New warnings with Clang10

[apollo13]

Issue description

After upgrading to Clang10 I see new warnings (which result in errors due to -Werror) on my builds with pybind11 2.5.0:

/home/florian/sources/bap_server/so/pybind11/include/pybind11/pybind11.h:1093:9: error: expression result unused [-Werror,-Wunused-value]
        PYBIND11_EXPAND_SIDE_EFFECTS(add_base<options>(record));
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/florian/sources/bap_server/so/pybind11/include/pybind11/detail/common.h:662:47: note: expanded from macro 'PYBIND11_EXPAND_SIDE_EFFECTS'
#define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) pybind11::detail::expand_side_effects{ ((PATTERN), void(), false)..., false }
                                              ^                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/florian/sources/bap_server/so/gina/gina.cpp:176:4: note: in instantiation of function template specialization 'pybind11::class_<bap::GINACallbackWrapper>::class_<>' requested here
   py::class_<bap::GINACallbackWrapper>(m, "GINACallbackWrapper").def("callback", &bap::GINACallbackWrapper::callback);

Reproducible example code

As far as I can see this would happen for every usage of class_ and should immediately show up as soon as Clang10 is used and -Wunused-value is enabled. I was able to locally disable it via:

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunused-value"
        /* Register base classes specified via template arguments to class_, if any */
        PYBIND11_EXPAND_SIDE_EFFECTS(add_base<options>(record));
#pragma clang diagnostic pop

[philbucher]

I can confirm that this problem happens and that the proposed solution silences the warning
I don't know what happens in the background and if the warning should rather be fixed than just silenced

Thanks

[bstaletic]

I can't reproduce this "just by using clang++ 10 and -Wunused-value". Can someone provide actual code that triggers this warning?

As far as I can see, the solution would be to just slap (void) or [[maybe_unused]], but it would be nice to confirm this is the actual problem.

[apollo13]

@bstaletic Mhm, good question; I have uploaded https://github.com/apollo13/pybind11-example which triggers the issue for me. Ie just execute build.sh in the checkout (don't forget to initialize submodules):

-- The C compiler identification is Clang 10.0.0
-- The CXX compiler identification is Clang 10.0.0
-- Check for working C compiler: /usr/bin/clang
-- Check for working C compiler: /usr/bin/clang - works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/clang++
-- Check for working CXX compiler: /usr/bin/clang++ - works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found PythonInterp: /usr/bin/python3.8 (found version "3.8.3") 
-- Found PythonLibs: /usr/lib64/libpython3.8.so
-- pybind11 v2.5.dev1
-- Performing Test HAS_FLTO
-- Performing Test HAS_FLTO - Success
-- LTO enabled
-- Configuring done
-- Generating done
-- Build files have been written to: /home/florian/sources/pybind11-example
/usr/bin/cmake -S/home/florian/sources/pybind11-example -B/home/florian/sources/pybind11-example --check-build-system CMakeFiles/Makefile.cmake 0
/usr/bin/cmake -E cmake_progress_start /home/florian/sources/pybind11-example/CMakeFiles /home/florian/sources/pybind11-example/CMakeFiles/progress.marks
make  -f CMakeFiles/Makefile2 all
make[1]: Entering directory '/home/florian/sources/pybind11-example'
make  -f CMakeFiles/example.dir/build.make CMakeFiles/example.dir/depend
make[2]: Entering directory '/home/florian/sources/pybind11-example'
cd /home/florian/sources/pybind11-example && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /home/florian/sources/pybind11-example /home/florian/sources/pybind11-example /home/florian/sources/pybind11-example /home/florian/sources/pybind11-example /home/florian/sources/pybind11-example/CMakeFiles/example.dir/DependInfo.cmake --color=
Dependee "/home/florian/sources/pybind11-example/CMakeFiles/example.dir/DependInfo.cmake" is newer than depender "/home/florian/sources/pybind11-example/CMakeFiles/example.dir/depend.internal".
Dependee "/home/florian/sources/pybind11-example/CMakeFiles/CMakeDirectoryInformation.cmake" is newer than depender "/home/florian/sources/pybind11-example/CMakeFiles/example.dir/depend.internal".
Scanning dependencies of target example
make[2]: Leaving directory '/home/florian/sources/pybind11-example'
make  -f CMakeFiles/example.dir/build.make CMakeFiles/example.dir/build
make[2]: Entering directory '/home/florian/sources/pybind11-example'
[ 50%] Building CXX object CMakeFiles/example.dir/example.cpp.o
/usr/bin/clang++  -Dexample_EXPORTS -I/home/florian/sources/pybind11-example/pybind11/include -I/usr/include/python3.8  -fPIC -fvisibility=hidden   -flto -o CMakeFiles/example.dir/example.cpp.o -c /home/florian/sources/pybind11-example/example.cpp
In file included from /home/florian/sources/pybind11-example/example.cpp:1:
/home/florian/sources/pybind11-example/pybind11/include/pybind11/pybind11.h:1120:9: warning: expression result unused [-Wunused-value]
        PYBIND11_EXPAND_SIDE_EFFECTS(add_base<options>(record));
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/florian/sources/pybind11-example/pybind11/include/pybind11/detail/common.h:679:47: note: expanded from macro 'PYBIND11_EXPAND_SIDE_EFFECTS'
#define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) pybind11::detail::expand_side_effects{ ((PATTERN), void(), false)..., false }
                                              ^                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/florian/sources/pybind11-example/example.cpp:19:3: note: in instantiation of function template specialization 'pybind11::class_<Example>::class_<>' requested here
  py::class_<Example>(m, "Example").def( "abc", &Example::abc );
  ^
1 warning generated.
[100%] Linking CXX shared module example.cpython-38-x86_64-linux-gnu.so
/usr/bin/cmake -E cmake_link_script CMakeFiles/example.dir/link.txt --verbose=1
/usr/bin/clang++ -fPIC   -shared  -o example.cpython-38-x86_64-linux-gnu.so CMakeFiles/example.dir/example.cpp.o  -flto 
make[2]: Leaving directory '/home/florian/sources/pybind11-example'
[100%] Built target example
make[1]: Leaving directory '/home/florian/sources/pybind11-example'
/usr/bin/cmake -E cmake_progress_start /home/florian/sources/pybind11-example/CMakeFiles 0

