Special-case value-expression inference of special form subscriptions

crates/red_knot_python_semantic/resources/mdtest/annotations/annotated.md

@@ -73,12 +73,10 @@ Inheriting from `Annotated[T, ...]` is equivalent to inheriting from `T` itself.
 ```py
 from typing_extensions import Annotated
 
-# TODO: False positive
-# error: [invalid-base]
 class C(Annotated[int, "foo"]): ...
 
 # TODO: Should be `tuple[Literal[C], Literal[int], Literal[object]]`
-reveal_type(C.__mro__)  # revealed: tuple[Literal[C], Unknown, Literal[object]]
+reveal_type(C.__mro__)  # revealed: tuple[Literal[C], @Todo(Inference of subscript on special form), Literal[object]]
 ```
 
 ### Not parameterized

crates/red_knot_python_semantic/src/types/class.rs

@@ -851,6 +851,10 @@ impl<'db> KnownClass {
         matches!(self, Self::Bool)
     }
 
+    pub(crate) const fn is_special_form(self) -> bool {
+        matches!(self, Self::SpecialForm)
+    }
+
     /// Determine whether instances of this class are always truthy, always falsy,
     /// or have an ambiguous truthiness.
     pub(crate) const fn bool(self) -> Truthiness {

crates/red_knot_python_semantic/src/types/infer.rs

@@ -5631,6 +5631,11 @@ impl<'db> TypeInferenceBuilder<'db> {
             (Type::KnownInstance(KnownInstanceType::Protocol), _) => {
                 Type::Dynamic(DynamicType::TodoProtocol)
             }
+            (Type::KnownInstance(known_instance), _)
+                if known_instance.class().is_special_form() =>
+            {
+                todo_type!("Inference of subscript on special form")
+            }
             (value_ty, slice_ty) => {
                 // If the class defines `__getitem__`, return its return type.
                 //