[ty] Infer parameter specializations of generic aliases (#18021)

This updates our function specialization inference to infer type
mappings from parameters that are generic aliases, e.g.:

```py
def f[T](x: list[T]) -> T: ...

reveal_type(f(["a", "b"]))  # revealed: str
```

Though note that we're still inferring the type of list literals as
`list[Unknown]`, so for now we actually need something like the
following in our tests:

```py
def _(x: list[str]):
    reveal_type(f(x))  # revealed: str
```

crates/ty_python_semantic/resources/mdtest/binary/tuples.md

@@ -17,7 +17,6 @@ def _(x: tuple[int, str], y: tuple[None, tuple[int]]):
 
 ```py
 def _(x: tuple[int, ...], y: tuple[str, ...]):
-    # TODO: should be `tuple[int | str, ...]`
-    reveal_type(x + y)  # revealed: tuple[int | Unknown, ...]
+    reveal_type(x + y)  # revealed: tuple[int | str, ...]
     reveal_type(x + (1, 2))  # revealed: tuple[int, ...]
 ```

crates/ty_python_semantic/resources/mdtest/generics/legacy/functions.md

@@ -88,14 +88,12 @@ def takes_in_protocol(x: CanIndex[T]) -> T:
     return x[0]
 
 def deep_list(x: list[str]) -> None:
-    # TODO: revealed: list[str]
-    reveal_type(takes_in_list(x))  # revealed: list[Unknown]
+    reveal_type(takes_in_list(x))  # revealed: list[str]
     # TODO: revealed: str
     reveal_type(takes_in_protocol(x))  # revealed: Unknown
 
 def deeper_list(x: list[set[str]]) -> None:
-    # TODO: revealed: list[set[str]]
-    reveal_type(takes_in_list(x))  # revealed: list[Unknown]
+    reveal_type(takes_in_list(x))  # revealed: list[set[str]]
     # TODO: revealed: set[str]
     reveal_type(takes_in_protocol(x))  # revealed: Unknown
 
@@ -119,13 +117,11 @@ This also works when passing in arguments that are subclasses of the parameter t
 class Sub(list[int]): ...
 class GenericSub(list[T]): ...
 
-# TODO: revealed: list[int]
-reveal_type(takes_in_list(Sub()))  # revealed: list[Unknown]
+reveal_type(takes_in_list(Sub()))  # revealed: list[int]
 # TODO: revealed: int
 reveal_type(takes_in_protocol(Sub()))  # revealed: Unknown
 
-# TODO: revealed: list[str]
-reveal_type(takes_in_list(GenericSub[str]()))  # revealed: list[Unknown]
+reveal_type(takes_in_list(GenericSub[str]()))  # revealed: list[str]
 # TODO: revealed: str
 reveal_type(takes_in_protocol(GenericSub[str]()))  # revealed: Unknown
 

crates/ty_python_semantic/resources/mdtest/generics/pep695/functions.md

@@ -83,14 +83,12 @@ def takes_in_protocol[T](x: CanIndex[T]) -> T:
     return x[0]
 
 def deep_list(x: list[str]) -> None:
-    # TODO: revealed: list[str]
-    reveal_type(takes_in_list(x))  # revealed: list[Unknown]
+    reveal_type(takes_in_list(x))  # revealed: list[str]
     # TODO: revealed: str
     reveal_type(takes_in_protocol(x))  # revealed: Unknown
 
 def deeper_list(x: list[set[str]]) -> None:
-    # TODO: revealed: list[set[str]]
-    reveal_type(takes_in_list(x))  # revealed: list[Unknown]
+    reveal_type(takes_in_list(x))  # revealed: list[set[str]]
     # TODO: revealed: set[str]
     reveal_type(takes_in_protocol(x))  # revealed: Unknown
 
@@ -114,13 +112,11 @@ This also works when passing in arguments that are subclasses of the parameter t
 class Sub(list[int]): ...
 class GenericSub[T](list[T]): ...
 
