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From: "mario at klebsch dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107019] New: -Wunused-but-set-variable false positive for
 static variable in lambda with boost
Date: Fri, 23 Sep 2022 15:03:51 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 11.3.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: mario at klebsch dot de
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107019

            Bug ID: 107019
           Summary: -Wunused-but-set-variable false positive for static
                    variable in lambda with boost
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mario at klebsch dot de
  Target Milestone: ---

I get a palse positive from -Wunused-but-set-variable with this little samp=
le:

#include <stdio.h>
#include <boost/asio.hpp>

int main(int argc, char *argv[])
{
    static const char *func =3D __FUNCTION__;
    boost::asio::io_service io_service;
    boost::asio::deadline_timer timer(io_service);
    timer.async_wait( [](auto) {
        printf("%s()\n", func);
    });
}

~/tmp $ g++  -Wall bug2.cpp   -c -I boost_1_80_0/
bug2.cpp: In function =E2=80=98int main(int, char**)=E2=80=99:
bug2.cpp:7:28: warning: variable =E2=80=98func=E2=80=99 set but not used
[-Wunused-but-set-variable]
    7 |         static const char *func =3D __FUNCTION__;
      |                            ^~~~
~/tmp $

If I do not call timer.async_wait() but some other function taking const
std::function<void(const boost::system::error_code&)>& as parameter, the us=
age
of func in the lambda is recognized correctly.

73, Mario=