[cfe-dev] libclang c API will not find c++ system headers

[Per Bull Holmen via cfe-dev]

Hi

I wanted to sort out a bug/limitation in a color coding plugin for neovim (not maintained by me), and tried to set up a vanilla c++ project to find out how libclang works. This is what I've done:

1) Download prebuilt binaries of clang+llvm-8.0.0 for macOS. I later downloaded and tried 7.0.0 as well
2) Create command line tool project in Xcode
3) Link to libclang.dylib under Linked Frameworks and Libraries
4) Add /path/to/clang+llvm/lib under Library search paths
5) Add /path/to/clang+llvm/include under Header search paths
6) Add /path/to/clang+llvm/lib under runpath search paths
7) Create new c++ target with only the auto-generated code to test parsing
8) Build the new target from command line, and copy the clang invocation from the build log
9) Run regex replace on the clang invocation to be sure I have an exact copy of the command line args during build of the test target, as an array of c strings (const char *)
10) Call clang_parseTranslationUnit2 with those parameters, with and without argv[0] (the executable path).

This is my main implementation:

int main(int argc, const char * argv[]) {
    CXIndex idx = clang_createIndex(0,1);
    CXTranslationUnit tu;

    CXErrorCode errorCode;

    if( !( errorCode = clang_parseTranslationUnit2( idx,
                                    NULL,
                                    options,
                                    opt_count,
                                    NULL,
                                    0,
                                    0,
                                    &tu) ) )

        std::cout << "Success\n";
    else
        std::cout << "error: " << errorCode << std::endl;

    clang_disposeTranslationUnit(tu);
    clang_disposeIndex(idx);
    return 0;
}

When I try it on a c file, with system header includes, it produces the following output:
Success

With a c++ file, it produces the following output:
/Users/pbholmen/Projects/ColorCodingTest/Vanilla/main.cpp:9:10: fatal error: 'iostream' file not found
Success

This is the file it tries to parse:
#include <iostream>

int main(int argc, const char * argv[]) {
     // insert code here...
     std::cout << "Hello, World!\n";
     return 0;
}

I have tried to invoke the clang binary inside the downloaded binaries folder, from the command line, compiling the same file with the exact same options as given to
clang_parseTranslationUnit2, and it compiles without issues.

I can post the list of command line options input to clang_parseTranslationUnit2 as well, but the message is getting long and I'd feel I'm spamming you with tons of Xcode-generated 
options, but if you ask for it I'll post it as well. For argv[0] I tried both with the path to the Xcode-provided clang, the path to the dowloaded clang, and omitting the path. I tried to provide
the source file both as an argument to clang_parseTranslationUnit2, and embedded in my argv. Same result either way. I have not deleted any files or folders inside the downloaded prebuilt
binaries folder or relocated libclang.dylib.

Per