-# TODO: revealed: list[int]
-reveal_type(takes_in_list(Sub()))  # revealed: list[Unknown]
+reveal_type(takes_in_list(Sub()))  # revealed: list[int]
 # TODO: revealed: int
 reveal_type(takes_in_protocol(Sub()))  # revealed: Unknown
 
-# TODO: revealed: list[str]
-reveal_type(takes_in_list(GenericSub[str]()))  # revealed: list[Unknown]
+reveal_type(takes_in_list(GenericSub[str]()))  # revealed: list[str]
 # TODO: revealed: str
 reveal_type(takes_in_protocol(GenericSub[str]()))  # revealed: Unknown
 

crates/ty_python_semantic/src/types.rs

@@ -562,25 +562,22 @@ impl<'db> Type<'db> {
 
     fn is_none(&self, db: &'db dyn Db) -> bool {
         self.into_nominal_instance()
-            .is_some_and(|instance| instance.class().is_known(db, KnownClass::NoneType))
+            .is_some_and(|instance| instance.class.is_known(db, KnownClass::NoneType))
     }
 
     fn is_bool(&self, db: &'db dyn Db) -> bool {
         self.into_nominal_instance()
-            .is_some_and(|instance| instance.class().is_known(db, KnownClass::Bool))
+            .is_some_and(|instance| instance.class.is_known(db, KnownClass::Bool))
     }
 
     pub fn is_notimplemented(&self, db: &'db dyn Db) -> bool {
-        self.into_nominal_instance().is_some_and(|instance| {
-            instance
-                .class()
-                .is_known(db, KnownClass::NotImplementedType)
-        })
+        self.into_nominal_instance()
+            .is_some_and(|instance| instance.class.is_known(db, KnownClass::NotImplementedType))
     }
 
     pub fn is_object(&self, db: &'db dyn Db) -> bool {
         self.into_nominal_instance()
-            .is_some_and(|instance| instance.class().is_object(db))
+            .is_some_and(|instance| instance.class.is_object(db))
     }
 
     pub const fn is_todo(&self) -> bool {
@@ -1063,7 +1060,7 @@ impl<'db> Type<'db> {
             (_, Type::Never) => false,
 
             // Everything is a subtype of `object`.
-            (_, Type::NominalInstance(instance)) if instance.class().is_object(db) => true,
+            (_, Type::NominalInstance(instance)) if instance.class.is_object(db) => true,
 
             // In general, a TypeVar `T` is not a subtype of a type `S` unless one of the two conditions is satisfied:
             // 1. `T` is a bound TypeVar and `T`'s upper bound is a subtype of `S`.
@@ -1373,7 +1370,7 @@ impl<'db> Type<'db> {
 
             // All types are assignable to `object`.
             // TODO this special case might be removable once the below cases are comprehensive
-            (_, Type::NominalInstance(instance)) if instance.class().is_object(db) => true,
+            (_, Type::NominalInstance(instance)) if instance.class.is_object(db) => true,
 
             // In general, a TypeVar `T` is not assignable to a type `S` unless one of the two conditions is satisfied:
             // 1. `T` is a bound TypeVar and `T`'s upper bound is assignable to `S`.
@@ -1547,7 +1544,7 @@ impl<'db> Type<'db> {
             }
 
             (Type::NominalInstance(instance), Type::Callable(_))
-                if instance.class().is_subclass_of_any_or_unknown(db) =>
+                if instance.class.is_subclass_of_any_or_unknown(db) =>
             {
                 true
             }
@@ -1616,7 +1613,7 @@ impl<'db> Type<'db> {
             }
             (Type::ProtocolInstance(protocol), nominal @ Type::NominalInstance(n))
             | (nominal @ Type::NominalInstance(n), Type::ProtocolInstance(protocol)) => {
-                n.class().is_object(db) && protocol.normalized(db) == nominal
+                n.class.is_object(db) && protocol.normalized(db) == nominal
             }
             _ => self == other && self.is_fully_static(db) && other.is_fully_static(db),
         }
@@ -1671,7 +1668,7 @@ impl<'db> Type<'db> {
             }
             (Type::ProtocolInstance(protocol), nominal @ Type::NominalInstance(n))
             | (nominal @ Type::NominalInstance(n), Type::ProtocolInstance(protocol)) => {
-                n.class().is_object(db) && protocol.normalized(db) == nominal
+                n.class.is_object(db) && protocol.normalized(db) == nominal
             }
             _ => false,
         }
@@ -1883,7 +1880,7 @@ impl<'db> Type<'db> {
             // member on `protocol`.
             (Type::ProtocolInstance(protocol), nominal @ Type::NominalInstance(n))
             | (nominal @ Type::NominalInstance(n), Type::ProtocolInstance(protocol)) => {
-                n.class().is_final(db) && !nominal.satisfies_protocol(db, protocol)
+                n.class.is_final(db) && !nominal.satisfies_protocol(db, protocol)
             }
 
