Fix metaclass lookup for class object __class__

Summary:
Pyrefly could infer `C.__class__` as `type[type[C]]` when `C` was represented as a `ClassDef`, but attribute-base conversion had no case for `type[ClassDef]`. That caused ordinary metaclass method lookups such as `C.__class__.__setattr__` to report an internal `attribute base undefined` error. Treat `type[ClassDef(C)]` as `C`'s metaclass viewed as a class object, preserving custom metaclass method signatures while leaving the broader `type[ClassType]` / generic alias behavior as the existing known bug.

Fixes #4093

Reviewed By: kinto0

Differential Revision: D111341385

fbshipit-source-id: b610104a8e39887b7cda66f3c608832d00b2cd72

Author: grievejia

pyrefly/lib/alt/attr.rs

@@ -2356,6 +2356,17 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                     acc.push(class_base)
                 }
             }
+            Type::Type(f) if matches!(&*f, Type::ClassDef(_)) => {
+                let Type::ClassDef(class) = *f else {
+                    unreachable!("guarded by matches! above")
+                };
+                // type[ClassDef(C)] is C.__class__: C's metaclass, viewed as a class object.
+                let metaclass = self
+                    .get_metadata_for_class(&class)
+                    .metaclass(self.stdlib)
+                    .clone();
+                acc.push(AttributeBase1::ClassObject(ClassBase::ClassType(metaclass)))
+            }
             Type::QuantifiedValue(q) => acc.push(AttributeBase1::QuantifiedValue(*q)),
             Type::Type(f) if matches!(&*f, Type::Quantified(_)) => {
                 let Type::Quantified(quantified) = *f else {

pyrefly/lib/test/attributes.rs

@@ -1730,7 +1730,7 @@ def test_union(x: A  | B):
 );
 
 testcase!(
-    bug = "type[ClassDef(..)] and type[ClassType(..)] should be type (or the direct metaclass?)",
+    bug = "type[ClassType(..)] should be type (or the direct metaclass?)",
     test_attribute_access_on_type_class,
     r#"
 # handy hack to get a type[X] for any X
@@ -1744,11 +1744,32 @@ class D[T]:
     @classmethod
     def m(cls, x: T): ...
 
-ty(C).m(0) # E: Expr::attr_infer_for_type attribute base undefined
+ty(C).m(0) # E: Class `type` has no class attribute `m`
 ty(D[int]).m(0) # E: Expr::attr_infer_for_type attribute base undefined
 "#,
 );
 
+testcase!(
+    test_attribute_access_on_classdef_metaclass,
+    r#"
+class Meta(type):
+    def __setattr__(cls, name: str, value: int) -> None: ...
+    def __getattribute__(cls, name: str) -> object: ...
+
+class C(metaclass=Meta):
+    pass
+
+C.__class__.__setattr__(C, "x", 0)
+C.__class__.__setattr__(C, "x", "bad") # E: Argument `Literal['bad']` is not assignable to parameter `value` with type `int`
+C.__class__.__getattribute__(C, "x")
+
+class DefaultMeta:
+    pass
+
+DefaultMeta.__class__.mro(DefaultMeta)
+"#,
+);
+
 testcase!(
     bug = "type[TypedDict()] should be type",
     test_attribute_access_on_type_typeddict,
@@ -1762,7 +1783,7 @@ class TD(TypedDict):
     x: int
 
 ty(TD(x = 0))(x = 0)
-ty(TD).mro() # E: Expr::attr_infer_for_type attribute base undefined
+ty(TD).mro() # E: Missing argument `self` in function `type.mro`
 "#,
 );