[ty] Specialize bound methods and nominal instances (#17865)

Fixes
https://github.com/astral-sh/ruff/pull/17832#issuecomment-2851224968. We
had a comment that we did not need to apply specializations to generic
aliases, or to the bound `self` of a bound method, because they were
already specialized. But they might be specialized with a type variable,
which _does_ need to be specialized, in the case of a "multi-step"
specialization, such as:

```py
class LinkedList[T]: ...

class C[U]:
    def method(self) -> LinkedList[U]:
        return LinkedList[U]()
```

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ty_python_semantic/src/types.rs

@@ -4962,20 +4962,16 @@ impl<'db> Type<'db> {
                 Type::FunctionLiteral(function.apply_specialization(db, specialization))
             }
 
-            // Note that we don't need to apply the specialization to `self_instance`, since it
-            // must either be a non-generic class literal (which cannot have any typevars to
-            // specialize) or a generic alias (which has already been fully specialized). For a
-            // generic alias, the specialization being applied here must be for some _other_
-            // generic context nested within the generic alias's class literal, which the generic
-            // alias's context cannot refer to. (The _method_ does need to be specialized, since it
-            // might be a nested generic method, whose generic context is what is now being
-            // specialized.)
             Type::BoundMethod(method) => Type::BoundMethod(BoundMethodType::new(
                 db,
                 method.function(db).apply_specialization(db, specialization),
-                method.self_instance(db),
+                method.self_instance(db).apply_specialization(db, specialization),
             )),
 
+            Type::NominalInstance(instance) => Type::NominalInstance(
+                instance.apply_specialization(db, specialization),
+            ),
+
             Type::MethodWrapper(MethodWrapperKind::FunctionTypeDunderGet(function)) => {
                 Type::MethodWrapper(MethodWrapperKind::FunctionTypeDunderGet(
                     function.apply_specialization(db, specialization),
@@ -5060,9 +5056,6 @@ impl<'db> Type<'db> {
             | Type::BytesLiteral(_)
             | Type::SliceLiteral(_)
             | Type::BoundSuper(_)
-            // `NominalInstance` contains a ClassType, which has already been specialized if needed,
-            // like above with BoundMethod's self_instance.
-            | Type::NominalInstance(_)
             // Same for `ProtocolInstance`
             | Type::ProtocolInstance(_)
             | Type::KnownInstance(_) => self,

crates/ty_python_semantic/src/types/class.rs

@@ -146,6 +146,19 @@ impl<'db> GenericAlias<'db> {
     pub(crate) fn definition(self, db: &'db dyn Db) -> Definition<'db> {
         self.origin(db).definition(db)
     }
+
+    pub(super) fn apply_specialization(
+        self,
+        db: &'db dyn Db,
+        specialization: Specialization<'db>,
+    ) -> Self {
+        Self::new(
+            db,
+            self.origin(db),
+            self.specialization(db)
+                .apply_specialization(db, specialization),
+        )
+    }
 }
 
 impl<'db> From<GenericAlias<'db>> for Type<'db> {
@@ -210,6 +223,19 @@ impl<'db> ClassType<'db> {
         self.is_known(db, KnownClass::Object)
     }
 
+    pub(super) fn apply_specialization(
+        self,
+        db: &'db dyn Db,
+        specialization: Specialization<'db>,
+    ) -> Self {
+        match self {
+            Self::NonGeneric(_) => self,
+            Self::Generic(generic) => {
+                Self::Generic(generic.apply_specialization(db, specialization))
+            }
+        }
+    }
+
     /// Iterate over the [method resolution order] ("MRO") of the class.
     ///
     /// If the MRO could not be accurately resolved, this method falls back to iterating

crates/ty_python_semantic/src/types/instance.rs

@@ -3,6 +3,7 @@
 use super::protocol_class::ProtocolInterface;
 use super::{ClassType, KnownClass, SubclassOfType, Type};
 use crate::symbol::{Symbol, SymbolAndQualifiers};
+use crate::types::generics::Specialization;
 use crate::Db;
 
 pub(super) use synthesized_protocol::SynthesizedProtocolType;
@@ -111,6 +112,16 @@ impl<'db> NominalInstanceType<'db> {
     pub(super) fn to_meta_type(self, db: &'db dyn Db) -> Type<'db> {
         SubclassOfType::from(db, self.class)
     }
+
+    pub(super) fn apply_specialization(
+        self,
+        db: &'db dyn Db,
+        specialization: Specialization<'db>,
+    ) -> Self {
+        Self {
+            class: self.class.apply_specialization(db, specialization),
+        }
+    }
 }
 
 impl<'db> From<NominalInstanceType<'db>> for Type<'db> {