title: error when calling operator inside FUNCTION const
body: <p>now i am stumbling with c++ code problem.</p>
  
  <p>i made a simple structure 'Vector3' in that i defined the operator+.
  and when i use that operator inside const function, it show a red line.</p>
  
  <pre><code>struct Vector3 {
      float x, y, z;
      Vector3 operator+ (const Vector3 v) {
          return Vector3(x + v.x, y + v.y, z + v.z);
      };
      Vector3(float x, float y, float z) : x(x), y(y), z(z) {};
  };
  
  // and i use it inside some const function
  struct SomeST {
    Vector3 a,b;
  
    Vector3 Function() const
    {
       return a + b; // error
    }
  };
  </code></pre>
  
  <p>and if i delete the const from function, it works!
  so it would be amazing if someone can explain what's going on under the hood.
  thanks.</p>