[ty] Prefer declared base class attribute over inferred attribute on subclass

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -865,6 +865,42 @@ reveal_type(Derived().declared_in_body)  # revealed: int | None
 reveal_type(Derived().defined_in_init)  # revealed: str | None
 ```
 
+#### Incorrect assignments in derived classes
+
+```py
+class Base:
+    declared_on_body_overwritten_on_body: str
+    declared_on_body_overwritten_in_init: str
+
+    def __init__(self) -> None:
+        self.declared_in_init_overwritten_on_body: str
+        self.declared_in_init_overwritten_in_init: str
+
+class Derived(Base):
+    # TODO: this should be an error
+    declared_on_body_overwritten_on_body = None
+
+    # TODO: this should be an error
+    declared_in_init_overwritten_on_body = None
+
+    def f(self) -> None:
+        # TODO: this should be an error
+        self.declared_on_body_overwritten_in_init = None
+
+        # TODO: this should be an error
+        self.declared_in_init_overwritten_in_init = None
+
+reveal_type(Derived().declared_on_body_overwritten_in_init)  # revealed: str
+
+reveal_type(Derived().declared_in_init_overwritten_in_init)  # revealed: str
+
+# TODO: This should be `str`
+reveal_type(Derived().declared_on_body_overwritten_on_body)  # revealed: Unknown | None | str
+
+# TODO: This should be `str`
+reveal_type(Derived().declared_in_init_overwritten_on_body)  # revealed: Unknown | None | str
+```
+
 ## Accessing attributes on class objects
 
 When accessing attributes on class objects, they are always looked up on the type of the class

crates/ty_python_semantic/src/types/class.rs

@@ -2863,7 +2863,10 @@ impl<'db> ClassLiteral<'db> {
         let mut union = UnionBuilder::new(db);
         let mut union_qualifiers = TypeQualifiers::empty();
 
-        for superclass in self.iter_mro(db, specialization) {
+        let mro = self.try_mro(db, specialization);
+        let mro = mro.unwrap_or_else(|e| e.fallback_mro());
+
+        for superclass in mro.iter().rev() {
             match superclass {
                 ClassBase::Generic | ClassBase::Protocol => {
                     // Skip over these very special class bases that aren't really classes.