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

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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.
This commit is contained in:
Jonathan Wakely 2023-12-11 15:33:59 +00:00
parent 988dd6384c
commit a01462ae8b
2 changed files with 12 additions and 4 deletions
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9
libstdc++-v3/include/bits/chrono_io.h
View file

@ -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);

7
libstdc++-v3/testsuite/std/time/year/io.cc
View file

@ -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" );