[ty] Propagate specializations to ancestor base classes (#17892)

@AlexWaygood discovered that even though we've been propagating
specializations to _parent_ base classes correctly, we haven't been
passing them on to _grandparent_ base classes:
https://github.com/astral-sh/ruff/pull/17832#issuecomment-2854360969

```py
class Bar[T]:
    x: T

class Baz[T](Bar[T]): ...
class Spam[T](Baz[T]): ...

reveal_type(Spam[int]().x) # revealed: `T`, but should be `int`
```

This PR updates the MRO machinery to apply the current specialization
when starting to iterate the MRO of each base class.

crates/ty_python_semantic/src/types.rs

@@ -4948,6 +4948,19 @@ impl<'db> Type<'db> {
         }
     }
 
+    #[must_use]
+    pub fn apply_optional_specialization(
+        self,
+        db: &'db dyn Db,
+        specialization: Option<Specialization<'db>>,
+    ) -> Type<'db> {
+        if let Some(specialization) = specialization {
+            self.apply_specialization(db, specialization)
+        } else {
+            self
+        }
+    }
+
     /// Applies a specialization to this type, replacing any typevars with the types that they are
     /// specialized to.
     ///
@@ -7979,7 +7992,9 @@ pub enum SuperOwnerKind<'db> {
 impl<'db> SuperOwnerKind<'db> {
     fn iter_mro(self, db: &'db dyn Db) -> impl Iterator<Item = ClassBase<'db>> {
         match self {
-            SuperOwnerKind::Dynamic(dynamic) => Either::Left(ClassBase::Dynamic(dynamic).mro(db)),
+            SuperOwnerKind::Dynamic(dynamic) => {
+                Either::Left(ClassBase::Dynamic(dynamic).mro(db, None))
+            }
             SuperOwnerKind::Class(class) => Either::Right(class.iter_mro(db)),
             SuperOwnerKind::Instance(instance) => Either::Right(instance.class().iter_mro(db)),
         }
@@ -8106,7 +8121,7 @@ impl<'db> BoundSuperType<'db> {
         mro_iter: impl Iterator<Item = ClassBase<'db>>,
     ) -> impl Iterator<Item = ClassBase<'db>> {
         let Some(pivot_class) = self.pivot_class(db).into_class() else {
-            return Either::Left(ClassBase::Dynamic(DynamicType::Unknown).mro(db));
+            return Either::Left(ClassBase::Dynamic(DynamicType::Unknown).mro(db, None));
         };
 
         let mut pivot_found = false;

crates/ty_python_semantic/src/types/class.rs

@@ -191,6 +191,26 @@ impl<'db> ClassType<'db> {
         }
     }
 
+    /// Returns the class literal and specialization for this class, with an additional
+    /// specialization applied if the class is generic.
+    pub(crate) fn class_literal_specialized(
+        self,
+        db: &'db dyn Db,
+        additional_specialization: Option<Specialization<'db>>,
+    ) -> (ClassLiteral<'db>, Option<Specialization<'db>>) {
+        match self {
+            Self::NonGeneric(non_generic) => (non_generic, None),
+            Self::Generic(generic) => (
+                generic.origin(db),
+                Some(
+                    generic
+                        .specialization(db)
+                        .apply_optional_specialization(db, additional_specialization),
+                ),
+            ),
+        }
+    }
+
     pub(crate) fn name(self, db: &'db dyn Db) -> &'db ast::name::Name {
         let (class_literal, _) = self.class_literal(db);
         class_literal.name(db)
@@ -206,13 +226,6 @@ impl<'db> ClassType<'db> {
         class_literal.definition(db)
     }
 
-    fn specialize_type(self, db: &'db dyn Db, ty: Type<'db>) -> Type<'db> {
-        match self {
-            Self::NonGeneric(_) => ty,
-            Self::Generic(generic) => ty.apply_specialization(db, generic.specialization(db)),
-        }
-    }
-
     /// Return `true` if this class represents `known_class`
     pub(crate) fn is_known(self, db: &'db dyn Db, known_class: KnownClass) -> bool {
         self.known(db) == Some(known_class)
@@ -249,6 +262,18 @@ impl<'db> ClassType<'db> {
         class_literal.iter_mro(db, specialization)
     }
 