             (
@@ -1948,7 +1945,7 @@ impl<'db> Type<'db> {
 
             (Type::KnownInstance(known_instance), Type::NominalInstance(instance))
             | (Type::NominalInstance(instance), Type::KnownInstance(known_instance)) => {
-                !known_instance.is_instance_of(db, instance.class())
+                !known_instance.is_instance_of(db, instance.class)
             }
 
             (known_instance_ty @ Type::KnownInstance(_), Type::Tuple(tuple))
@@ -1960,7 +1957,7 @@ impl<'db> Type<'db> {
             | (Type::NominalInstance(instance), Type::BooleanLiteral(..)) => {
                 // A `Type::BooleanLiteral()` must be an instance of exactly `bool`
                 // (it cannot be an instance of a `bool` subclass)
-                !KnownClass::Bool.is_subclass_of(db, instance.class())
+                !KnownClass::Bool.is_subclass_of(db, instance.class)
             }
 
             (Type::BooleanLiteral(..), _) | (_, Type::BooleanLiteral(..)) => true,
@@ -1969,7 +1966,7 @@ impl<'db> Type<'db> {
             | (Type::NominalInstance(instance), Type::IntLiteral(..)) => {
                 // A `Type::IntLiteral()` must be an instance of exactly `int`
                 // (it cannot be an instance of an `int` subclass)
-                !KnownClass::Int.is_subclass_of(db, instance.class())
+                !KnownClass::Int.is_subclass_of(db, instance.class)
             }
 
             (Type::IntLiteral(..), _) | (_, Type::IntLiteral(..)) => true,
@@ -1981,7 +1978,7 @@ impl<'db> Type<'db> {
             | (Type::NominalInstance(instance), Type::StringLiteral(..) | Type::LiteralString) => {
                 // A `Type::StringLiteral()` or a `Type::LiteralString` must be an instance of exactly `str`
                 // (it cannot be an instance of a `str` subclass)
-                !KnownClass::Str.is_subclass_of(db, instance.class())
+                !KnownClass::Str.is_subclass_of(db, instance.class)
             }
 
             (Type::LiteralString, Type::LiteralString) => false,
@@ -1991,7 +1988,7 @@ impl<'db> Type<'db> {
             | (Type::NominalInstance(instance), Type::BytesLiteral(..)) => {
                 // A `Type::BytesLiteral()` must be an instance of exactly `bytes`
                 // (it cannot be an instance of a `bytes` subclass)
-                !KnownClass::Bytes.is_subclass_of(db, instance.class())
+                !KnownClass::Bytes.is_subclass_of(db, instance.class)
             }
 
             // A class-literal type `X` is always disjoint from an instance type `Y`,
@@ -2012,7 +2009,7 @@ impl<'db> Type<'db> {
             | (Type::NominalInstance(instance), Type::FunctionLiteral(..)) => {
                 // A `Type::FunctionLiteral()` must be an instance of exactly `types.FunctionType`
                 // (it cannot be an instance of a `types.FunctionType` subclass)
-                !KnownClass::FunctionType.is_subclass_of(db, instance.class())
+                !KnownClass::FunctionType.is_subclass_of(db, instance.class)
             }
 
             (Type::BoundMethod(_), other) | (other, Type::BoundMethod(_)) => KnownClass::MethodType
@@ -2440,7 +2437,7 @@ impl<'db> Type<'db> {
             // i.e. Type::NominalInstance(type). So looking up a name in the MRO of
             // `Type::NominalInstance(type)` is equivalent to looking up the name in the
             // MRO of the class `object`.
-            Type::NominalInstance(instance) if instance.class().is_known(db, KnownClass::Type) => {
+            Type::NominalInstance(instance) if instance.class.is_known(db, KnownClass::Type) => {
                 KnownClass::Object
                     .to_class_literal(db)
                     .find_name_in_mro_with_policy(db, name, policy)
@@ -2530,7 +2527,7 @@ impl<'db> Type<'db> {
 
