[Bug libstdc++/117085] chrono formatting: %c does not honor locale after expansion

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117085

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
   Last reconfirmed|                            |2024-10-11
             Status|UNCONFIRMED                 |ASSIGNED
   Target Milestone|---                         |13.4
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Thanks for identifying exactly where the bug is!

I'll test this patch:

--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -912,17 +912,20 @@ namespace __format
          // %c  Locale's date and time representation.
          // %Ec Locale's alternate date and time representation.

+         basic_string<_CharT> __fmt;
          auto __t = _S_floor_seconds(__tt);
          locale __loc = _M_locale(__ctx);
          const auto& __tp = use_facet<__timepunct<_CharT>>(__loc);
          const _CharT* __formats[2];
          __tp._M_date_time_formats(__formats);
          const _CharT* __rep = __formats[__mod];
-         if (!*__rep) [[unlikely]]
-           __rep = _GLIBCXX_WIDEN("%a %b %e %T %Y");
-         basic_string<_CharT> __fmt(_S_empty_spec);
-         __fmt.insert(1u, 1u, _S_colon);
-         __fmt.insert(2u, __rep);
+         if (const _CharT* __rep = __formats[__mod]; *__rep) [[likely]]
+           {
+             __fmt = _GLIBCXX_WIDEN("{:L}");
+             __fmt.insert(3u, __rep);
+           }
+         else
+           __fmt = _GLIBCXX_WIDEN("{:L%a %b %e %T %Y}");
          return std::vformat_to(std::move(__out), __loc, __fmt,
                                 std::make_format_args<_FormatContext>(__t));
        }