Hi, your problem seems to be related to a mismatch between the compiler and the standard library headers.
The error comes from a header file that probably belongs to libc++ (coming from /usr/include/c++ rather than from you compiler’s own standard library implementation). I believe that’s why when you add -stdlib=libc++ it solves your problem. I don’t think this is really an issue with ROOT per se, but just a configuration problem of the compiler. Maybe ROOT should just add -stdlib=libc++ to root-config --cflags when clang is using that as its own standard library. For now, you can just add that by hand as you mentioned to work around the problem.