[Bug middle-end/83239] False positive from -Wstringop-overflow on simple std::vector code

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83239

--- Comment #6 from Martin Sebor <msebor at gcc dot gnu.org> ---
This libstdc++ patch helps avoid both the warning and the bogus memset.  if
Jonathan isn't opposed to this kind of annotation I think there might be other
places in vector where this approach could improve the emitted object code.

diff --git a/libstdc++-v3/include/bits/vector.tcc
b/libstdc++-v3/include/bits/vector.tcc
index eadce3c..8093f5e 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -582,8 +582,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
     {
       if (__n != 0)
        {
-         if (size_type(this->_M_impl._M_end_of_storage
-                       - this->_M_impl._M_finish) >= __n)
+         size_type __navail = size_type(this->_M_impl._M_end_of_storage
+                                          - this->_M_impl._M_finish);
+
+         if (__navail > max_size () - size ())
+           __builtin_unreachable ();
+
+         if (__navail >= __n)
            {
              _GLIBCXX_ASAN_ANNOTATE_GROW(__n);
              this->_M_impl._M_finish =