             Type::Dynamic(_) | Type::Never => Symbol::bound(self).into(),
 
-            Type::NominalInstance(instance) => instance.class().instance_member(db, name),
+            Type::NominalInstance(instance) => instance.class.instance_member(db, name),
 
             Type::ProtocolInstance(protocol) => protocol.instance_member(db, name),
 
@@ -2978,7 +2975,7 @@ impl<'db> Type<'db> {
 
             Type::NominalInstance(instance)
                 if matches!(name.as_str(), "major" | "minor")
-                    && instance.class().is_known(db, KnownClass::VersionInfo) =>
+                    && instance.class.is_known(db, KnownClass::VersionInfo) =>
             {
                 let python_version = Program::get(db).python_version(db);
                 let segment = if name == "major" {
@@ -3050,7 +3047,7 @@ impl<'db> Type<'db> {
                     // resolve the attribute.
                     if matches!(
                         self.into_nominal_instance()
-                            .and_then(|instance| instance.class().known(db)),
+                            .and_then(|instance| instance.class.known(db)),
                         Some(KnownClass::ModuleType | KnownClass::GenericAlias)
                     ) {
                         return Symbol::Unbound.into();
@@ -3309,7 +3306,7 @@ impl<'db> Type<'db> {
                 }
             },
 
-            Type::NominalInstance(instance) => match instance.class().known(db) {
+            Type::NominalInstance(instance) => match instance.class.known(db) {
                 Some(known_class) => known_class.bool(),
                 None => try_dunder_bool()?,
             },
@@ -4863,7 +4860,7 @@ impl<'db> Type<'db> {
 
             Type::Dynamic(_) => Ok(*self),
 
-            Type::NominalInstance(instance) => match instance.class().known(db) {
+            Type::NominalInstance(instance) => match instance.class.known(db) {
                 Some(KnownClass::TypeVar) => Ok(todo_type!(
                     "Support for `typing.TypeVar` instances in type expressions"
                 )),
@@ -5291,7 +5288,7 @@ impl<'db> Type<'db> {
             }
             Self::GenericAlias(alias) => Some(TypeDefinition::Class(alias.definition(db))),
             Self::NominalInstance(instance) => {
-                Some(TypeDefinition::Class(instance.class().definition(db)))
+                Some(TypeDefinition::Class(instance.class.definition(db)))
             }
             Self::KnownInstance(instance) => match instance {
                 KnownInstanceType::TypeVar(var) => {
@@ -8046,7 +8043,7 @@ impl<'db> SuperOwnerKind<'db> {
                 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)),
+            SuperOwnerKind::Instance(instance) => Either::Right(instance.class.iter_mro(db)),
         }
     }
 
@@ -8062,7 +8059,7 @@ impl<'db> SuperOwnerKind<'db> {
         match self {
             SuperOwnerKind::Dynamic(_) => None,
             SuperOwnerKind::Class(class) => Some(class),
-            SuperOwnerKind::Instance(instance) => Some(instance.class()),
+            SuperOwnerKind::Instance(instance) => Some(instance.class),
         }
     }
 
@@ -8240,7 +8237,7 @@ impl<'db> BoundSuperType<'db> {
                     .expect("Calling `find_name_in_mro` on dynamic type should return `Some`")
             }
             SuperOwnerKind::Class(class) => class,
-            SuperOwnerKind::Instance(instance) => instance.class(),
+            SuperOwnerKind::Instance(instance) => instance.class,
         };
 
         let (class_literal, _) = class.class_literal(db);

crates/ty_python_semantic/src/types/builder.rs

@@ -614,7 +614,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
             _ => {
                 let known_instance = new_positive
                     .into_nominal_instance()
-                    .and_then(|instance| instance.class().known(db));
+                    .and_then(|instance| instance.class.known(db));
 
                 if known_instance == Some(KnownClass::Object) {
                     // `object & T` -> `T`; it is always redundant to add `object` to an intersection
@@ -634,7 +634,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                             new_positive = Type::BooleanLiteral(false);
                         }
                         Type::NominalInstance(instance)
-                            if instance.class().is_known(db, KnownClass::Bool) =>
+                            if instance.class.is_known(db, KnownClass::Bool) =>
                         {
                             match new_positive {
                                 // `bool & AlwaysTruthy` -> `Literal[True]`
@@ -728,7 +728,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
             self.positive
                 .iter()
                 .filter_map(|ty| ty.into_nominal_instance())
-                .filter_map(|instance| instance.class().known(db))
+                .filter_map(|instance| instance.class.known(db))
                 .any(KnownClass::is_bool)
         };
 
