Fix crash overriding partial-type attribute with method (#12943)

Attributes can still be partially typed (e.g. `<partial list[?]>`) after
a parent-class definition if the block they are declared in is deferred.
The first pass for child classes might then encounter this type when
considering method overrides, which could cause a crash when attempting
to determine subtype compatibility.

Fixes #11686
Fixes #11981

mypy/checker.py

@@ -1579,7 +1579,9 @@ def check_method_override_for_base_with_name(
             # it can be checked for compatibility.
             original_type = get_proper_type(base_attr.type)
             original_node = base_attr.node
-            if original_type is None:
+            # `original_type` can be partial if (e.g.) it is originally an
+            # instance variable from an `__init__` block that becomes deferred.
+            if original_type is None or isinstance(original_type, PartialType):
                 if self.pass_num < self.last_pass:
                     # If there are passes left, defer this node until next pass,
                     # otherwise try reconstructing the method type from available information.

test-data/unit/check-classes.test

@@ -151,6 +151,24 @@ class Derived(Base):
     __hash__ = 1  # E: Incompatible types in assignment (expression has type "int", base class "Base" defined the type as "Callable[[Base], int]")
 
 
+[case testOverridePartialAttributeWithMethod]
+# This was crashing: https://github.com/python/mypy/issues/11686.
+class Base:
+    def __init__(self, arg: int):
+        self.partial_type = []  # E: Need type annotation for "partial_type" (hint: "partial_type: List[<type>] = ...")
+        self.force_deferral = []
+
+    # Force inference of the `force_deferral` attribute in `__init__` to be
+    # deferred to a later pass by providing a definition in another context,
+    # which means `partial_type` remains only partially inferred.
+    force_deferral = []  # E: Need type annotation for "force_deferral" (hint: "force_deferral: List[<type>] = ...")
+
+
+class Derived(Base):
+    def partial_type(self) -> int:  # E: Signature of "partial_type" incompatible with supertype "Base"
+        ...
+
+
 -- Attributes
 -- ----------