libstdc++: Fix formatting of negative chrono::hh_mm_ss

When formatting with an empty chrono spec ("{}") two minus signs were
being added to hh_mm_ss values. This is because the __is_neg flag was
checked to add one explicitly, and then the ostream operator added
another one.

We should only check the __is_neg flag for durations, because those are
the only types which are modified to be non-negative before calling
_M_format. We don't change hh_mm_ss values to be negative, because that
would require performing arithmetic on the hh_mm_ss members to sum them,
and then again to construct a new hh_mm_ss object with the positive
value.  Instead, we can just be careful about using the __is_neg flag
correctly.

To fix the bug, _M_format_to_ostream no longer checks the __is_neg flag
for non-durations, and _M_format doesn't set it for hh_mm_ss until after
the call to _M_format_to_ostream. We can also avoid setting it for types
that it doesn't apply to, by making the __print_sign lambda only inspect
it for duration and hh_mm_ss types.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_format):
	Do not set __is_neg for hh_mm_ss before calling
	_M_format_to_ostream. Change __print_sign lambda to only check
	__is_neg for durations and hh_mm_ss types.
	(__formatter_chrono::_M_format_to_ostream): Only check __is_neg
	for duration types.
	* testsuite/std/time/hh_mm_ss/io.cc: Check negative values.

2 files changed, 19 insertions, 12 deletions

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index 0c5f9f5..c953013 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -486,11 +486,6 @@ namespace __format
 	_M_format(const _Tp& __t, _FormatContext& __fc,
 		  bool __is_neg = false) const
 	{
-	  if constexpr (__is_specialization_of<_Tp, chrono::hh_mm_ss>)
-	    __is_neg = __t.is_negative();
-	  else if constexpr (!chrono::__is_duration_v<_Tp>)
-	    __is_neg = false;
-
 	  auto __first = _M_spec._M_chrono_specs.begin();
 	  const auto __last = _M_spec._M_chrono_specs.end();
 	  if (__first == __last)
@@ -513,12 +508,19 @@ namespace __format
 	  else
 	    __out = __sink.out();
 
+	  // formatter<duration> passes the correct value of __is_neg
+	  // for durations but for hh_mm_ss we decide it here.
+	  if constexpr (__is_specialization_of<_Tp, chrono::hh_mm_ss>)
+	    __is_neg = __t.is_negative();
+
 	  auto __print_sign = [&__is_neg, &__out] {
-	    if (__is_neg)
-	      {
-		*__out++ = _S_plus_minus[1];
-		__is_neg = false;
-	      }
+	    if constexpr (chrono::__is_duration_v<_Tp>
+			    || __is_specialization_of<_Tp, chrono::hh_mm_ss>)
+	      if (__is_neg)
+		{
+		  *__out++ = _S_plus_minus[1];
+		  __is_neg = false;
+		}
 	    return std::move(__out);
 	  };
 
@@ -708,8 +710,9 @@ namespace __format
 		__os << __t._M_date << ' ' << __t._M_time;
 	      else
 		{
-		  if (__is_neg) [[unlikely]]
-		    __os << _S_plus_minus[1];
+		  if constexpr (chrono::__is_duration_v<_Tp>)
+		    if (__is_neg) [[unlikely]]
+		      __os << _S_plus_minus[1];
 		  __os << __t;
 		}
 
diff --git a/libstdc++-v3/testsuite/std/time/hh_mm_ss/io.cc b/libstdc++-v3/testsuite/std/time/hh_mm_ss/io.cc
index 0722343..ddb1ad7 100644
--- a/libstdc++-v3/testsuite/std/time/hh_mm_ss/io.cc
+++ b/libstdc++-v3/testsuite/std/time/hh_mm_ss/io.cc
@@ -47,9 +47,13 @@ test_format()
 
   auto s = std::format("{}", hh_mm_ss{1h + 23min + 45s});
   VERIFY( s == "01:23:45" );
+  s = std::format("{}", hh_mm_ss{-42min});
+  VERIFY( s == "-00:42:00" );
 
   auto ws = std::format(L"{}", hh_mm_ss{1h + 23min + 45s});
   VERIFY( ws == L"01:23:45" );
+  ws = std::format(L"{}", hh_mm_ss{-42min});
+  VERIFY( ws == L"-00:42:00" );
 
   // Locale-specific formats:
   auto loc = std::locale::classic();