files changed, 115 insertions, 12 deletions

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index 412e8b83fb7..eaa36b8a074 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -2164,6 +2164,8 @@ namespace __detail
       year_month_day _M_ymd{};
       weekday _M_wd{};
       __format::_ChronoParts _M_need;
+      unsigned _M_is_leap_second : 1 {};
+      unsigned _M_reserved : 15 {};
 
       template<typename _CharT, typename _Traits, typename _Alloc>
 	basic_istream<_CharT, _Traits>&
@@ -2742,8 +2744,13 @@ namespace __detail
       __detail::_Parser_t<_Duration> __p(__need);
       if (__p(__is, __fmt, __abbrev, __offset))
 	{
-	  auto __st = __p._M_sys_days + __p._M_time - *__offset;
-	  __tp = chrono::time_point_cast<_Duration>(__st);
+	  if (__p._M_is_leap_second)
+	    __is.setstate(ios_base::failbit);
+	  else
+	    {
+	      auto __st = __p._M_sys_days + __p._M_time - *__offset;
+	      __tp = chrono::time_point_cast<_Duration>(__st);
+	    }
 	}
       return __is;
     }
@@ -2765,9 +2772,21 @@ namespace __detail
 		basic_string<_CharT, _Traits, _Alloc>* __abbrev = nullptr,
 		minutes* __offset = nullptr)
     {
-      sys_time<_Duration> __st;
-      if (chrono::from_stream(__is, __fmt, __st, __abbrev, __offset))
-	__tp = utc_clock::from_sys(__st);
+      minutes __off{};
+      if (!__offset)
+	__offset = &__off;
+      using __format::_ChronoParts;
+      auto __need = _ChronoParts::_Year | _ChronoParts::_Month
+		    | _ChronoParts::_Day | _ChronoParts::_TimeOfDay;
+      __detail::_Parser_t<_Duration> __p(__need);
+      if (__p(__is, __fmt, __abbrev, __offset))
+	{
+	  // Converting to utc_time before adding _M_time is necessary for
+	  // "23:59:60" to correctly produce a time within a leap second.
+	  auto __ut = utc_clock::from_sys(__p._M_sys_days) + __p._M_time
+			- *__offset;
+	  __tp = chrono::time_point_cast<_Duration>(__ut);
+	}
       return __is;
     }
 
@@ -2788,9 +2807,24 @@ namespace __detail
 		basic_string<_CharT, _Traits, _Alloc>* __abbrev = nullptr,
 		minutes* __offset = nullptr)
     {
-      utc_time<_Duration> __ut;
-      if (chrono::from_stream(__is, __fmt, __ut, __abbrev, __offset))
-	__tp = tai_clock::from_utc(__ut);
+      minutes __off{};
+      if (!__offset)
+	__offset = &__off;
+      using __format::_ChronoParts;
+      auto __need = _ChronoParts::_Year | _ChronoParts::_Month
+		    | _ChronoParts::_Day | _ChronoParts::_TimeOfDay;
+      __detail::_Parser_t<_Duration> __p(__need);
+      if (__p(__is, __fmt, __abbrev, __offset))
+	{
+	  if (__p._M_is_leap_second)
+	    __is.setstate(ios_base::failbit);
+	  else
+	    {
+	      auto __st = __p._M_sys_days + __p._M_time - *__offset;
+	      auto __tt = tai_clock::from_utc(utc_clock::from_sys(__st));
+	      __tp = chrono::time_point_cast<_Duration>(__tt);
+	    }
+	}
       return __is;
     }
 
