Merge pull request #26590 from Kumataro:fix26589

Support C++20 standard #26590

Close https://github.com/opencv/opencv/issues/26589
Related https://github.com/opencv/opencv_contrib/pull/3842
Related: https://github.com/opencv/opencv/issues/20269

- do not arithmetic enums and ( different enums or floating numeric) 
- remove unused variable

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/calib3d/test/test_stereomatching.cpp

@@ -789,7 +789,7 @@ protected:
         calcROI = getValidDisparityROI(cROI, cROI, params.mindisp, params.ndisp, params.winSize);
 
         bm->compute( leftImg, rightImg, tempDisp );
-        tempDisp.convertTo(leftDisp, CV_32F, 1./StereoMatcher::DISP_SCALE);
+        tempDisp.convertTo(leftDisp, CV_32F, 1./static_cast<double>(StereoMatcher::DISP_SCALE));
 
         //check for fixed-type disparity data type
         Mat_<float> fixedFloatDisp;
@@ -802,7 +802,7 @@ protected:
                 for (int x = 0; x < leftDisp.cols; x++)
                 {
                     if (leftDisp.at<float>(y, x) < params.mindisp)
-                        leftDisp.at<float>(y, x) = -1./StereoMatcher::DISP_SCALE; // treat disparity < mindisp as no disparity
+                        leftDisp.at<float>(y, x) = -1./static_cast<double>(StereoMatcher::DISP_SCALE); // treat disparity < mindisp as no disparity
                 }
 
         return params.winSize/2;