[Bug c++/105580] New: False warning "potential null pointer dereference" raised when using istreambuf_iterator and any "-O" flag

["j.gaffiot at laposte dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105580

            Bug ID: 105580
           Summary: False warning "potential null pointer dereference"
                    raised when using istreambuf_iterator and any "-O"
                    flag
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: j.gaffiot at laposte dot net
  Target Milestone: ---

I found what I believe is a false positive with g++ 12 when using the
"null-dereference" warning and istreambuf_iterator with any level of
optimisation. And of course the "-Werror" flag is mandatory in my company
process.
I have searched for such similar bug report.

This behavior does not show up with g++ 11.2.0 or g++ 9.4.0.

Minimal example (basically the first lines of the cppreference example for
istreambuf_iterator):

#include <string>
#include <sstream>

int main()
{
    std::istringstream in{"Hello, world"};
    std::istreambuf_iterator<char> it(in), end;
    std::string ss{it, end};
    return 0;
}

Compiled with:
g++-12 -O -Wnull-dereference <my_file>

Compiler version (default g++-12 on Ubuntu 22.04):
g++ (Ubuntu 12-20220319-1ubuntu1) 12.0.1 20220319 (experimental) [master
r12-7719-g8ca61ad148f]



Full version:
Using built-in specs.
COLLECT_GCC=g++-12
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
12-20220319-1ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-12
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch
--disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-12-OcsLtf/gcc-12-12-20220319/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-OcsLtf/gcc-12-12-20220319/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f]
(Ubuntu 12-20220319-1ubuntu1)