@@ -2811,9 +2845,24 @@ namespace __detail
 		basic_string<_CharT, _Traits, _Alloc>* __abbrev = nullptr,
 		minutes* __offset = nullptr)
     {
-      utc_time<_Duration> __ut;
-      if (chrono::from_stream(__is, __fmt, __ut, __abbrev, __offset))
-	__tp = gps_clock::from_utc(__ut);
+      minutes __off{};
+      if (!__offset)
+	__offset = &__off;
+      using __format::_ChronoParts;
+      auto __need = _ChronoParts::_Year | _ChronoParts::_Month
+		    | _ChronoParts::_Day | _ChronoParts::_TimeOfDay;
+      __detail::_Parser_t<_Duration> __p(__need);
+      if (__p(__is, __fmt, __abbrev, __offset))
+	{
+	  if (__p._M_is_leap_second)
+	    __is.setstate(ios_base::failbit);
+	  else
+	    {
+	      auto __st = __p._M_sys_days + __p._M_time - *__offset;
+	      auto __tt = gps_clock::from_utc(utc_clock::from_sys(__st));
+	      __tp = chrono::time_point_cast<_Duration>(__tt);
+	    }
+	}
       return __is;
     }
 
@@ -2836,7 +2885,7 @@ namespace __detail
     {
       sys_time<_Duration> __st;
       if (chrono::from_stream(__is, __fmt, __st, __abbrev, __offset))
-	__tp = file_clock::from_sys(__st);
+	__tp = chrono::time_point_cast<_Duration>(file_clock::from_sys(__st));
       return __is;
     }
 
@@ -4209,6 +4258,7 @@ namespace __detail
 		    {
 		      __ok = true;
 		      __t += __s;
+		      _M_is_leap_second = __s >= seconds(60);
 		    }
 
 		  if (__ok)
diff --git a/libstdc++-v3/testsuite/std/time/parse/114279.cc b/libstdc++-v3/testsuite/std/time/parse/114279.cc
new file mode 100644
index 00000000000..67e5cba23bd
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/parse/114279.cc
@@ -0,0 +1,53 @@
+// { dg-do run { target c++20 } }
+
+#include <chrono>
+#include <sstream>
+#include <testsuite_hooks.h>
+
+template<class Clock>
+void
+test_leap_second_parsing()
+{
+  std::chrono::time_point<Clock, std::chrono::milliseconds> tp, tp2;
+
+  std::istringstream ss("20161231-23:59:60.05");
+  ss >> std::chrono::parse("%Y%m%d-%T", tp);
+
+  if constexpr (std::is_same_v<Clock, std::chrono::local_t>)
+    VERIFY( ss ); // We allow parsing "23:59:60" as local_time.
+  else
+  {
+    if constexpr (std::is_same_v<Clock, std::chrono::utc_clock>)
+    {
+      // Entire input was consumed.
+      VERIFY( ss );
+      VERIFY( ss.eof() );
+      // The parsed value is the leap second inserted on Jan 1 2017.
+      VERIFY( std::chrono::get_leap_second_info(tp).is_leap_second );
+    }
+    else
+      VERIFY( !ss ); // Other clocks do not allow "HH:MM:60"
+
+    ss.clear();
+    ss.str("20161231-22:59:60.05 -0100"); // Same time at -1h offset.
+    ss >> std::chrono::parse("%Y%m%d-%T %z", tp2);
+
+    if constexpr (std::is_same_v<Clock, std::chrono::utc_clock>)
+    {
+      VERIFY( ss );
+      VERIFY( tp2 == tp );
+    }
+    else
+      VERIFY( !ss );
+  }
+}
+
+int main()
+{
+  test_leap_second_parsing<std::chrono::system_clock>();
+  test_leap_second_parsing<std::chrono::utc_clock>();
+  test_leap_second_parsing<std::chrono::tai_clock>();
+  test_leap_second_parsing<std::chrono::gps_clock>();
+  test_leap_second_parsing<std::chrono::file_clock>();
+  test_leap_second_parsing<std::chrono::local_t>();
+}
diff --git a/libstdc++-v3/testsuite/std/time/parse.cc b/libstdc++-v3/testsuite/std/time/parse/parse.cc
index 8bb0fd0beba..8bb0fd0beba 100644
--- a/libstdc++-v3/testsuite/std/time/parse.cc
+++ b/libstdc++-v3/testsuite/std/time/parse/parse.cc