+    /// Iterate over the method resolution order ("MRO") of the class, optionally applying an
+    /// additional specialization to it if the class is generic.
+    pub(super) fn iter_mro_specialized(
+        self,
+        db: &'db dyn Db,
+        additional_specialization: Option<Specialization<'db>>,
+    ) -> MroIterator<'db> {
+        let (class_literal, specialization) =
+            self.class_literal_specialized(db, additional_specialization);
+        class_literal.iter_mro(db, specialization)
+    }
+
     /// Is this class final?
     pub(super) fn is_final(self, db: &'db dyn Db) -> bool {
         let (class_literal, _) = self.class_literal(db);
@@ -372,8 +397,10 @@ impl<'db> ClassType<'db> {
 
     /// Return the metaclass of this class, or `type[Unknown]` if the metaclass cannot be inferred.
     pub(super) fn metaclass(self, db: &'db dyn Db) -> Type<'db> {
-        let (class_literal, _) = self.class_literal(db);
-        self.specialize_type(db, class_literal.metaclass(db))
+        let (class_literal, specialization) = self.class_literal(db);
+        class_literal
+            .metaclass(db)
+            .apply_optional_specialization(db, specialization)
     }
 
     /// Return a type representing "the set of all instances of the metaclass of this class".
@@ -396,9 +423,7 @@ impl<'db> ClassType<'db> {
         policy: MemberLookupPolicy,
     ) -> SymbolAndQualifiers<'db> {
         let (class_literal, specialization) = self.class_literal(db);
-        class_literal
-            .class_member_inner(db, specialization, name, policy)
-            .map_type(|ty| self.specialize_type(db, ty))
+        class_literal.class_member_inner(db, specialization, name, policy)
     }
 
     /// Returns the inferred type of the class member named `name`. Only bound members
@@ -411,7 +436,7 @@ impl<'db> ClassType<'db> {
         let (class_literal, specialization) = self.class_literal(db);
         class_literal
             .own_class_member(db, specialization, name)
-            .map_type(|ty| self.specialize_type(db, ty))
+            .map_type(|ty| ty.apply_optional_specialization(db, specialization))
     }
 
     /// Returns the `name` attribute of an instance of this class.
@@ -424,16 +449,16 @@ impl<'db> ClassType<'db> {
         let (class_literal, specialization) = self.class_literal(db);
         class_literal
             .instance_member(db, specialization, name)
-            .map_type(|ty| self.specialize_type(db, ty))
+            .map_type(|ty| ty.apply_optional_specialization(db, specialization))
     }
 
     /// A helper function for `instance_member` that looks up the `name` attribute only on
     /// this class, not on its superclasses.
     fn own_instance_member(self, db: &'db dyn Db, name: &str) -> SymbolAndQualifiers<'db> {
-        let (class_literal, _) = self.class_literal(db);
+        let (class_literal, specialization) = self.class_literal(db);
         class_literal
             .own_instance_member(db, name)
-            .map_type(|ty| self.specialize_type(db, ty))
+            .map_type(|ty| ty.apply_optional_specialization(db, specialization))
     }
 }
 

crates/ty_python_semantic/src/types/class_base.rs

@@ -1,4 +1,4 @@
-use crate::types::generics::GenericContext;
+use crate::types::generics::{GenericContext, Specialization};
 use crate::types::{
     todo_type, ClassType, DynamicType, KnownClass, KnownInstanceType, MroIterator, Type,
 };
@@ -202,8 +202,35 @@ impl<'db> ClassBase<'db> {
         }
     }
 
+    pub(crate) fn apply_specialization(
+        self,
+        db: &'db dyn Db,
+        specialization: Specialization<'db>,
+    ) -> Self {
+        match self {
+            Self::Class(class) => Self::Class(class.apply_specialization(db, specialization)),
+            Self::Dynamic(_) | Self::Generic(_) | Self::Protocol => self,
+        }
+    }
+
+    pub(crate) fn apply_optional_specialization(
+        self,
+        db: &'db dyn Db,
+        specialization: Option<Specialization<'db>>,
+    ) -> Self {
+        if let Some(specialization) = specialization {
+            self.apply_specialization(db, specialization)
+        } else {
+            self
+        }
+    }
+
     /// Iterate over the MRO of this base
-    pub(super) fn mro(self, db: &'db dyn Db) -> impl Iterator<Item = ClassBase<'db>> {
+    pub(super) fn mro(
+        self,
+        db: &'db dyn Db,
+        additional_specialization: Option<Specialization<'db>>,
+    ) -> impl Iterator<Item = ClassBase<'db>> {
         match self {
             ClassBase::Protocol => {
                 ClassBaseMroIterator::length_3(db, self, ClassBase::Generic(None))
@@ -214,7 +241,9 @@ impl<'db> ClassBase<'db> {
             ClassBase::Dynamic(_) | ClassBase::Generic(_) => {
                 ClassBaseMroIterator::length_2(db, self)
             }
-            ClassBase::Class(class) => ClassBaseMroIterator::from_class(db, class),
+            ClassBase::Class(class) => {
+                ClassBaseMroIterator::from_class(db, class, additional_specialization)
+            }
         }
     }
 }
@@ -263,8 +292,12 @@ impl<'db> ClassBaseMroIterator<'db> {
     }
 
