Instance variables typed as `Callable` on the class body don't get assignment warnings when missing in `__slots__`

[bzoracler]

Bug Report, To Reproduce, & Actual Behaviour

See mypy Playground:

from collections.abc import Callable

class A:
    __slots__ = ()
    a: int
    b: Callable[[int], str]
    
    def __init__(self, /) -> None:
        self.a = 1  # E: Trying to assign name "a" that is not in "__slots__" of type "__main__.A"  [misc]
        self.b = lambda _: str(_)  # No warning

Expected Behavior

from collections.abc import Callable

class A:
    __slots__ = ()
    a: int
    b: Callable[[int], str]
    
    def __init__(self, /) -> None:
        self.a = 1  # E: Trying to assign name "a" that is not in "__slots__" of type "__main__.A"  [misc]
        self.b = lambda _: str(_)  # E: Trying to assign name "b" that is not in "__slots__" of type "__main__.A"  [misc]

Your Environment

• Mypy version used: (Occurs as far back as 1.0.0)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9, 3.12

[sterliakov]

mypy/mypy/checker.py

Lines 3756 to 3757 in 68233f6

	if isinstance(typ, FunctionLike):
	return True # Can be a property, or some other magic

We allow assignments to any callable-typed attribute not in slots. Probably using FunctionLike was an oversight. AFAIC something function-ish can only be assigned if it's a property-like inline definition that has a setter, so only a Decorator (which isn't even a FunctionLike!) should be relevant?

[bzoracler]

Decorator is on the mypy.nodes.Node AST while FunctionLike is on the mypy.types.Type AST, so I don't think they preclude each other (d.type can be a FunctionLike where d: Decorator). Also, I believe that properties with setters are normally OverloadedFuncDefs rather than Decorators.

But anyway, the only kind of item which should be allowed here is something that's a class variable with __set__. IIRC @property is not represented as a descriptor in mypy, so that's a special casing needed to be accounted for.

[sterliakov]

Whoops, sorry, my bad, I was looking at the parent node, not at the type, because with our property representation it's usually easier.

Properties with setters are settable, so we really don't want this warning for them, and that can only be OverloadedFuncDef with is_property=True. Properties without setters can't be set, and current implementation should already produce an error similar to "can't assign to a property without setter", so we want to allow Decorator nodes too.