CLion seems to not recognize operator overloading

Hello,

I have a problem with CLion's C++ code inspection.

There is my main function :

#include <iostream>
#include <string>

using namespace std;

int main()
{
  string s1 ("Hello World!");
  string s2 ("Test");
  string s3 = s1 + s2;

  if (s1 == s2)
    cout << "No !" << endl;
  cout << s3 << endl;
}

CLion underlines the section "s1 + s2" in red and says "binary operator '+' can't be applied to the expression of type 'string' and 'string'", and the section "s1 == s2", saying "Can't compare structures", but the code is compiling.

There is my CMakeLists.txt :

cmake_minimum_required(VERSION 3.3)
project(Test2)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror")

set(SOURCE_FILES main.cpp)
add_executable(Test2 ${SOURCE_FILES})

When I turn on c++11, the error on "s1 == s2" disapear, but not the error on "s1 + s2".

I am using CLion 1.2.4 build #CL-143.1186

Thank you.