CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_EXPORT_NO_PACKAGE_REGISTRY
    CMAKE_INSTALL_DATAROOTDIR
    CMAKE_INSTALL_LIBEXECDIR
    CMAKE_INSTALL_OLDINCLUDEDIR
    CMAKE_INSTALL_SBINDIR


g++: internal compiler error: Terminated (program cc1plus)
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-6/README.Bugs> for instructions.
make[2]: *** [source_subfolder/CMakeFiles/mailio.dir/src/dialog.cpp.o] Error 4
make[2]: *** Waiting for unfinished jobs....
Terminated
make[2]: *** [source_subfolder/CMakeFiles/mailio.dir/src/imap.cpp.o] Error 143
g++: internal compiler error: Terminated (program cc1plus)
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-6/README.Bugs> for instructions.
make[2]: *** [source_subfolder/CMakeFiles/mailio.dir/src/codec.cpp.o] Error 4
make[1]: *** [source_subfolder/CMakeFiles/mailio.dir/all] Error 2
make: *** [all] Error 2
mailio/0.20.0: WARN: Build folder is dirty, removing it: /home/conan/w/cci_PR-4017/.conan/data/mailio/0.20.0/_/_/build/5d32df612713089b3fb3787b9080f777aefa9bf4
mailio/0.20.0: ERROR: Package '5d32df612713089b3fb3787b9080f777aefa9bf4' build failed
mailio/0.20.0: WARN: Build folder /home/conan/w/cci_PR-4017/.conan/data/mailio/0.20.0/_/_/build/5d32df612713089b3fb3787b9080f777aefa9bf4
ERROR: Exiting with code: 5
In file included from /home/conan/w/cci_PR-4017/.conan/data/mailio/0.20.0/_/_/build/5d32df612713089b3fb3787b9080f777aefa9bf4/source_subfolder/src/imap.cpp:27:0:
/home/conan/w/cci_PR-4017/.conan/data/mailio/0.20.0/_/_/build/5d32df612713089b3fb3787b9080f777aefa9bf4/source_subfolder/include/mailio/imap.hpp:19:20: fatal error: optional: No such file or directory
 #include <optional>
                    ^
compilation terminated.