From: Jonathan Wakely <jwakely@redhat.com>
Date: Mon, 11 Dec 2023 15:33:59 +0000 (+0000)
Subject: libstdc++: Fix std::format output of %C for negative years
X-Git-Tag: basepoints/gcc-15~3671
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=a01462ae8bafa86e7df47a252917ba6899d587cf;p=thirdparty%2Fgcc.git

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 --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index 16e8fc58dffb..b63b8592ebac 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 6157afae2533..a6683ae20df4 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" );