@@ -744,7 +744,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
             Type::Never => {
                 // Adding ~Never to an intersection is a no-op.
             }
-            Type::NominalInstance(instance) if instance.class().is_object(db) => {
+            Type::NominalInstance(instance) if instance.class.is_object(db) => {
                 // Adding ~object to an intersection results in Never.
                 *self = Self::default();
                 self.positive.insert(Type::Never);

crates/ty_python_semantic/src/types/class.rs

@@ -2733,7 +2733,7 @@ impl<'db> Type<'db> {
     /// The type must be a specialization of the `slice` builtin type, where the specialized
     /// typevars are statically known integers or `None`.
     pub(crate) fn slice_literal(self, db: &'db dyn Db) -> Option<SliceLiteral> {
-        let ClassType::Generic(alias) = self.into_nominal_instance()?.class() else {
+        let ClassType::Generic(alias) = self.into_nominal_instance()?.class else {
             return None;
         };
         if !alias.origin(db).is_known(db, KnownClass::Slice) {
@@ -2747,7 +2747,7 @@ impl<'db> Type<'db> {
             Type::IntLiteral(n) => i32::try_from(*n).map(Some).ok(),
             Type::BooleanLiteral(b) => Some(Some(i32::from(*b))),
             Type::NominalInstance(instance)
-                if instance.class().is_known(db, KnownClass::NoneType) =>
+                if instance.class.is_known(db, KnownClass::NoneType) =>
             {
                 Some(None)
             }

crates/ty_python_semantic/src/types/class_base.rs

@@ -103,7 +103,7 @@ impl<'db> ClassBase<'db> {
             }
             Type::GenericAlias(generic) => Some(Self::Class(ClassType::Generic(generic))),
             Type::NominalInstance(instance)
-                if instance.class().is_known(db, KnownClass::GenericAlias) =>
+                if instance.class.is_known(db, KnownClass::GenericAlias) =>
             {
                 Self::try_from_type(db, todo_type!("GenericAlias instance"))
             }

crates/ty_python_semantic/src/types/display.rs

@@ -69,7 +69,7 @@ impl Display for DisplayRepresentation<'_> {
             Type::Dynamic(dynamic) => dynamic.fmt(f),
             Type::Never => f.write_str("Never"),
             Type::NominalInstance(instance) => {
-                match (instance.class(), instance.class().known(self.db)) {
+                match (instance.class, instance.class.known(self.db)) {
                     (_, Some(KnownClass::NoneType)) => f.write_str("None"),
                     (_, Some(KnownClass::NoDefaultType)) => f.write_str("NoDefault"),
                     (ClassType::NonGeneric(class), _) => f.write_str(class.name(self.db)),

crates/ty_python_semantic/src/types/generics.rs

@@ -2,6 +2,9 @@ use ruff_python_ast as ast;
 use rustc_hash::FxHashMap;
 
 use crate::semantic_index::SemanticIndex;
+use crate::types::class::ClassType;
+use crate::types::class_base::ClassBase;
+use crate::types::instance::NominalInstanceType;
 use crate::types::signatures::{Parameter, Parameters, Signature};
 use crate::types::{
     declaration_type, todo_type, KnownInstanceType, Type, TypeVarBoundOrConstraints,
@@ -671,6 +674,35 @@ impl<'db> SpecializationBuilder<'db> {
                 }
             }
 
+            (
+                Type::NominalInstance(NominalInstanceType {
+                    class: ClassType::Generic(formal_alias),
+                    ..
+                }),
+                Type::NominalInstance(NominalInstanceType {
+                    class: actual_class,
+                    ..
+                }),
+            ) => {
+                let formal_origin = formal_alias.origin(self.db);
+                for base in actual_class.iter_mro(self.db) {
+                    let ClassBase::Class(ClassType::Generic(base_alias)) = base else {
+                        continue;
+                    };
+                    if formal_origin != base_alias.origin(self.db) {
+                        continue;
+                    }
+                    let formal_specialization = formal_alias.specialization(self.db).types(self.db);
+                    let base_specialization = base_alias.specialization(self.db).types(self.db);
+                    for (formal_ty, base_ty) in
+                        formal_specialization.iter().zip(base_specialization)
+                    {
+                        self.infer(*formal_ty, *base_ty)?;
+                    }
+                    return Ok(());
+                }
+            }
+
             (Type::Union(formal), _) => {
                 // TODO: We haven't implemented a full unification solver yet. If typevars appear
                 // in multiple union elements, we ideally want to express that _only one_ of them

