Simplest possible test case for Objective-C in YouCompleteMe.

.ycm_extra_conf.py

import os
import ycm_core

flags = [
'-resource-dir',
'/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/6.0',
'-isysroot',
'/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk',
'-I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include',
'-F/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/System/Library/Frameworks',
'-fblocks',
'-fobjc-runtime=macosx-10.10.0',
'-fencode-extended-block-signature',
'-fobjc-arc',
'-fobjc-exceptions',
'-fexceptions',
'-x',
'objective-c'
]

SOURCE_EXTENSIONS = [ '.cpp', '.cxx', '.cc', '.c', '.m', '.mm' ]

def FlagsForFile( filename, **kwargs ):
  return {
    'flags': flags,
    'do_cache': True
  }

main.m

#import <Foundation/Foundation.h>

int main(int argc, char *argv[]) {
  int retVal;
  @autoreleasepool {
    NSString *string = @"hihi";
    NSLog(string);
    retVal = 0;
  }
  return retVal;
}