Ruff version: 0.0.261

When writing type-checked code, it is sometimes necessary to let the type checker know about conditions it cannot be aware of using if TYPE_CHECKING blocks. Here's a (contrived) example:

from __future__ import annotations

from typing import TYPE_CHECKING


class Thing:
    some_prop: str | None

    def _init(self) -> None:
        self.some_prop = 'set'

    def _internal_method(self, arg: str) -> None:
        print(arg)

    def start(self) -> None:
        self._init()

        if TYPE_CHECKING:
            assert self.some_prop is not None

        self._internal_method(self.some_prop)

There is no way for the type checker to know that _init() set some_prop, so it still thinks some_prop is str | None, so we have to help it out with an assert statement. Obviously, if TYPE_CHECKING blocks don't run at runtime, so the assert will never run. It would be nice if ruff ignored assert statements in if TYPE_CHECKING blocks for the S101 rule.