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/ml/src/rtrees.cpp

@@ -158,7 +158,7 @@ public:
         float* psamples = samples.ptr<float>();
         size_t sstep0 = samples.step1(), sstep1 = 1;
         Mat sample0, sample(nallvars, 1, CV_32F, &samplebuf[0]);
-        int predictFlags = _isClassifier ? (PREDICT_MAX_VOTE + RAW_OUTPUT) : PREDICT_SUM;
+        int predictFlags = _isClassifier ? (+PREDICT_MAX_VOTE + RAW_OUTPUT) : PREDICT_SUM;
 
         bool calcOOBError = eps > 0 || rparams.calcVarImportance;
         double max_response = 0.;