“Decorator @X can only be used on methods” is too strict

[grievejia]

Describe the Bug

Pyrefly reports "Decorator @x can only be used on methods" when @property, @classmethod, or @staticmethod are applied to functions outside a class body. This is valid Python -- these descriptors work correctly when the resulting objects are later assigned to class attributes.

Real-world: Django uses this pattern extensively in ModelAdmin and FormSet classes. Libraries like attrs and SQLAlchemy also create properties outside class bodies via factory functions.

# Pattern 1: @property factory function
#
# A helper creates a property descriptor, which is later assigned to a class.
# This is common when multiple classes share the same computed attributes.
def make_property(attr_name: str):
    @property
    def prop(self):
        return getattr(self, "_" + attr_name)

    return prop


class Point:
    def __init__(self, x: int, y: int):
        self._x = x
        self._y = y

    x = make_property("x")
    y = make_property("y")


p = Point(1, 2)
assert p.x == 1
assert p.y == 2


# Pattern 2: @staticmethod outside class body
#
# A utility function decorated with @staticmethod at module level,
# then assigned as a class attribute.
@staticmethod
def utility(x: int) -> int:
    return x * 2


class Helper:
    compute = utility


result = Helper.compute(5)


# Pattern 3: @classmethod outside class body
#
# A classmethod created at module level, then assigned to a class.
@classmethod
def from_value(cls, val: int) -> "Container":
    obj = cls()
    obj.value = val
    return obj


class Container:
    value: int = 0
    from_value = from_value  # type: ignore[assignment]


c = Container.from_value(42)

The easiest way to address this issue might be to lower the default severity of the problem?

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(Only applicable for extension issues) IDE Information

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[asukaminato0721]

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