Worked on enhancing type-checking correctness in the python/mypy repository, focusing on strict_optional scenarios and protocol handling. Addressed a bug affecting join and meet symmetry when strict_optional was enabled, ensuring more accurate type inference. Improved the resolution of protocol __call__ methods by detecting protocol callees and handling incompatible argument types, which led to better error reporting and analysis accuracy for users. Utilized Python and static analysis techniques to refine the type-checking process, contributing to overall code quality and maintainability. The work demonstrated depth in protocol implementation and type checking, directly improving the reliability of static analysis in Python projects.