[Bug c++/107019] New: -Wunused-but-set-variable false positive for static variable in lambda with boost

[mario at klebsch dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107019

            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 sample:

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

int main(int argc, char *argv[])
{
    static const char *func = __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 ‘int main(int, char**)’:
bug2.cpp:7:28: warning: variable ‘func’ set but not used
[-Wunused-but-set-variable]
    7 |         static const char *func = __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 usage
of func in the lambda is recognized correctly.

73, Mario