     /// Iterate over the MRO of an arbitrary class. The MRO may be of any length.
-    fn from_class(db: &'db dyn Db, class: ClassType<'db>) -> Self {
-        ClassBaseMroIterator::FromClass(class.iter_mro(db))
+    fn from_class(
+        db: &'db dyn Db,
+        class: ClassType<'db>,
+        additional_specialization: Option<Specialization<'db>>,
+    ) -> Self {
+        ClassBaseMroIterator::FromClass(class.iter_mro_specialized(db, additional_specialization))
     }
 }
 

crates/ty_python_semantic/src/types/generics.rs

@@ -202,6 +202,19 @@ impl<'db> Specialization<'db> {
         Specialization::new(db, self.generic_context(db), types)
     }
 
+    /// Applies an optional specialization to this specialization.
+    pub(crate) fn apply_optional_specialization(
+        self,
+        db: &'db dyn Db,
+        other: Option<Specialization<'db>>,
+    ) -> Self {
+        if let Some(other) = other {
+            self.apply_specialization(db, other)
+        } else {
+            self
+        }
+    }
+
     /// Combines two specializations of the same generic context. If either specialization maps a
     /// typevar to `Type::Unknown`, the other specialization's mapping is used. If both map the
     /// typevar to a known type, those types are unioned together.

crates/ty_python_semantic/src/types/mro.rs

@@ -80,12 +80,14 @@ impl<'db> Mro<'db> {
             ));
         }
 
+        let class_type = class.apply_optional_specialization(db, specialization);
+
         match class_bases {
             // `builtins.object` is the special case:
             // the only class in Python that has an MRO with length <2
             [] if class.is_object(db) => Ok(Self::from([
                 // object is not generic, so the default specialization should be a no-op
-                ClassBase::Class(class.apply_optional_specialization(db, specialization)),
+                ClassBase::Class(class_type),
             ])),
 
             // All other classes in Python have an MRO with length >=2.
@@ -102,7 +104,7 @@ impl<'db> Mro<'db> {
             // (<class '__main__.Foo'>, <class 'object'>)
             // ```
             [] => Ok(Self::from([
-                ClassBase::Class(class.apply_optional_specialization(db, specialization)),
+                ClassBase::Class(class_type),
                 ClassBase::object(db),
             ])),
 
@@ -114,11 +116,9 @@ impl<'db> Mro<'db> {
             [single_base] => ClassBase::try_from_type(db, *single_base).map_or_else(
                 || Err(MroErrorKind::InvalidBases(Box::from([(0, *single_base)]))),
                 |single_base| {
-                    Ok(std::iter::once(ClassBase::Class(
-                        class.apply_optional_specialization(db, specialization),
-                    ))
-                    .chain(single_base.mro(db))
-                    .collect())
+                    Ok(std::iter::once(ClassBase::Class(class_type))
+                        .chain(single_base.mro(db, specialization))
+                        .collect())
                 },
             ),
 
@@ -142,13 +142,16 @@ impl<'db> Mro<'db> {
                     return Err(MroErrorKind::InvalidBases(invalid_bases.into_boxed_slice()));
                 }
 
-                let mut seqs = vec![VecDeque::from([ClassBase::Class(
-                    class.apply_optional_specialization(db, specialization),
-                )])];
+                let mut seqs = vec![VecDeque::from([ClassBase::Class(class_type)])];
                 for base in &valid_bases {
-                    seqs.push(base.mro(db).collect());
+                    seqs.push(base.mro(db, specialization).collect());
                 }
-                seqs.push(valid_bases.iter().copied().collect());
+                seqs.push(
+                    valid_bases
+                        .iter()
+                        .map(|base| base.apply_optional_specialization(db, specialization))
+                        .collect(),
+                );
 
                 c3_merge(seqs).ok_or_else(|| {
                     let mut seen_bases = FxHashSet::default();