[ty] Add support for `Optional` and `Annotated` in implicit type aliases (#21321)

## Summary

Add support for `Optional` and `Annotated` in implicit type aliases

part of https://github.com/astral-sh/ty/issues/221

## Typing conformance changes

New expected diagnostics.

## Ecosystem

A lot of true positives, some known limitations unrelated to this PR.

## Test Plan

New Markdown tests

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

@@ -76,8 +76,7 @@ from ty_extensions import reveal_mro
 
 class C(Annotated[int, "foo"]): ...
 
-# TODO: Should be `(<class 'C'>, <class 'int'>, <class 'object'>)`
+reveal_mro(C)  # revealed: (<class 'C'>, <class 'int'>, <class 'object'>)
-reveal_mro(C)  # revealed: (<class 'C'>, @Todo(Inference of subscript on special form), <class 'object'>)
 ```
 
 ### Not parameterized

crates/ty_python_semantic/resources/mdtest/implicit_type_aliases.md

@@ -33,7 +33,7 @@ g(None)
 We also support unions in type aliases:
 
 ```py
-from typing_extensions import Any, Never, Literal
+from typing_extensions import Any, Never, Literal, LiteralString, Tuple, Annotated, Optional
 from ty_extensions import Unknown
 
 IntOrStr = int | str
@@ -56,6 +56,14 @@ UnknownOrInt = Unknown | int
 IntOrUnknown = int | Unknown
 StrOrZero = str | Literal[0]
 ZeroOrStr = Literal[0] | str
+LiteralStringOrInt = LiteralString | int
+IntOrLiteralString = int | LiteralString
+NoneOrTuple = None | Tuple[int, str]
+TupleOrNone = Tuple[int, str] | None
+IntOrAnnotated = int | Annotated[str, "meta"]
+AnnotatedOrInt = Annotated[str, "meta"] | int
+IntOrOptional = int | Optional[str]
+OptionalOrInt = Optional[str] | int
 
 reveal_type(IntOrStr)  # revealed: types.UnionType
 reveal_type(IntOrStrOrBytes1)  # revealed: types.UnionType
@@ -77,6 +85,14 @@ reveal_type(UnknownOrInt)  # revealed: types.UnionType
 reveal_type(IntOrUnknown)  # revealed: types.UnionType
 reveal_type(StrOrZero)  # revealed: types.UnionType
 reveal_type(ZeroOrStr)  # revealed: types.UnionType
+reveal_type(IntOrLiteralString)  # revealed: types.UnionType
+reveal_type(LiteralStringOrInt)  # revealed: types.UnionType
+reveal_type(NoneOrTuple)  # revealed: types.UnionType
+reveal_type(TupleOrNone)  # revealed: types.UnionType
+reveal_type(IntOrAnnotated)  # revealed: types.UnionType
+reveal_type(AnnotatedOrInt)  # revealed: types.UnionType
+reveal_type(IntOrOptional)  # revealed: types.UnionType
+reveal_type(OptionalOrInt)  # revealed: types.UnionType
 
 def _(
     int_or_str: IntOrStr,
@@ -99,6 +115,14 @@ def _(
     int_or_unknown: IntOrUnknown,
     str_or_zero: StrOrZero,
     zero_or_str: ZeroOrStr,
+    literal_string_or_int: LiteralStringOrInt,
+    int_or_literal_string: IntOrLiteralString,
+    none_or_tuple: NoneOrTuple,
+    tuple_or_none: TupleOrNone,
+    int_or_annotated: IntOrAnnotated,
+    annotated_or_int: AnnotatedOrInt,
+    int_or_optional: IntOrOptional,
+    optional_or_int: OptionalOrInt,
 ):
     reveal_type(int_or_str)  # revealed: int | str
     reveal_type(int_or_str_or_bytes1)  # revealed: int | str | bytes
@@ -120,6 +144,14 @@ def _(
     reveal_type(int_or_unknown)  # revealed: int | Unknown
     reveal_type(str_or_zero)  # revealed: str | Literal[0]
     reveal_type(zero_or_str)  # revealed: Literal[0] | str
+    reveal_type(literal_string_or_int)  # revealed: LiteralString | int
+    reveal_type(int_or_literal_string)  # revealed: int | LiteralString
+    reveal_type(none_or_tuple)  # revealed: None | tuple[int, str]
+    reveal_type(tuple_or_none)  # revealed: tuple[int, str] | None
+    reveal_type(int_or_annotated)  # revealed: int | str
+    reveal_type(annotated_or_int)  # revealed: str | int
+    reveal_type(int_or_optional)  # revealed: int | str | None
+    reveal_type(optional_or_int)  # revealed: str | None | int
 ```
 
 If a type is unioned with itself in a value expression, the result is just that type. No
@@ -325,6 +357,115 @@ def _(weird: IntLiteral1[int]):
     reveal_type(weird)  # revealed: Unknown
 ```
 