[apollo13]

Note that I am on Fedora 32, executing the clang++ invocation manually with -v yields:

clang version 10.0.0 (Fedora 10.0.0-2.fc32)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/10
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/10
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
 (in-process)
 "/usr/bin/clang-10" -cc1 -triple x86_64-unknown-linux-gnu -emit-llvm-bc -flto -flto-unit -disable-free -disable-llvm-verifier -discard-value-names -main-file-name example.cpp -mrelocation-model pic -pic-level 2 -mthread-model posix -mframe-pointer=all -fmath-errno -fno-rounding-math -masm-verbose -mconstructor-aliases -munwind-tables -target-cpu x86-64 -dwarf-column-info -fno-split-dwarf-inlining -debugger-tuning=gdb -v -resource-dir /usr/lib64/clang/10.0.0 -D example_EXPORTS -I /home/florian/sources/pybind11-example/pybind11/include -I /usr/include/python3.8 -internal-isystem /usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10 -internal-isystem /usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/x86_64-redhat-linux -internal-isystem /usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/backward -internal-isystem /usr/local/include -internal-isystem /usr/lib64/clang/10.0.0/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -fdebug-compilation-dir /home/florian/sources/pybind11-example -ferror-limit 19 -fmessage-length 0 -fvisibility hidden -fgnuc-version=4.2.1 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -faddrsig -o CMakeFiles/example.dir/example.cpp.o -x c++ /home/florian/sources/pybind11-example/example.cpp
clang -cc1 version 10.0.0 based upon LLVM 10.0.0 default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /home/florian/sources/pybind11-example/pybind11/include
 /usr/include/python3.8
 /usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10
 /usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/x86_64-redhat-linux
 /usr/bin/../lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/backward
 /usr/local/include
 /usr/lib64/clang/10.0.0/include
 /usr/include
End of search list.
In file included from /home/florian/sources/pybind11-example/example.cpp:1:
/home/florian/sources/pybind11-example/pybind11/include/pybind11/pybind11.h:1120:9: warning: expression result unused [-Wunused-value]
        PYBIND11_EXPAND_SIDE_EFFECTS(add_base<options>(record));
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/florian/sources/pybind11-example/pybind11/include/pybind11/detail/common.h:679:47: note: expanded from macro 'PYBIND11_EXPAND_SIDE_EFFECTS'
#define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) pybind11::detail::expand_side_effects{ ((PATTERN), void(), false)..., false }
                                              ^                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/florian/sources/pybind11-example/example.cpp:19:3: note: in instantiation of function template specialization 'pybind11::class_<Example>::class_<>' requested here
  py::class_<Example>(m, "Example").def( "abc", &Example::abc );
  ^
1 warning generated.

If you have the same "default" flags the issue should be reproducible.

[apollo13]

diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index 8b609e7..33805e0 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -676,7 +676,7 @@ inline void ignore_unused(const int *) { }
 #define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) (((PATTERN), void()), ...)
 #else
 using expand_side_effects = bool[];
-#define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) pybind11::detail::expand_side_effects{ ((PATTERN), void(), false)..., false }
+#define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) (void)pybind11::detail::expand_side_effects{ ((PATTERN), void(), false)..., false }
 #endif
 
 PYBIND11_NAMESPACE_END(detail)

Adding (void) seems to fix it for me, cannot say if this causes other issues though (ie I just tried to ensure that the warnings are gone, the full testsuite might see issues with it).

[bstaletic]

I think that is fine. Mind submitting a pull request?

[YannickJadoul]

Merged #2225. Thanks again! :-)