https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80472
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Another place where I'd like to selectively enable warnings is for this code
(from PR 78830):
#include <forward_list>
#include <algorithm>
int main()
{
std::forward_list<int> il = {1, 2, 3, 4, 5, 6, 7};
auto iter = std::prev(il.end());
}
With -Wall -O1 -Wsystem-headers GCC detects the undefined behaviour:
In file included from
/home/jwakely/gcc/9/include/c++/9.0.1/bits/stl_algobase.h:66,
from
/home/jwakely/gcc/9/include/c++/9.0.1/bits/forward_list.h:38,
from /home/jwakely/gcc/9/include/c++/9.0.1/forward_list:38,
from prev.cc:1:
/home/jwakely/gcc/9/include/c++/9.0.1/bits/stl_iterator_base_funcs.h:153:7:
warning: iteration 9223372036854775807 invokes undefined behavior
[-Waggressive-loop-optimizations]
153 | while (__n--)
| ^~~~~
/home/jwakely/gcc/9/include/c++/9.0.1/bits/stl_iterator_base_funcs.h:153:7:
note: within this loop
but that very useful warning is suppressed by default. This bug prevents me
from locally enabling -Wsystem-headers around the relevant functions in
<bits/stl_iterator_base_funcs.h>.