+## `Annotated`
+
+Basic usage:
+
+```py
+from typing import Annotated
+
+MyAnnotatedInt = Annotated[int, "some metadata", 1, 2, 3]
+
+def _(annotated_int: MyAnnotatedInt):
+    reveal_type(annotated_int)  # revealed: int
+```
+
+Usage with generics:
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T")
+
+Deprecated = Annotated[T, "deprecated attribute"]
+
+class C:
+    old: Deprecated[int]
+
+# TODO: Should be `int`
+reveal_type(C().old)  # revealed: @Todo(Generic specialization of typing.Annotated)
+```
+
+If the metadata argument is missing, we emit an error (because this code fails at runtime), but
+still use the first element as the type, when used in annotations:
+
+```py
+# error: [invalid-type-form] "Special form `typing.Annotated` expected at least 2 arguments (one type and at least one metadata element)"
+WronglyAnnotatedInt = Annotated[int]
+
+def _(wrongly_annotated_int: WronglyAnnotatedInt):
+    reveal_type(wrongly_annotated_int)  # revealed: int
+```
+
+## `Optional`
+
+Starting with Python 3.14, `Optional[int]` creates an instance of `typing.Union`, which is an alias
+for `types.UnionType`. We only support this new behavior and do not attempt to model the details of
+the pre-3.14 behavior:
+
+```py
+from typing import Optional
+
+MyOptionalInt = Optional[int]
+
+reveal_type(MyOptionalInt)  # revealed: types.UnionType
+
+def _(optional_int: MyOptionalInt):
+    reveal_type(optional_int)  # revealed: int | None
+```
+
+A special case is `Optional[None]`, which is equivalent to `None`:
+
+```py
+JustNone = Optional[None]
+
+reveal_type(JustNone)  # revealed: None
+
+def _(just_none: JustNone):
+    reveal_type(just_none)  # revealed: None
+```
+
+Invalid uses:
+
+```py
+# error: [invalid-type-form] "`typing.Optional` requires exactly one argument"
+Optional[int, str]
+```
+
+## `LiteralString`, `NoReturn`, `Never`
+
+```py
+from typing_extensions import LiteralString, NoReturn, Never
+
+MyLiteralString = LiteralString
+MyNoReturn = NoReturn
+MyNever = Never
+
+reveal_type(MyLiteralString)  # revealed: typing.LiteralString
+reveal_type(MyNoReturn)  # revealed: typing.NoReturn
+reveal_type(MyNever)  # revealed: typing.Never
+
+def _(
+    ls: MyLiteralString,
+    nr: MyNoReturn,
+    nv: MyNever,
+):
+    reveal_type(ls)  # revealed: LiteralString
+    reveal_type(nr)  # revealed: Never
+    reveal_type(nv)  # revealed: Never
+```
+
+## `Tuple`
+
+```py
+from typing import Tuple
+
+IntAndStr = Tuple[int, str]
+
+def _(int_and_str: IntAndStr):
+    reveal_type(int_and_str)  # revealed: tuple[int, str]
+```
+
 ## Stringified annotations?
 
 From the [typing spec on type aliases](https://typing.python.org/en/latest/spec/aliases.html):

crates/ty_python_semantic/resources/mdtest/snapshots/isinstance.md_-_Narrowing_for_`isins…_-_`classinfo`_is_an_in…_(eeef56c0ef87a30b).snap

