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 16e8fc58dffb12b1b65910af8ad6c10da7071f8c..b63b8592ebac022caf79f22bfab90912921e6ae7 100644 (file)
--- 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 6157afae2533d57f2ec5e4f7e603371339f67547..a6683ae20df4d2bd18d3631d1e6276b633d14668 100644 (file)
--- 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" );