[red-knot] fix detecting a metaclass on a not-explicitly-specialized generic base

crates/red_knot_python_semantic/resources/mdtest/metaclass.md

@@ -53,6 +53,25 @@ class B(A): ...
 reveal_type(B.__class__)  # revealed: Literal[M]
 ```
 
+## Linear inheritance with PEP 695 generic class
+
+The same is true if the base with the metaclass is a generic class.
+
+```toml
+[environment]
+python-version = "3.13"
+```
+
+```py
+class M(type): ...
+class A[T](metaclass=M): ...
+class B(A): ...
+class C(A[int]): ...
+
+reveal_type(B.__class__)  # revealed: Literal[M]
+reveal_type(C.__class__)  # revealed: Literal[M]
+```
+
 ## Conflict (1)
 
 The metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its

crates/red_knot_python_semantic/src/types.rs

@@ -688,20 +688,21 @@ impl<'db> Type<'db> {
         matches!(self, Type::ClassLiteral(..))
     }
 
-    pub const fn into_class_type(self) -> Option<ClassType<'db>> {
+    /// Turn a class literal (`Type::ClassLiteral` or `Type::GenericAlias`) into a `ClassType`.
+    /// Since a `ClassType` must be specialized, apply the default specialization to any
+    /// unspecialized generic class literal.
+    pub fn to_class_type(self, db: &'db dyn Db) -> Option<ClassType<'db>> {
         match self {
-            Type::ClassLiteral(ClassLiteralType::NonGeneric(non_generic)) => {
-                Some(ClassType::NonGeneric(non_generic))
-            }
+            Type::ClassLiteral(class_literal) => Some(class_literal.default_specialization(db)),
             Type::GenericAlias(alias) => Some(ClassType::Generic(alias)),
             _ => None,
         }
     }
 
     #[track_caller]
-    pub fn expect_class_type(self) -> ClassType<'db> {
-        self.into_class_type()
-            .expect("Expected a Type::GenericAlias or non-generic Type::ClassLiteral variant")
+    pub fn expect_class_type(self, db: &'db dyn Db) -> ClassType<'db> {
+        self.to_class_type(db)
+            .expect("Expected a Type::GenericAlias or Type::ClassLiteral variant")
     }
 
     pub const fn is_class_type(&self) -> bool {

crates/red_knot_python_semantic/src/types/class.rs

@@ -577,12 +577,13 @@ impl<'db> ClassLiteralType<'db> {
         self.explicit_bases_query(db)
     }
 
-    /// Iterate over this class's explicit bases, filtering out any bases that are not class objects.
+    /// Iterate over this class's explicit bases, filtering out any bases that are not class
+    /// objects, and applying default specialization to any unspecialized generic class literals.
     fn fully_static_explicit_bases(self, db: &'db dyn Db) -> impl Iterator<Item = ClassType<'db>> {
         self.explicit_bases(db)
             .iter()
             .copied()
-            .filter_map(Type::into_class_type)
+            .filter_map(|ty| ty.to_class_type(db))
     }
 
     #[salsa::tracked(return_ref, cycle_fn=explicit_bases_cycle_recover, cycle_initial=explicit_bases_cycle_initial)]
@@ -767,7 +768,7 @@ impl<'db> ClassLiteralType<'db> {
             (KnownClass::Type.to_class_literal(db), self)
         };
 
-        let mut candidate = if let Some(metaclass_ty) = metaclass.into_class_type() {
+        let mut candidate = if let Some(metaclass_ty) = metaclass.to_class_type(db) {
             MetaclassCandidate {
                 metaclass: metaclass_ty,
                 explicit_metaclass_of: class_metaclass_was_from,
@@ -809,7 +810,7 @@ impl<'db> ClassLiteralType<'db> {
         // - https://github.com/python/cpython/blob/83ba8c2bba834c0b92de669cac16fcda17485e0e/Objects/typeobject.c#L3629-L3663
         for base_class in base_classes {
             let metaclass = base_class.metaclass(db);
-            let Some(metaclass) = metaclass.into_class_type() else {
+            let Some(metaclass) = metaclass.to_class_type(db) else {
                 continue;
             };
             if metaclass.is_subclass_of(db, candidate.metaclass) {
@@ -2164,7 +2165,7 @@ impl<'db> KnownClass {
     /// If the class cannot be found in typeshed, a debug-level log message will be emitted stating this.
     pub(crate) fn to_instance(self, db: &'db dyn Db) -> Type<'db> {
         self.to_class_literal(db)
-            .into_class_type()
+            .to_class_type(db)
             .map(Type::instance)
             .unwrap_or_else(Type::unknown)
     }
@@ -2231,7 +2232,7 @@ impl<'db> KnownClass {
     /// If the class cannot be found in typeshed, a debug-level log message will be emitted stating this.
     pub(crate) fn to_subclass_of(self, db: &'db dyn Db) -> Type<'db> {
         self.to_class_literal(db)
-            .into_class_type()
+            .to_class_type(db)
             .map(|class| SubclassOfType::from(db, class))
             .unwrap_or_else(SubclassOfType::subclass_of_unknown)
     }

crates/red_knot_python_semantic/src/types/class_base.rs

@@ -62,7 +62,7 @@ impl<'db> ClassBase<'db> {
     pub(super) fn object(db: &'db dyn Db) -> Self {
         KnownClass::Object
             .to_class_literal(db)
-            .into_class_type()
+            .to_class_type(db)
             .map_or(Self::unknown(), Self::Class)
     }