libstdc++: Fix parsing of fractional seconds [PR114244]

When converting a chrono::duration<long double> to a result type with an
integer representation we should use chrono::round<_Duration> so that we
don't truncate towards zero. Rounding ensures that e.g. 0.001999s
becomes 2ms not 1ms.

We can also remove some redundant uses of chrono::duration_cast to
convert from seconds to _Duration, because the _Parser class template
requires _Duration type to be able to represent seconds without loss of
precision.

This also fixes a bug where no fractional part would be parsed for
chrono::duration<long double> because its period is ratio<1>. We should
also consider treat_as_floating_point<rep> when deciding whether to skip
reading a fractional part.

libstdc++-v3/ChangeLog:

	PR libstdc++/114244
	* include/bits/chrono_io.h (_Parser::operator()): Remove
	redundant uses of duration_cast. Use chrono::round to convert
	long double value to durations with integer representations.
	Check represenation type when deciding whether to skip parsing
	fractional seconds.
	* testsuite/20_util/duration/114244.cc: New test.
	* testsuite/20_util/duration/io.cc: Check that a floating-point
	duration with ratio<1> precision can be parsed.

libstdc++-v3/include/bits/chrono_io.h

@@ -3113,6 +3113,9 @@ namespace __detail
 	  unsigned __num = 0; // Non-zero for N modifier.
 	  bool __is_flag = false; // True if we're processing a % flag.
 
+	  constexpr bool __is_floating
+	    = treat_as_floating_point_v<typename _Duration::rep>;
+
 	  // If an out-of-range value is extracted (e.g. 61min for %M),
 	  // do not set failbit immediately because we might not need it
 	  // (e.g. parsing chrono::year doesn't care about invalid %M values).
@@ -3195,7 +3198,7 @@ namespace __detail
 			  __d = day(__tm.tm_mday);
 			  __h = hours(__tm.tm_hour);
 			  __min = minutes(__tm.tm_min);
-			  __s = duration_cast<_Duration>(seconds(__tm.tm_sec));
+			  __s = seconds(__tm.tm_sec);
 			}
 		    }
 		  __parts |= _ChronoParts::_DateTime;
@@ -3564,8 +3567,8 @@ namespace __detail
 		      if (!__is_failed(__err))
 			__s = seconds(__tm.tm_sec);
 		    }
-		  else if constexpr (ratio_equal_v<typename _Duration::period,
-						   ratio<1>>)
+		  else if constexpr (_Duration::period::den == 1
+				       && !__is_floating)
 		    {
 		      auto __val = __read_unsigned(__num ? __num : 2);
 		      if (0 <= __val && __val <= 59) [[likely]]
@@ -3577,7 +3580,7 @@ namespace __detail
 			  break;
 			}
 		    }
-		  else
+		  else // Read fractional seconds
 		    {
 		      basic_stringstream<_CharT> __buf;
 		      auto __digit = _S_try_read_digit(__is, __err);
@@ -3626,7 +3629,10 @@ namespace __detail
 			  else
 			    {
 			      duration<long double> __fs(__val);
-			      __s = duration_cast<_Duration>(__fs);
+			      if constexpr (__is_floating)
+				__s = __fs;
+			      else
+				__s = chrono::round<_Duration>(__fs);
 			    }
 			}
 		    }
@@ -3737,7 +3743,7 @@ namespace __detail
 			{
 			  __h = hours(__tm.tm_hour);
 			  __min = minutes(__tm.tm_min);
-			  __s = duration_cast<_Duration>(seconds(__tm.tm_sec));
+			  __s = seconds(__tm.tm_sec);
 			}
 		    }
 		  __parts |= _ChronoParts::_TimeOfDay;

libstdc++-v3/testsuite/20_util/duration/114244.cc

@@ -0,0 +1,36 @@
+// { dg-do run { target c++20 } }
+// { dg-timeout-factor 2 }
+// { dg-require-namedlocale "en_US.ISO8859-1" }
+
+// PR libstdc++/114244 Need to use round when parsing fractional seconds
+
+#include <chrono>
+#include <sstream>
+#include <testsuite_hooks.h>
+
+void
+test_pr114244()
+{
+  using namespace std::chrono;
+  seconds s;
+  milliseconds ms;
+  microseconds us;
+
+  std::istringstream is;
+
+  is.clear();
+  is.str("0.002");
+  VERIFY( is >> parse("%S", ms) );
+  VERIFY( ms == 2ms ); // not 1ms
+
+  is.imbue(std::locale(ISO_8859(1,en_US)));
+  is.clear();
+  is.str("0.002");
+  VERIFY( is >> parse("%S", us) );
+  VERIFY( us == 2000us ); // not 1999us
+}
+
+int main()
+{
+  test_pr114244();
+}

libstdc++-v3/testsuite/20_util/duration/io.cc

@@ -200,6 +200,18 @@ test_parse()
   VERIFY( is >> parse("%S", us) );
   VERIFY( us == 976us );
   VERIFY( is.get() == '5' );
+
+  is.clear();
+  is.str("0.5");
+  std::chrono::duration<double> ds;
+  VERIFY( is >> parse("%S", ds) );
+  VERIFY( ds == 0.5s );
+
+  is.clear();
+  is.str("0.125");
+  std::chrono::duration<double, std::milli> dms;
+  VERIFY( is >> parse("%S", dms) );
+  VERIFY( dms == 0.125s );
 }
 
 int main()