[gcc r15-9273] libstdc++: Fix use-after-free in std::format [PR119671]

[Jonathan Wakely]

https://gcc.gnu.org/g:e33b62eed7fd0a82d758b23252d288585b6790d2

commit r15-9273-ge33b62eed7fd0a82d758b23252d288585b6790d2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Apr 7 19:52:55 2025 +0100

    libstdc++: Fix use-after-free in std::format [PR119671]
    
    When formatting floating-point values to wide strings there's a case
    where we invalidate a std::wstring buffer while a std::wstring_view is
    still referring to it.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/119671
            * include/std/format (__formatter_fp::format): Do not invalidate
            __wstr unless _M_localized returns a valid string.
            * testsuite/std/format/functions/format.cc: Check wide string
            formatting of floating-point types with classic locale.
    
    Reviewed-by: Tomasz Kaminski <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/include/std/format                       |  6 +++---
 libstdc++-v3/testsuite/std/format/functions/format.cc | 12 ++++++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 01a53143d1c5..2e9319cdda63 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -1838,9 +1838,9 @@ namespace __format
 
 	  if (_M_spec._M_localized && __builtin_isfinite(__v))
 	    {
-	      __wstr = _M_localize(__str, __expc, __fc.locale());
-	      if (!__wstr.empty())
-		__str = __wstr;
+	      auto __s = _M_localize(__str, __expc, __fc.locale());
+	      if (!__s.empty())
+		__str = __wstr = std::move(__s);
 	    }
 
 	  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 000f2671816d..93c33b456e64 100644
--- a/libstdc++-v3/testsuite/std/format/functions/format.cc
+++ b/libstdc++-v3/testsuite/std/format/functions/format.cc
@@ -370,6 +370,18 @@ test_wchar()
   // P2909R4 Fix formatting of code units as integers (Dude, where’s my char?)
   s = std::format(L"{:d} {:d}", wchar_t(-1), char(-1));
   VERIFY( s.find('-') == std::wstring::npos );
+
+  auto ws = std::format(L"{:L}", 0.5);
+  VERIFY( ws == L"0.5" );
+  // The default C locale.
+  std::locale cloc = std::locale::classic();
+  // PR libstdc++/119671 use-after-free formatting floating-point to wstring
+  ws = std::format(cloc, L"{:L}", 0.5);
+  VERIFY( ws == L"0.5" );
+  // A locale with no name, but with the same facets as the C locale.
+  std::locale locx(cloc, &std::use_facet<std::ctype<char>>(cloc));
+  ws = std::format(locx, L"{:L}", 0.5);
+  VERIFY( ws == L"0.5" );
 }
 
 void