https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95545
--- Comment #3 from David Edelsohn <dje at gcc dot gnu.org> --- #include <stdio.h> #include <thread> using namespace std; int main(void) { int maxThreads = thread::hardware_concurrency(); printf("maxThreads: %d\n", maxThreads); return(0); } $ g++ -pthread /tmp/nameclash.cpp -o /tmp/nc /tmp/nameclash.cpp: In function 'int main()': /tmp/nameclash.cpp:7:19: error: reference to 'thread' is ambiguous int maxThreads = thread::hardware_concurrency(); ^~~~~~ In file included from /tmp/nameclash.cpp:2: /opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/thread:62:9: note: candidates are: 'class std::thread' class thread ^~~~~~ In file included from /usr/include/sys/ptrace.h:28, from /usr/include/sys/proc.h:44, from /usr/include/sys/pri.h:43, from /usr/include/sys/sched.h:38, from /usr/include/sched.h:51, from /usr/include/pthread.h:63, from /opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/powerpc-ibm-aix7.2.0.0/pthread/bits/gthr-posix.h:35, from /opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/powerpc-ibm-aix7.2.0.0/pthread/bits/gthr-default.h:30, from /opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/powerpc-ibm-aix7.2.0.0/pthread/bits/gthr.h:148, from /opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/ext/atomicity.h:35, from /opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/memory:73, from /opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/thread:39, from /tmp/nameclash.cpp:2: /usr/include/sys/thread.h:106:8: note: 'struct thread' struct thread { ^~~~~~