https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67632
--- Comment #6 from Markus Trippelsdorf <trippels at gcc dot gnu.org> --- (In reply to Jonathan Wakely from comment #5) > Not sure how it can be a libstdc++ issue, the copy constructor exists, the > library can't force the front-end to instantiate it. > > I'm travelling without access to proper computers, but as far as I can tell > from coliru.stacked-crooked.com it links fine using clang++ > -stdlib=libstdc++ (with libstdc++ 5.2.0). No, it doesn't for -O0. Higher -O values make the issue go away. markus@x4 /tmp % clang++ -stdlib=libc++ -O0 -std=c++11 t.cpp x.cpp markus@x4 /tmp % clang++ -Wl,--no-demangle -O0 -std=c++11 t.cpp x.cpp /tmp/x-5ef538.o:x.cpp:function _Z4copyRKSt13unordered_mapIiiSt4hashIiESt8equal_toIiESaISt4pairIKiiEEE: error: undefined reference to '_ZNSt13unordered_mapIiiSt4hashIiESt8equal_toIiESaISt4pairIKiiEEEC1ERKS8_' clang-3.8: error: linker command failed with exit code 1 (use -v to see invocation) markus@x4 /tmp % icpc -Wl,--no-demangle -O0 -std=c++11 t.cpp x.cpp /tmp/icpcMamJXz.o:x.cpp:function _Z4copyRKSt13unordered_mapIiiSt4hashIiESt8equal_toIiESaISt4pairIKiiEEE: error: undefined reference to '_ZNSt13unordered_mapIiiSt4hashIiESt8equal_toIiESaISt4pairIKiiEEEC1ERKS8_'
