[gcc r14-6468] libstdc++: Fix std::format output of %C for negative years

[Jonathan Wakely]

https://gcc.gnu.org/g:a01462ae8bafa86e7df47a252917ba6899d587cf

commit r14-6468-ga01462ae8bafa86e7df47a252917ba6899d587cf
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Dec 11 15:33:59 2023 +0000

    libstdc++: Fix std::format output of %C for negative years
    
    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.

Diff:
---
 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" );