[gcc r16-4299] libstdc++: Fix type in computation of _M_weekday_index.

[Tomasz Kaminski]

https://gcc.gnu.org/g:43f524c1c4f1aa79e3c443b45af9464a8b8943b1

commit r16-4299-g43f524c1c4f1aa79e3c443b45af9464a8b8943b1
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Wed Oct 8 15:14:04 2025 +0200

    libstdc++: Fix type in computation of _M_weekday_index.
    
    The value should use divide instead of modulo, as given 1st of month
    being weekday X (Mon, Tue, ...), 01 is always X[1], 08 is X[2], e.t.c.
    
    This values is currently not observable, as there is no user-accessible
    format specifier that will print it, however it may be exposed in future.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/chrono_io.h (_ChronoData::_M_fill_day): Replace
            '%' by '/'.

Diff:
---
 libstdc++-v3/include/bits/chrono_io.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index 79a44d128b12..690c10d79ce5 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -463,7 +463,7 @@ namespace __format
       {
 	_M_day = __d;
 	__parts -= _ChronoParts::_Day;
-	_M_weekday_index = ((unsigned)__d + 6u) % 7u;
+	_M_weekday_index = ((unsigned)__d + 6u) / 7u;
 	__parts -= _ChronoParts::_WeekdayIndex;
 	return __parts;
       }