[Bug libstdc++/107852] [12/13 Regression] Spurious warnings stringop-overflow and array-bounds copying data as bytes into vector

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107852

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
With that compiler patch for the missed-optimization one of the two bogus
warnings goes away. The second one goes away with this change:

--- a/libstdc++-v3/include/bits/stl_vector.h
+++ b/libstdc++-v3/include/bits/stl_vector.h
@@ -374,8 +374,19 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       pointer
       _M_allocate(size_t __n)
       {
-       typedef __gnu_cxx::__alloc_traits<_Tp_alloc_type> _Tr;
-       return __n != 0 ? _Tr::allocate(_M_impl, __n) : pointer();
+        pointer __p = pointer();
+        if (__n != 0)
+          {
+            typedef __gnu_cxx::__alloc_traits<_Tp_alloc_type> _Tr;
+            const pointer __s = _M_impl._M_start;
+            const pointer __f = _M_impl._M_finish;
+            const pointer __e = _M_impl._M_end_of_storage;
+            __p = _Tr::allocate(_M_impl, __n);
+            if (__s != _M_impl._M_start || __f != _M_impl._M_finish
+               || __e != _M_impl._M_end_of_storage)
+              __builtin_unreachable();
+          }
+        return __p;
       }

       _GLIBCXX20_CONSTEXPR