`not isinstance(x, cls)` inside classmethod does not narrow type

[robsdedude]

Bug Report

Inside a @classmethod, not isinstance(x, cls) does not appear to narrow the type, while isinstance(x, cls) does.

To Reproduce

This is what I wanted to write:

class Foo:
    def __init__(self, x: float) -> None:
        ...
    
    @classmethod
    def from_foo_or_float_cls(cls, x: Self | float) -> Self:
        if isinstance(x, cls):
            return x
        return cls(x)  # error: Argument 1 to "Foo" has incompatible type "Self | float"; expected "float"  [arg-type]

Further reduced:

class Foo:
    @classmethod
    def foo_1(cls, x: Self | float) -> None:
        assert isinstance(x, cls)
        reveal_type(x)  # expected: "Self"; was: "Self`0"; ✅

    @classmethod
    def foo_2(cls, x: Self | float) -> None:
        assert not isinstance(x, cls)
        reveal_type(x)  # expected: "float"; was: "Self`0" | builtins.float"; ❌

Expected Behavior

not isinstance(x, cls) should narrow the type union to remove Self.

Actual Behavior

Self is still part of the type union.

Your Environment

• Mypy version used: 1.19.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10

Same behavior can be observers on mypy master with Python 3.14 (tested on the playground).