Libc++ and old Clang versions


I recently broke a libc++ build bot that was running the libc++ tests using Clang 4 on Debian [1].

I'm not sure I understand why we're running the libc++ tests under old Clang versions like that. I can understand that libc++ should _build_ with Clang 4, but it's not clear to me that recent libc++ headers should be usable under old Clangs, given that we ship libc++ and Clang together. Is this a configuration we want to support? Why?