crates/ty_python_semantic/src/types/infer.rs

@@ -1384,7 +1384,7 @@ impl<'db> TypeInferenceBuilder<'db> {
             Type::BooleanLiteral(_) | Type::IntLiteral(_) => {}
             Type::NominalInstance(instance)
                 if matches!(
-                    instance.class().known(self.db()),
+                    instance.class.known(self.db()),
                     Some(KnownClass::Float | KnownClass::Int | KnownClass::Bool)
                 ) => {}
             _ => return false,
@@ -2477,7 +2477,7 @@ impl<'db> TypeInferenceBuilder<'db> {
         definition: Definition<'db>,
     ) {
         fn extract_tuple_specialization<'db>(db: &'db dyn Db, ty: Type<'db>) -> Option<Type<'db>> {
-            let class = ty.into_nominal_instance()?.class();
+            let class = ty.into_nominal_instance()?.class;
             if !class.is_known(db, KnownClass::Tuple) {
                 return None;
             }
@@ -2933,7 +2933,7 @@ impl<'db> TypeInferenceBuilder<'db> {
             }
 
             // Super instances do not allow attribute assignment
-            Type::NominalInstance(instance) if instance.class().is_known(db, KnownClass::Super) => {
+            Type::NominalInstance(instance) if instance.class.is_known(db, KnownClass::Super) => {
                 if emit_diagnostics {
                     if let Some(builder) = self.context.report_lint(&INVALID_ASSIGNMENT, target) {
                         builder.into_diagnostic(format_args!(
@@ -3462,10 +3462,7 @@ impl<'db> TypeInferenceBuilder<'db> {
 
         // Handle various singletons.
         if let Type::NominalInstance(instance) = declared_ty.inner_type() {
-            if instance
-                .class()
-                .is_known(self.db(), KnownClass::SpecialForm)
-            {
+            if instance.class.is_known(self.db(), KnownClass::SpecialForm) {
                 if let Some(name_expr) = target.as_name_expr() {
                     if let Some(known_instance) = KnownInstanceType::try_from_file_and_name(
                         self.db(),
@@ -6593,9 +6590,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                 range,
             ),
             (Type::Tuple(_), Type::NominalInstance(instance))
-                if instance
-                    .class()
-                    .is_known(self.db(), KnownClass::VersionInfo) =>
+                if instance.class.is_known(self.db(), KnownClass::VersionInfo) =>
             {
                 self.infer_binary_type_comparison(
                     left,
@@ -6605,9 +6600,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                 )
             }
             (Type::NominalInstance(instance), Type::Tuple(_))
-                if instance
-                    .class()
-                    .is_known(self.db(), KnownClass::VersionInfo) =>
+                if instance.class.is_known(self.db(), KnownClass::VersionInfo) =>
             {
                 self.infer_binary_type_comparison(
                     Type::version_info_tuple(self.db()),
@@ -6999,9 +6992,7 @@ impl<'db> TypeInferenceBuilder<'db> {
     ) -> Type<'db> {
         match (value_ty, slice_ty, slice_ty.slice_literal(self.db())) {
             (Type::NominalInstance(instance), _, _)
-                if instance
-                    .class()
-                    .is_known(self.db(), KnownClass::VersionInfo) =>
+                if instance.class.is_known(self.db(), KnownClass::VersionInfo) =>
             {
                 self.infer_subscript_expression_types(
                     value_node,
@@ -7362,7 +7353,7 @@ impl<'db> TypeInferenceBuilder<'db> {
         let type_to_slice_argument = |ty: Option<Type<'db>>| match ty {
             Some(ty @ (Type::IntLiteral(_) | Type::BooleanLiteral(_))) => SliceArg::Arg(ty),
             Some(ty @ Type::NominalInstance(instance))
-                if instance.class().is_known(self.db(), KnownClass::NoneType) =>
+                if instance.class.is_known(self.db(), KnownClass::NoneType) =>
             {
                 SliceArg::Arg(ty)
             }

crates/ty_python_semantic/src/types/instance.rs

@@ -1,5 +1,7 @@
 //! Instance types: both nominal and structural.
 
