I believe that for clang 3.4 and later, USR support is part of clang, not libclang.  Therefore when we compile our version of libclang, which lacks USR support since it came from post-3.4 clang, we can do USR stuff only if we're compiling against a version of clang from after the move.  So I think CLANG_VERSION_GE(3,4) is correct.  However, you're also correct that it doesn't work in that case either because we've disabled all of CIndexUSR.cpp.  The correct fix is likely to make that code unconditionally call XDRESS_FATAL.

Can you show me the error you're seeing to make sure?