The C++ compiler and mypy tell you about the same thing: your typing is consistent. Neither is even close to perfect about finding defects. You have to write

The C++ compiler and mypy tell you about the same thing: your typing is consistent. Neither is even close to perfect about finding defects. You have to write unit tests anyway to validate other useful properties about your code. If you’re not close to 100% you’re doing it wrong. Even 100% coverage is a fairly low bar these days.