Overriding __bool__ in NamedTuple doesn't work

[Stannislav]

Even if __bool__ is overridden in a subclass of NamedTuple, mypy thinks instances of this subclass can't evaluate to False.

This possibility was mentioned in #3601 (comment) before, but it appears this wasn't fixed.

To Reproduce
Consider this code example.

# x.py
from typing import NamedTuple


class C(NamedTuple):
    x: int

    def __bool__(self) -> bool:
        return self.x > 0


def hello(c: C) -> None:
    if c:
        print("True branch")
    else:
        print("False branch")


hello(C(0))
hello(C(1))

Mypy reports a "Statement is unreachable"

$ mypy x.py 
x.py:15: error: Statement is unreachable  [unreachable]
Found 1 error in 1 file (checked 1 source file)

while the statement clearly is reachable:

$ python x.py
False branch
True branch

Your Environment

• Mypy version used: 0.930
• Mypy command-line flags: -
• Mypy configuration options from mypy.ini (and other config files): -
• Python version used: 3.10.0
• Operating system and version: macOS Big Sur