I don't understand why mypy likes this code but doesn't like the previous revision of this gist.

I don't understand python type annotations.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from dataclasses import dataclass
from typing import Optional, Protocol, TypeVar

class ProtoA(Protocol):
    def a(self) -> None:
        ...

Why_Though = TypeVar('Why_Though', bound=ProtoA)

class ProtoB(Protocol[Why_Though]):
    a: Why_Though

class RealA:
    def a(self) -> None:
        pass

@dataclass
class RealB:
    a: RealA

def main(b: ProtoB) -> None:
    if b.a is not None:
        b.a.a()

if __name__ == '__main__':
    main(RealB(a=RealA()))