[committed] libstdc++: Fix std::format output of %C for negative years

[Jonathan Wakely <jwakely@redhat.com>]

Tested x86_64-linux. Pushed to trunk.

-- >8--

During discussion of LWG 4022 I noticed that we do not correctly
implement floored division for the century. We were just truncating
towards zero, rather than applying the floor function. For negative
values that rounds the wrong way.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_C_y_Y): Fix
	rounding for negative centuries.
	* testsuite/std/time/year/io.cc: Check %C for negative years.
---
 libstdc++-v3/include/bits/chrono_io.h      | 9 +++++++--
 libstdc++-v3/testsuite/std/time/year/io.cc | 7 +++++--
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index 16e8fc58dff..b63b8592eba 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -820,9 +820,14 @@ namespace __format
 
 	  if (__conv == 'Y' || __conv == 'C')
 	    {
-	      if (__is_neg)
-		__s.assign(1, _S_plus_minus[1]);
 	      int __ci = __yi / 100;
+	      if (__is_neg) [[unlikely]]
+		{
+		  __s.assign(1, _S_plus_minus[1]);
+		  // For floored division -123//100 is -2 and -100//100 is -1
+		  if ((__ci * 100) != __yi)
+		    ++__ci;
+		}
 	      if (__ci >= 100) [[unlikely]]
 		{
 		  __s += std::format(_S_empty_spec, __ci / 100);
diff --git a/libstdc++-v3/testsuite/std/time/year/io.cc b/libstdc++-v3/testsuite/std/time/year/io.cc
index 6157afae253..a6683ae20df 100644
--- a/libstdc++-v3/testsuite/std/time/year/io.cc
+++ b/libstdc++-v3/testsuite/std/time/year/io.cc
@@ -43,8 +43,11 @@ test_format()
   s = std::format("{}", --year::min()); // formatted via ostream
   VERIFY( s == "-32768 is not a valid year" );
 
-  s = std::format("{:%y} {:%y}", 1976y, -1976y);
-  VERIFY( s == "76 76" ); // LWG 3831
+  s = std::format("{:%C %y} {:%C %y}", 1976y, -1976y);
+  VERIFY( s == "19 76 -20 76" ); // LWG 3831
+
+  s = std::format("{:%C %y} {:%C %y} {:%C %y}", -9y, -900y, -555y);
+  VERIFY( s == "-01 09 -09 00 -06 55" ); // LWG 4022
 
   s = std::format("{0:%EC}{0:%Ey} = {0:%EY}", 1642y);
   VERIFY( s == "1642 = 1642" );
-- 
2.43.0