YCM is not working with .ycm_extra_conf nor compile_comands.json on Windows

Question

I have problems using YCM plugged for C-family languages. Here are what I have done:

1. project layout

-- build
-- src
   -- source file dir0
      -- include
      main.cpp
   -- source file dir1
      -- include
         -- dir1.h
      -- dir1.cpp

2. generate compile database running command on windows in directory build as below.

cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON ..\src\ -G "MinGW Makefiles"

3. copy compile_commands.json to directory src.

4. I also generate configure file to src/.ycm_extra_conf.py with contents as below.

flags = [
    '-x',
    'c++',
    '-I',
    'dir1/include',
]


def Settings(**kwargs):
    return {'flags': flags}

I compiled both clangd and libclang during installation. Since clangd is default selected by YCM, I switch clangd and libclang by manipulating vimrc setting ycm_use_clangd.

After doing all these above, The completion and syntax checking seem not working at all.

#include "dir1.h"  ==> diagnosed as header file not FOUND

And those functions and variables defined in dir1.h are diagnosed as undefined.