Currently in alpha, Pyrefly is a new open-source Python type checker developed by Meta in Rust for maximum performance. Inspired by tools like Pyre, Pyright, and MyPy, Pyrefly is intended to replace ...

Python 3.11 introduced the Specializing Adaptive Interpreter. When the interpreter detects that some operations predictably involve the same types, those operations are “specialized.” ...

But in actuality, Python will ignore these type annotations completely ... Notice that I've modified the annotation to be Sequence[int]. In the wake of that change, mypy has now found lots of problems ...