@@ -63,7 +63,7 @@ error[invalid-argument-type]: Invalid second argument to `isinstance`
 10 |     # error: [invalid-argument-type]
    |
 info: A `UnionType` instance can only be used as the second argument to `isinstance` if all elements are class objects
-info: Elements `typing.Literal` and `<class 'list[int]'>` in the union are not class objects
+info: Elements `<typing.Literal special form>` and `<class 'list[int]'>` in the union are not class objects
 info: rule `invalid-argument-type` is enabled by default
 
 ```

crates/ty_python_semantic/src/types.rs

@@ -6462,7 +6462,12 @@ impl<'db> Type<'db> {
                     }
                     Ok(builder.build())
                 }
-                KnownInstanceType::Literal(list) => Ok(list.to_union(db)),
+                KnownInstanceType::Literal(ty) => Ok(ty.inner(db)),
+                KnownInstanceType::Annotated(ty) => {
+                    Ok(ty
+                        .inner(db)
+                        .in_type_expression(db, scope_id, typevar_binding_context)?)
+                }
             },
 
             Type::SpecialForm(special_form) => match special_form {
@@ -7676,10 +7681,13 @@ pub enum KnownInstanceType<'db> {
 
     /// A single instance of `types.UnionType`, which stores the left- and
     /// right-hand sides of a PEP 604 union.
-    UnionType(TypeList<'db>),
+    UnionType(InternedTypes<'db>),
 
     /// A single instance of `typing.Literal`
-    Literal(TypeList<'db>),
+    Literal(InternedType<'db>),
+
+    /// A single instance of `typing.Annotated`
+    Annotated(InternedType<'db>),
 }
 
 fn walk_known_instance_type<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
@@ -7706,11 +7714,14 @@ fn walk_known_instance_type<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
                 visitor.visit_type(db, default_ty);
             }
         }
-        KnownInstanceType::UnionType(list) | KnownInstanceType::Literal(list) => {
+        KnownInstanceType::UnionType(list) => {
             for element in list.elements(db) {
                 visitor.visit_type(db, *element);
             }
         }
+        KnownInstanceType::Literal(ty) | KnownInstanceType::Annotated(ty) => {
+            visitor.visit_type(db, ty.inner(db));
+        }
     }
 }
 
@@ -7748,7 +7759,8 @@ impl<'db> KnownInstanceType<'db> {
                 Self::ConstraintSet(set)
             }
             Self::UnionType(list) => Self::UnionType(list.normalized_impl(db, visitor)),
-            Self::Literal(list) => Self::Literal(list.normalized_impl(db, visitor)),
+            Self::Literal(ty) => Self::Literal(ty.normalized_impl(db, visitor)),
+            Self::Annotated(ty) => Self::Annotated(ty.normalized_impl(db, visitor)),
         }
     }
 
@@ -7768,6 +7780,7 @@ impl<'db> KnownInstanceType<'db> {
             Self::ConstraintSet(_) => KnownClass::ConstraintSet,
             Self::UnionType(_) => KnownClass::UnionType,
             Self::Literal(_) => KnownClass::GenericAlias,
+            Self::Annotated(_) => KnownClass::GenericAlias,
         }
     }
 
@@ -7848,7 +7861,10 @@ impl<'db> KnownInstanceType<'db> {
                         )
                     }
                     KnownInstanceType::UnionType(_) => f.write_str("types.UnionType"),
-                    KnownInstanceType::Literal(_) => f.write_str("typing.Literal"),
+                    KnownInstanceType::Literal(_) => f.write_str("<typing.Literal special form>"),
+                    KnownInstanceType::Annotated(_) => {
+                        f.write_str("<typing.Annotated special form>")
+                    }
                 }
             }
         }
@@ -8045,7 +8061,7 @@ impl<'db> InvalidTypeExpressionError<'db> {
     fn into_fallback_type(
         self,
         context: &InferContext,
-        node: &ast::Expr,
+        node: &impl Ranged,
         is_reachable: bool,
     ) -> Type<'db> {
         let InvalidTypeExpressionError {
@@ -8984,29 +9000,25 @@ impl<'db> TypeVarBoundOrConstraints<'db> {
 /// # Ordering
 /// Ordering is based on the context's salsa-assigned id and not on its values.
 /// The id may change between runs, or when the context was garbage collected and recreated.
-#[salsa::interned(debug)]
+#[salsa::interned(debug, heap_size=ruff_memory_usage::heap_size)]
 #[derive(PartialOrd, Ord)]
-pub struct TypeList<'db> {
+pub struct InternedTypes<'db> {
