https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71856
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- You can't mix Debug Mode and Parallel Mode anyway. With previous versions you get In file included from /usr/include/c++/5.3.1/utility:68:0, from /usr/include/c++/5.3.1/algorithm:60, from par.cc:1: /usr/include/c++/5.3.1/x86_64-redhat-linux/bits/c++config.h:2092:4: error: #error illegal use of multiple inlined namespaces # error illegal use of multiple inlined namespaces ^ Now you get that followed by a cascade of other errors, which should be fixed, but you'll never be able to mix them.