[red-knot] Use type[Unknown] rather than Unknown as the fallback metaclass for invalid classes (#14961)

Author: AlexWaygood

crates/red_knot_python_semantic/resources/mdtest/metaclass.md

@@ -68,7 +68,7 @@ class B(metaclass=M2): ...
 # error: [conflicting-metaclass] "The metaclass of a derived class (`C`) must be a subclass of the metaclasses of all its bases, but `M1` (metaclass of base class `A`) and `M2` (metaclass of base class `B`) have no subclass relationship"
 class C(A, B): ...
 
-reveal_type(C.__class__)  # revealed: Unknown
+reveal_type(C.__class__)  # revealed: type[Unknown]
 ```
 
 ## Conflict (2)
@@ -85,7 +85,7 @@ class A(metaclass=M1): ...
 # error: [conflicting-metaclass] "The metaclass of a derived class (`B`) must be a subclass of the metaclasses of all its bases, but `M2` (metaclass of `B`) and `M1` (metaclass of base class `A`) have no subclass relationship"
 class B(A, metaclass=M2): ...
 
-reveal_type(B.__class__)  # revealed: Unknown
+reveal_type(B.__class__)  # revealed: type[Unknown]
 ```
 
 ## Common metaclass
@@ -129,7 +129,7 @@ class C(metaclass=M12): ...
 # error: [conflicting-metaclass] "The metaclass of a derived class (`D`) must be a subclass of the metaclasses of all its bases, but `M1` (metaclass of base class `A`) and `M2` (metaclass of base class `B`) have no subclass relationship"
 class D(A, B, C): ...
 
-reveal_type(D.__class__)  # revealed: Unknown
+reveal_type(D.__class__)  # revealed: type[Unknown]
 ```
 
 ## Unknown
@@ -183,7 +183,7 @@ class A(B): ...  # error: [cyclic-class-definition]
 class B(C): ...  # error: [cyclic-class-definition]
 class C(A): ...  # error: [cyclic-class-definition]
 
-reveal_type(A.__class__)  # revealed: Unknown
+reveal_type(A.__class__)  # revealed: type[Unknown]
 ```
 
 ## PEP 695 generic

crates/red_knot_python_semantic/src/types.rs

@@ -2925,10 +2925,10 @@ impl<'db> Class<'db> {
         Some(metaclass_ty)
     }
 
-    /// Return the metaclass of this class, or `Unknown` if the metaclass cannot be inferred.
+    /// Return the metaclass of this class, or `type[Unknown]` if the metaclass cannot be inferred.
     pub(crate) fn metaclass(self, db: &'db dyn Db) -> Type<'db> {
-        // TODO: `type[Unknown]` would be a more precise fallback
-        self.try_metaclass(db).unwrap_or(Type::Unknown)
+        self.try_metaclass(db)
+            .unwrap_or_else(|_| Type::subclass_of_base(ClassBase::Unknown))
     }
 
     /// Return the metaclass of this class, or an error if the metaclass cannot be inferred.
@@ -2941,9 +2941,7 @@ impl<'db> Class<'db> {
             // We emit diagnostics for cyclic class definitions elsewhere.
             // Avoid attempting to infer the metaclass if the class is cyclically defined:
             // it would be easy to enter an infinite loop.
-            //
-            // TODO: `type[Unknown]` might be better here?
-            return Ok(Type::Unknown);
+            return Ok(Type::subclass_of_base(ClassBase::Unknown));
         }
 
         let explicit_metaclass = self.explicit_metaclass(db);