OPENCV LNK 2001 Error: "unresolved external symbol" while adding to/modifying OPENCV source files

Question

I need to modfiy some OPENCV source files (VS 2019, C++) but get this error. I have OpenCV version 4.6. which is installed with CMAKE and it works well untill I try to add a new function in source files. More precilsely, I have performed the following

1. I have modified source feature2d.cpp by adding the following test function

void mynewtest()
{
    std::cout<<" this is  opencv";
} 

2. Added in source features2d.hpp the following

void  mynewtest();

3. Then I build ALL_BUILD and build INSTALL using OpenCV.sln in build folder

4. Next, I have tried to run the following standard code, which gives LNK 2001 Error

#include <opencv2/core.hpp>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/features2d.hpp>
#include <iostream>


using namespace cv;
int main()
{
    mynewtest(); //--- ADDED FUNCTION

    //-----------------------------------------------------------;
    std::string image_path = samples::findFile("starry_night.jpg");
    Mat img = imread(image_path, IMREAD_COLOR);
    if (img.empty())
    {
        std::cout << "Could not read the image: " << image_path << std::endl;
        return 1;
    }
    imshow("Display window", img);
    int k = waitKey(0); // Wait for a keystroke in the window
    if (k == 's')
    {
        imwrite("starry_night.png", img);
    }
    return 0;
}

If I comment mynewtest(); then the code starts normally. As I understand smth is with linking lib libriries but all the libs are linked well

have tried to re-build opencv using cmake