Merge pull request #20370 from ddacw:stub-gen-next

Python typing stub generation #20370

Add stub generation to `gen2.py`, addressing #14590.

### 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 other license that is incompatible with OpenCV
- [x] The PR is proposed to proper branch
- [x] There is reference to original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/python/python_loader.cmake

@@ -45,8 +45,6 @@ foreach(fname ${PYTHON_LOADER_FILES})
   endif()
 endforeach()
 
-
-
 if(WIN32)
   if(CMAKE_GENERATOR MATCHES "Visual Studio")
     list(APPEND CMAKE_PYTHON_BINARIES_PATH "'${EXECUTABLE_OUTPUT_PATH}/Release'")  # TODO: CMAKE_BUILD_TYPE is not defined
@@ -126,3 +124,20 @@ if(NOT "${OPENCV_PYTHON_EXTRA_MODULES_PATH}" STREQUAL "")
     ocv_add_python_files_from_path(${extra_ocv_py_modules_path})
   endforeach()
 endif()
+
+if(${PYTHON}_VERSION_STRING VERSION_GREATER "3.6" AND PYTHON_DEFAULT_VERSION VERSION_GREATER "3.6")
+  add_custom_target(copy_opencv_typing_stubs)
+  # Copy all generated stub files to python_loader directory only if
+  # generation succeeds, this behvoir can't be achieved with default
+  # CMake constructions, because failed generation produces a warning instead of
+  # halts on hard error.
+  add_custom_command(
+    TARGET copy_opencv_typing_stubs
+    COMMAND ${PYTHON_DEFAULT_EXECUTABLE} ${PYTHON_SOURCE_DIR}/src2/copy_typings_stubs_on_success.py
+            --stubs_dir ${OPENCV_PYTHON_BINDINGS_DIR}/cv2
+            --output_dir ${__loader_path}/cv2
+  )
+  if(DEFINED OPENCV_PYTHON_INSTALL_PATH)
+    install(DIRECTORY "${OPENCV_PYTHON_BINDINGS_DIR}/cv2" DESTINATION "${OPENCV_PYTHON_INSTALL_PATH}" COMPONENT python)
+  endif()
+endif()