Improve handling of attribute access on class objects (#14988)

Fixes #14056

#14056 was originally reported as a mypyc issue (because that's how it
presented itself to the user), but the underlying bug is really a bug to
do with how mypy understands metaclasses. On mypy `master`:

```py
class Meta(type):
    bar: str

class Foo(metaclass=Meta):
    bar: int

reveal_type(Foo().bar)  # Revealed type is int (correct!)
reveal_type(Foo.bar)  # Revealed type is int, but should be str
```

This PR fixes that incorrect behaviour.

Since this is really a mypy bug rather than a mypyc bug, I haven't added
a mypyc test, but I'm happy to if that would be useful. (I'll need some
guidance as to exactly where it should go -- I don't know much about
mypyc internals!)

Author: AlexWaygood

mypy/checkmember.py

@@ -388,7 +388,7 @@ def analyze_type_callable_member_access(name: str, typ: FunctionLike, mx: Member
             # See https://github.com/python/mypy/pull/1787 for more info.
             # TODO: do not rely on same type variables being present in all constructor overloads.
             result = analyze_class_attribute_access(
-                ret_type, name, mx, original_vars=typ.items[0].variables
+                ret_type, name, mx, original_vars=typ.items[0].variables, mcs_fallback=typ.fallback
             )
             if result:
                 return result
@@ -434,17 +434,21 @@ def analyze_type_type_member_access(
         if isinstance(typ.item.item, Instance):
             item = typ.item.item.type.metaclass_type
     ignore_messages = False
+
+    if item is not None:
+        fallback = item.type.metaclass_type or fallback
+
     if item and not mx.is_operator:
         # See comment above for why operators are skipped
-        result = analyze_class_attribute_access(item, name, mx, override_info)
+        result = analyze_class_attribute_access(
+            item, name, mx, mcs_fallback=fallback, override_info=override_info
+        )
         if result:
             if not (isinstance(get_proper_type(result), AnyType) and item.type.fallback_to_any):
                 return result
             else:
                 # We don't want errors on metaclass lookup for classes with Any fallback
                 ignore_messages = True
-    if item is not None:
-        fallback = item.type.metaclass_type or fallback
 
     with mx.msg.filter_errors(filter_errors=ignore_messages):
         return _analyze_member_access(name, fallback, mx, override_info)
@@ -893,6 +897,8 @@ def analyze_class_attribute_access(
     itype: Instance,
     name: str,
     mx: MemberContext,
+    *,
+    mcs_fallback: Instance,
     override_info: TypeInfo | None = None,
     original_vars: Sequence[TypeVarLikeType] | None = None,
 ) -> Type | None:
@@ -919,6 +925,22 @@ def analyze_class_attribute_access(
             return apply_class_attr_hook(mx, hook, AnyType(TypeOfAny.special_form))
         return None
 
+    if (
+        isinstance(node.node, Var)
+        and not node.node.is_classvar
+        and not hook
+        and mcs_fallback.type.get(name)
+    ):
+        # If the same attribute is declared on the metaclass and the class but with different types,
+        # and the attribute on the class is not a ClassVar,
+        # the type of the attribute on the metaclass should take priority
+        # over the type of the attribute on the class,
+        # when the attribute is being accessed from the class object itself.
+        #
+        # Return `None` here to signify that the name should be looked up
+        # on the class object itself rather than the instance.
+        return None
+
     is_decorated = isinstance(node.node, Decorator)
     is_method = is_decorated or isinstance(node.node, FuncBase)
     if mx.is_lvalue:

test-data/unit/check-class-namedtuple.test

@@ -325,7 +325,10 @@ class X(NamedTuple):
 reveal_type(X._fields)  # N: Revealed type is "Tuple[builtins.str, builtins.str]"
 reveal_type(X._field_types)  # N: Revealed type is "builtins.dict[builtins.str, Any]"
 reveal_type(X._field_defaults)  # N: Revealed type is "builtins.dict[builtins.str, Any]"
-reveal_type(X.__annotations__)  # N: Revealed type is "builtins.dict[builtins.str, Any]"
+
+# In typeshed's stub for builtins.pyi, __annotations__ is `dict[str, Any]`,
+# but it's inferred as `Mapping[str, object]` here due to the fixture we're using
+reveal_type(X.__annotations__)  # N: Revealed type is "typing.Mapping[builtins.str, builtins.object]"
 
 [builtins fixtures/dict.pyi]
 

test-data/unit/check-classes.test

@@ -4540,6 +4540,39 @@ def f(TA: Type[A]):
     reveal_type(TA)  # N: Revealed type is "Type[__main__.A]"
     reveal_type(TA.x)  # N: Revealed type is "builtins.int"
 
+[case testMetaclassConflictingInstanceVars]
+from typing import ClassVar
+
+class Meta(type):
+    foo: int
+    bar: int
+    eggs: ClassVar[int] = 42
+    spam: ClassVar[int] = 42
+
+class Foo(metaclass=Meta):
+    foo: str
+    bar: ClassVar[str] = 'bar'
+    eggs: str
+    spam: ClassVar[str] = 'spam'
+
+reveal_type(Foo.foo)  # N: Revealed type is "builtins.int"
+reveal_type(Foo.bar)  # N: Revealed type is "builtins.str"
+reveal_type(Foo.eggs)  # N: Revealed type is "builtins.int"
+reveal_type(Foo.spam)  # N: Revealed type is "builtins.str"
+
+class MetaSub(Meta): ...
+
+class Bar(metaclass=MetaSub):
+    foo: str
+    bar: ClassVar[str] = 'bar'
+    eggs: str
+    spam: ClassVar[str] = 'spam'
+
+reveal_type(Bar.foo)  # N: Revealed type is "builtins.int"
+reveal_type(Bar.bar)  # N: Revealed type is "builtins.str"
+reveal_type(Bar.eggs)  # N: Revealed type is "builtins.int"
+reveal_type(Bar.spam)  # N: Revealed type is "builtins.str"
+
 [case testSubclassMetaclass]
 class M1(type):
     x = 0

test-data/unit/pythoneval.test

@@ -1987,6 +1987,19 @@ def good9(foo1: Foo[Concatenate[int, P]], foo2: Foo[[int, str, bytes]], *args: P
 [out]
 _testStrictEqualitywithParamSpec.py:11: error: Non-overlapping equality check (left operand type: "Foo[[int]]", right operand type: "Bar[[int]]")
 
+[case testInferenceOfDunderDictOnClassObjects]
+class Foo: ...
+reveal_type(Foo.__dict__)
+reveal_type(Foo().__dict__)
+Foo.__dict__ = {}
+Foo().__dict__ = {}
+
+[out]
+_testInferenceOfDunderDictOnClassObjects.py:2: note: Revealed type is "types.MappingProxyType[builtins.str, Any]"
+_testInferenceOfDunderDictOnClassObjects.py:3: note: Revealed type is "builtins.dict[builtins.str, Any]"
+_testInferenceOfDunderDictOnClassObjects.py:4: error: Property "__dict__" defined in "type" is read-only
+_testInferenceOfDunderDictOnClassObjects.py:4: error: Incompatible types in assignment (expression has type "Dict[<nothing>, <nothing>]", variable has type "MappingProxyType[str, Any]")
+
 [case testTypeVarTuple]
 # flags: --enable-incomplete-feature=TypeVarTuple --enable-incomplete-feature=Unpack --python-version=3.11
 from typing import Any, Callable, Unpack, TypeVarTuple