With recent versions of Clang built from SVN in Ubuntu 12.04 Precise, you’ll receive the following error when trying to run clang:

clang: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.18' not found (required by /opt/clang-svn/bin/clang)

The solution is to symlink /opt/gcc-4.8.2/lib64/libstdc++.so.6 to /usr/lib/x86_64-linux-gnu/libstdc++.so.6:

sudo ln -sf /opt/gcc-4.8.2/lib64/libstdc++.so.6 /usr/lib/x86_64-linux-gnu/libstdc++.so.6

Now clang will run properly:

$ /opt/clang-svn/bin/clang -v
clang version 3.5.0 (211951)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.6
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.6.3
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6.3
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64