[gcc r13-7706] libstdc++: Fix std::format for localized floats [PR110968]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:15b3fc6c1c5296662d0ffd7d3cfb8b669421e5ea

commit r13-7706-g15b3fc6c1c5296662d0ffd7d3cfb8b669421e5ea
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 10 14:33:44 2023 +0100

    libstdc++: Fix std::format for localized floats [PR110968]
    
    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.
    
    (cherry picked from commit f48a5423964f72e2e1ba0ad6a14d9d1464a78bed)

Diff:
---
 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 96bf17e2aa17..8d6df7452fd7 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -1608,8 +1608,8 @@ 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
@@ -1632,7 +1632,8 @@ 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 3485535e3cba..ba9403be79e3 100644
--- a/libstdc++-v3/testsuite/std/format/functions/format.cc
+++ b/libstdc++-v3/testsuite/std/format/functions/format.cc
@@ -193,6 +193,9 @@ 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);
 }