Invalid type inferred for attribute initialized to None in class body #4416

The type inferred for attribute x in the fragment below is unexpected (I was using --strict-optional):

class A:
    x = None
    y = None

    def __init__(self):  # Note: no annotation
        self.x = 1

    def g(self) -> None:
        self.y = 1

a = A()
reveal_type(a.x)  # None  <---- unexpected
reveal_type(a.y)  # Union[int, None]

if a.x is not None:
    1 + ''  # No error, because mypy considers this unreachable

Union[None, Any] would be a better inferred type for A.x. It would work around the false negative. This would be consistent with how global variables work:

x = None

def f():
    global x
    x = 1

reveal_type(x)  # Union[Any, None]

When not using strict optional checking, the type of A.x is currently also None. Any would be a better inferred type in that case.

Another alternative would be to require annotations for both x and y, since inferring a useful (non-None) type requires non-local context. This would be my preference if we didn't have existing code to worry about -- this could require a large number of additional annotations for code that currently type checks cleanly. A final option would be to only require an annotation for y and infer Union[None, Any] for x, since x is initialized in an unannotated method.

[Note that use of non-local context (outside current scope) can make fine-grained incremental checking harder to implement.]