[PATCH] libstdc++: Fix use-after-free in std::format [PR119671]

[Jonathan Wakely]

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.
---

Tested x86_64-linux.

 libstdc++-v3/include/std/format                      |  6 +++---
 .../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 01a53143d1c..2e9319cdda6 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 000f2671816..93c33b456e6 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
-- 
2.49.0