[gcc r15-996] libstdc++: Fix -Wstringop-overflow warning coming from std::vector [PR109849]

[Francois Dumont]

https://gcc.gnu.org/g:0426be454448f8cfb9db21f4f669426afb7b57c8

commit r15-996-g0426be454448f8cfb9db21f4f669426afb7b57c8
Author: François Dumont <frs.dumont@gmail.com>
Date:   Sat Jun 1 22:17:19 2024 +0200

    libstdc++: Fix -Wstringop-overflow warning coming from std::vector [PR109849]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/109849
            * include/bits/vector.tcc
            (std::vector<>::_M_range_insert(iterator, _FwdIt, _FwdIt,
            forward_iterator_tag))[__cplusplus < 201103L]: Add __builtin_unreachable
            expression to tell the compiler that the allocated buffer is large enough to
            receive current elements plus the elements of the range to insert.

Diff:
---
 libstdc++-v3/include/bits/vector.tcc | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index 36b27dce7b9..c500aab9e56 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -933,6 +933,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
 		const size_type __len =
 		  _M_check_len(__n, "vector::_M_range_insert");
+#if __cplusplus < 201103LL
+		if (__len < (__n + (__old_start - __old_finish)))
+		  __builtin_unreachable();
+#endif
+
 		pointer __new_start(this->_M_allocate(__len));
 		pointer __new_finish(__new_start);
 		__try