From 1a0aafdc2cf94c53e6451ac76fbc875cb56408df Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Mon, 11 Dec 2023 15:33:59 +0000
Subject: [PATCH] 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.
(cherry picked from commit a01462ae8bafa86e7df47a252917ba6899d587cf)
---
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 c95301361d87..c422435f7f31 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -826,9 +826,14 @@ 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 89ab7b1e61e1..b50d9d89c614 100644
--- a/libstdc++-v3/testsuite/std/time/year/io.cc
+++ b/libstdc++-v3/testsuite/std/time/year/io.cc
@@ -44,8 +44,11 @@ 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" );
--
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