[Regression 1.19.1 -> 1.20.0] `value in container` based narrowing causes false negatives.

[randolf-scholz]

mypy seems to narrow the type of value in a if value in container branch, which leads to false negatives.

class Size(tuple[int, ...]):
    def numel(self) -> int:
        return 0  # math.prod(self)

sizes: list[Size] = [Size([1, 2, 3])]
print(sizes[0].numel())  # OK
value = (1, 2, 3)
if value in sizes:
    reveal_type(value)  # mypy: Size; pyright, ty, zuban, pyrefly: tuple
    print(value.numel())  # ERROR at runtime

• with 1.19.1, mypy reveals tuple[int, int, int] and correctly errors. https://mypy-play.net/?mypy=1.19.1&python=3.12&gist=12a961d6aab6b40ee13d895ab9ce037b
• with 1.20.0 onwards, mypy reveals Size and produces a false negative. https://mypy-play.net/?mypy=1.20.0&python=3.12&gist=76665c3096babff2c683164fdae02dbc

[randolf-scholz]

Bisects to b91f53c (#20602)

cc @hauntsaninja