difference between xdress clang and clang++

gistfile1.txt

Compiling with clang++ with the same options as cindex uses

❯ clang++ -v -std=c++11  -I. -c build/sodium_sodium.cpp-sodium_sodium.h.cpp
clang version 3.4 (tags/RELEASE_34/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.8.2
Found candidate GCC installation: /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-unknown-linux-gnu/4.8.2
Found candidate GCC installation: /usr/lib64/gcc/x86_64-unknown-linux-gnu/4.8.2
Selected GCC installation: /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2
 "/usr/bin/clang" -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -main-file-name sodium_sodium.cpp-sodium_sodium.h.cpp -mrelocation-model static -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -target-linker-version 2.24 -v -coverage-file /home/james/projects/silver/sodium_sodium.cpp-sodium_sodium.h.o -resource-dir /usr/bin/../lib/clang/3.4 -I . -internal-isystem /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2 -internal-isystem /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2/x86_64-unknown-linux-gnu -internal-isystem /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2/backward -internal-isystem /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/x86_64-unknown-linux-gnu/c++/4.8.2 -internal-isystem /usr/local/include -internal-isystem /usr/bin/../lib/clang/3.4/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /home/james/projects/silver -ferror-limit 19 -fmessage-length 198 -mstackrealign -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -vectorize-slp -o sodium_sodium.cpp-sodium_sodium.h.o -x c++ build/sodium_sodium.cpp-sodium_sodium.h.cpp
clang -cc1 version 3.4 based upon LLVM 3.4 default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/x86_64-unknown-linux-gnu/c++/4.8.2"
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 .
 /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2
 /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2/x86_64-unknown-linux-gnu
 /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2/backward
 /usr/local/include
 /usr/bin/../lib/clang/3.4/include
 /usr/include
End of search list.

Same thing with xdress (I have extra logging to show the parameters to cindex.Index.parse:
❯ xdress                                                                                                                                                          ✱ ◼
autoall: discovering API names
autoall: no API names to discover!
autodescribe: registering Event
dtypes: registering numpy dtypes
stlwrap: registering C++ standard library types
autodescribe: scraping C/C++ APIs from source
autodescribe: describing Event
clang version 3.4 (tags/RELEASE_34/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation: /../lib/gcc/x86_64-unknown-linux-gnu/4.8.2
Found candidate GCC installation: /../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-unknown-linux-gnu/4.8.2
Found candidate GCC installation: /usr/lib64/gcc/x86_64-unknown-linux-gnu/4.8.2
Selected GCC installation: /../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2
ignoring nonexistent directory "/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/x86_64-unknown-linux-gnu/c++/4.8.2"
ignoring nonexistent directory "../lib/clang/3.4/include"
ignoring nonexistent directory "/include"
ignoring duplicate directory "."
#include "..." search starts here:
#include <...> search starts here:
 .
 /../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2
 /../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2/x86_64-unknown-linux-gnu
 /../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2/backward
 /usr/local/include
 /usr/include
End of search list.
/usr/include/limits.h:123:16: fatal error: 'limits.h' file not found