Consider all class fields in stub files to be always magically initialized

Summary:
See [this github issue](#123). Pyrefly currently complains on the following code:
```
class Foo:
  x: int

Foo.x  # ERROR! Instance-only attribute `x` of class `Foo` is not visible on the class
```
Problem is, typeshed doesn't really differentiate between uninitialized and initialized classlevel field. Meaning that this:
```
class object:
  __dict__: dict[str, Any]
```
and this:
```
class object:
  __dict__: dict[str, Any] = ...
```
are treated as equivalent (see [this PR](python/typeshed#13875) for further details). Based on the feedback of the PR, this is unlikely to be something that's going to change in the short term. And the suggested action was to avoid reporting the error when the containing class of the field comes from a stub file.

Fixes #123

Reviewed By: rchen152

Differential Revision: D74035159

fbshipit-source-id: 72db88a9506294f4395d049549f3220e6fa2b4c1

Author: grievejia

conformance/third_party/conformance.exp

@@ -4763,46 +4763,6 @@
       "stop_column": 24,
       "stop_line": 28
     },
-    {
-      "code": -2,
-      "column": 1,
-      "concise_description": "assert_type(Any, str) failed",
-      "description": "assert_type(Any, str) failed",
-      "line": 35,
-      "name": "assert-type",
-      "stop_column": 29,
-      "stop_line": 35
-    },
-    {
-      "code": -2,
-      "column": 13,
-      "concise_description": "Instance-only attribute `genus` of class `Pet2` is not visible on the class",
-      "description": "Instance-only attribute `genus` of class `Pet2` is not visible on the class",
-      "line": 35,
-      "name": "missing-attribute",
-      "stop_column": 23,
-      "stop_line": 35
-    },
-    {
-      "code": -2,
-      "column": 1,
-      "concise_description": "assert_type(Any, str) failed",
-      "description": "assert_type(Any, str) failed",
-      "line": 36,
-      "name": "assert-type",
-      "stop_column": 31,
-      "stop_line": 36
-    },
-    {
-      "code": -2,
-      "column": 13,
-      "concise_description": "Instance-only attribute `species` of class `Pet2` is not visible on the class",
-      "description": "Instance-only attribute `species` of class `Pet2` is not visible on the class",
-      "line": 36,
-      "name": "missing-attribute",
-      "stop_column": 25,
-      "stop_line": 36
-    },
     {
       "code": -2,
       "column": 5,

conformance/third_party/conformance.result

@@ -270,8 +270,6 @@
   "enums_members.py": [
     "Line 27: Unexpected errors ['assert_type(Any, str) failed', 'Instance-only attribute `genus` of class `Pet` is not visible on the class']",
     "Line 28: Unexpected errors ['assert_type(Any, str) failed', 'Instance-only attribute `species` of class `Pet` is not visible on the class']",
-    "Line 35: Unexpected errors ['assert_type(Any, str) failed', 'Instance-only attribute `genus` of class `Pet2` is not visible on the class']",
-    "Line 36: Unexpected errors ['assert_type(Any, str) failed', 'Instance-only attribute `species` of class `Pet2` is not visible on the class']",
     "Line 100: Unexpected errors ['assert_type(Literal[TrafficLight.AMBER], Literal[TrafficLight.YELLOW]) failed']",
     "Line 128: Unexpected errors ['Could not find name `reveal_type`']"
   ],

conformance/third_party/results.json

@@ -3,7 +3,7 @@
   "pass": 49,
   "fail": 87,
   "pass_rate": 0.36,
-  "differences": 443,
+  "differences": 441,
   "passing": [
     "aliases_newtype.py",
     "annotations_coroutines.py",
@@ -87,7 +87,7 @@
     "directives_deprecated.py": 12,
     "directives_type_ignore_file1.py": 1,
     "enums_behaviors.py": 4,
-    "enums_members.py": 6,
+    "enums_members.py": 4,
     "exceptions_context_managers.py": 2,
     "generics_base_class.py": 3,
     "generics_basic.py": 7,

pyrefly/lib/alt/class/class_field.rs

@@ -544,7 +544,11 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
             }
         };
         let metadata = self.get_metadata_for_class(class);
-        let initialization = self.get_class_field_initialization(&metadata, initial_value);
+        let initialization = self.get_class_field_initialization(
+            &metadata,
+            initial_value,
+            class.module_info().path().is_interface(),
+        );
 
         // Ban typed dict from containing values; fields should be annotation-only.
         // TODO(stroxler): we ought to look into this more: class-level attributes make sense on a `TypedDict` class;
@@ -743,9 +747,14 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
         &self,
         metadata: &ClassMetadata,
         initial_value: &ClassFieldInitialValue,
+        is_stub: bool,
     ) -> ClassFieldInitialization {
         match initial_value {
-            ClassFieldInitialValue::Instance(_) => ClassFieldInitialization::Instance(false),
+            ClassFieldInitialValue::Instance(_) => {
+                // We consider fields to be always-initialized if it's defined within stub files.
+                // See https://github.com/python/typeshed/pull/13875 for reasoning.
+                ClassFieldInitialization::Instance(is_stub)
+            }
             ClassFieldInitialValue::Class(None) => ClassFieldInitialization::Class(None),
             ClassFieldInitialValue::Class(Some(e)) => {
                 // If this field was created via a call to a dataclass field specifier, extract field flags from the call.
@@ -919,7 +928,7 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                 )
             }
             ClassFieldInner::Simple {
-                initialization: ClassFieldInitialization::Instance(_),
+                initialization: ClassFieldInitialization::Instance(false),
                 annotation,
                 ..
             } if annotation

pyrefly/lib/test/attributes.rs

@@ -548,14 +548,14 @@ class A:
 }
 
 testcase!(
-    test_stub_initializes_attr_with_ellipses,
+    test_stub_initializes_attr,
     env_with_stub(),
     r#"
 from typing import assert_type
 from foo import A
 
 assert_type(A.x, int)
-A.y  # E: Instance-only attribute `y` of class `A` is not visible on the class
+assert_type(A.y, int)
     "#,
 );