Invalid type inferred for attribute initialized to `None` in class body

[JukkaL]

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.]