+use std::marker::PhantomData;
+
 use super::protocol_class::ProtocolInterface;
 use super::{ClassType, KnownClass, SubclassOfType, Type};
 use crate::symbol::{Symbol, SymbolAndQualifiers};
@@ -14,7 +16,10 @@ impl<'db> Type<'db> {
         if class.class_literal(db).0.is_protocol(db) {
             Self::ProtocolInstance(ProtocolInstanceType(Protocol::FromClass(class)))
         } else {
-            Self::NominalInstance(NominalInstanceType { class })
+            Self::NominalInstance(NominalInstanceType {
+                class,
+                _phantom: PhantomData,
+            })
         }
     }
 
@@ -56,16 +61,14 @@ impl<'db> Type<'db> {
 /// A type representing the set of runtime objects which are instances of a certain nominal class.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, salsa::Update)]
 pub struct NominalInstanceType<'db> {
+    pub(super) class: ClassType<'db>,
+
     // Keep this field private, so that the only way of constructing `NominalInstanceType` instances
     // is through the `Type::instance` constructor function.
-    class: ClassType<'db>,
+    _phantom: PhantomData<()>,
 }
 
 impl<'db> NominalInstanceType<'db> {
-    pub(super) fn class(self) -> ClassType<'db> {
-        self.class
-    }
-
     pub(super) fn is_subtype_of(self, db: &'db dyn Db, other: Self) -> bool {
         // N.B. The subclass relation is fully static
         self.class.is_subclass_of(db, other.class)
@@ -130,6 +133,7 @@ impl<'db> NominalInstanceType<'db> {
     ) -> Self {
         Self {
             class: self.class.apply_type_mapping(db, type_mapping),
+            _phantom: PhantomData,
         }
     }
 

crates/ty_python_semantic/src/types/narrow.rs

@@ -474,7 +474,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
                     }
                     // Treat `bool` as `Literal[True, False]`.
                     Type::NominalInstance(instance)
-                        if instance.class().is_known(db, KnownClass::Bool) =>
+                        if instance.class.is_known(db, KnownClass::Bool) =>
                     {
                         UnionType::from_elements(
                             db,
@@ -505,7 +505,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
     fn evaluate_expr_ne(&mut self, lhs_ty: Type<'db>, rhs_ty: Type<'db>) -> Option<Type<'db>> {
         match (lhs_ty, rhs_ty) {
             (Type::NominalInstance(instance), Type::IntLiteral(i))
-                if instance.class().is_known(self.db, KnownClass::Bool) =>
+                if instance.class.is_known(self.db, KnownClass::Bool) =>
             {
                 if i == 0 {
                     Some(Type::BooleanLiteral(false).negate(self.db))

crates/ty_python_semantic/src/types/type_ordering.rs

@@ -126,9 +126,7 @@ pub(super) fn union_or_intersection_elements_ordering<'db>(
         (Type::SubclassOf(_), _) => Ordering::Less,
         (_, Type::SubclassOf(_)) => Ordering::Greater,
 
-        (Type::NominalInstance(left), Type::NominalInstance(right)) => {
-            left.class().cmp(&right.class())
-        }
+        (Type::NominalInstance(left), Type::NominalInstance(right)) => left.class.cmp(&right.class),
         (Type::NominalInstance(_), _) => Ordering::Less,
         (_, Type::NominalInstance(_)) => Ordering::Greater,
 
@@ -171,7 +169,7 @@ pub(super) fn union_or_intersection_elements_ordering<'db>(
                 (SuperOwnerKind::Class(_), _) => Ordering::Less,
                 (_, SuperOwnerKind::Class(_)) => Ordering::Greater,
                 (SuperOwnerKind::Instance(left), SuperOwnerKind::Instance(right)) => {
-                    left.class().cmp(&right.class())
+                    left.class.cmp(&right.class)
                 }
                 (SuperOwnerKind::Instance(_), _) => Ordering::Less,
                 (_, SuperOwnerKind::Instance(_)) => Ordering::Greater,