[committed] libstdc++: Fix std::format for localized floats [PR110968]

[Jonathan Wakely via Gcc-patches]

Tested x86_64-linux. Pushed to trunk. Backport to gcc-13 to follow.

-- >8 --

The __formatter_fp::_M_localize function just returns an empty string if
the formatting locale is the C locale, as there is nothing to do. But
the caller was assuming that the returned string contains the localized
string. The caller should use the original string if _M_localize returns
an empty string.

libstdc++-v3/ChangeLog:

        PR libstdc++/110968
        * include/std/format (__formatter_fp::format): Check return
        value of _M_localize.
        * testsuite/std/format/functions/format.cc: Check classic
        locale.
---
 libstdc++-v3/include/std/format                       | 5 +++--
 libstdc++-v3/testsuite/std/format/functions/format.cc | 3 +++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 96eb4cd742e..5d7af53fc94 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -1610,8 +1610,8 @@ namespace __format
 
 
          _Optional_locale __loc;
-         basic_string_view<_CharT> __str;
          basic_string<_CharT> __wstr;
+         basic_string_view<_CharT> __str;
          if constexpr (is_same_v<_CharT, char>)
            __str = __narrow_str;
          else
@@ -1634,7 +1634,8 @@ namespace __format
                __wstr = _M_localize(__str, __expc, __fc.locale());
              else
                __wstr = _M_localize(__str, __expc, __loc.value());
-             __str = __wstr;
+             if (!__wstr.empty())
+               __str = __wstr;
            }
 
          size_t __width = _M_spec._M_get_width(__fc);
diff --git a/libstdc++-v3/testsuite/std/format/functions/format.cc 
b/libstdc++-v3/testsuite/std/format/functions/format.cc
index bd914df6d7c..471cffb2b36 100644
--- a/libstdc++-v3/testsuite/std/format/functions/format.cc
+++ b/libstdc++-v3/testsuite/std/format/functions/format.cc
@@ -197,6 +197,9 @@ test_locale()
   s = std::format(eloc, "{0:#Lg} {0:+#.3Lg} {0:#08.4Lg}", -1234.);
   VERIFY( s == "-1.234,00 -1,23e+03 -01.234," );
 
+  s = std::format(cloc, "{:05L}", -1.0); // PR libstdc++/110968
+  VERIFY( s == "-0001" );
+
   // Restore
   std::locale::global(cloc);
 }
-- 
2.41.0