     #[returns(deref)]
     elements: Box<[Type<'db>]>,
 }
 
-impl get_size2::GetSize for TypeList<'_> {}
+impl get_size2::GetSize for InternedTypes<'_> {}
 
-impl<'db> TypeList<'db> {
+impl<'db> InternedTypes<'db> {
     pub(crate) fn from_elements(
         db: &'db dyn Db,
         elements: impl IntoIterator<Item = Type<'db>>,
-    ) -> TypeList<'db> {
+    ) -> InternedTypes<'db> {
-        TypeList::new(db, elements.into_iter().collect::<Box<[_]>>())
+        InternedTypes::new(db, elements.into_iter().collect::<Box<[_]>>())
-    }
-
-    pub(crate) fn singleton(db: &'db dyn Db, element: Type<'db>) -> TypeList<'db> {
-        TypeList::from_elements(db, [element])
     }
 
     pub(crate) fn normalized_impl(self, db: &'db dyn Db, visitor: &NormalizedVisitor<'db>) -> Self {
-        TypeList::new(
+        InternedTypes::new(
             db,
             self.elements(db)
                 .iter()
@@ -9014,10 +9026,24 @@ impl<'db> TypeList<'db> {
                 .collect::<Box<[_]>>(),
         )
     }
+}
 
-    /// Turn this list of types `[T1, T2, ...]` into a union type `T1 | T2 | ...`.
+/// A salsa-interned `Type`
-    pub(crate) fn to_union(self, db: &'db dyn Db) -> Type<'db> {
+///
-        UnionType::from_elements(db, self.elements(db))
+/// # Ordering
+/// Ordering is based on the context's salsa-assigned id and not on its values.
+/// The id may change between runs, or when the context was garbage collected and recreated.
+#[salsa::interned(debug, heap_size=ruff_memory_usage::heap_size)]
+#[derive(PartialOrd, Ord)]
+pub struct InternedType<'db> {
+    inner: Type<'db>,
+}
+
+impl get_size2::GetSize for InternedType<'_> {}
+
+impl<'db> InternedType<'db> {
+    pub(crate) fn normalized_impl(self, db: &'db dyn Db, visitor: &NormalizedVisitor<'db>) -> Self {
+        InternedType::new(db, self.inner(db).normalized_impl(db, visitor))
     }
 }
 

crates/ty_python_semantic/src/types/class_base.rs

@@ -170,6 +170,7 @@ impl<'db> ClassBase<'db> {
                 | KnownInstanceType::ConstraintSet(_)
                 | KnownInstanceType::UnionType(_)
                 | KnownInstanceType::Literal(_) => None,
+                KnownInstanceType::Annotated(ty) => Self::try_from_type(db, ty.inner(db), subclass),
             },
 
             Type::SpecialForm(special_form) => match special_form {

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -69,12 +69,12 @@ use crate::types::diagnostic::{
     hint_if_stdlib_submodule_exists_on_other_versions, report_attempted_protocol_instantiation,
     report_bad_dunder_set_call, report_cannot_pop_required_field_on_typed_dict,
     report_duplicate_bases, report_implicit_return_type, report_index_out_of_bounds,
-    report_instance_layout_conflict, report_invalid_assignment,
+    report_instance_layout_conflict, report_invalid_arguments_to_annotated,
-    report_invalid_attribute_assignment, report_invalid_exception_caught,
+    report_invalid_assignment, report_invalid_attribute_assignment,
-    report_invalid_exception_cause, report_invalid_exception_raised,
+    report_invalid_exception_caught, report_invalid_exception_cause,
-    report_invalid_generator_function_return_type, report_invalid_key_on_typed_dict,
+    report_invalid_exception_raised, report_invalid_generator_function_return_type,
-    report_invalid_or_unsupported_base, report_invalid_return_type,
+    report_invalid_key_on_typed_dict, report_invalid_or_unsupported_base,
-    report_invalid_type_checking_constant,
+    report_invalid_return_type, report_invalid_type_checking_constant,
     report_namedtuple_field_without_default_after_field_with_default, report_non_subscriptable,
     report_possibly_missing_attribute, report_possibly_unresolved_reference,
     report_rebound_typevar, report_slice_step_size_zero,
@@ -100,10 +100,10 @@ use crate::types::typed_dict::{
 use crate::types::visitor::any_over_type;
 use crate::types::{
     CallDunderError, CallableBinding, CallableType, ClassLiteral, ClassType, DataclassParams,
-    DynamicType, IntersectionBuilder, IntersectionType, KnownClass, KnownInstanceType,
+    DynamicType, InternedType, InternedTypes, IntersectionBuilder, IntersectionType, KnownClass,
-    MemberLookupPolicy, MetaclassCandidate, PEP695TypeAliasType, Parameter, ParameterForm,
+    KnownInstanceType, MemberLookupPolicy, MetaclassCandidate, PEP695TypeAliasType, Parameter,
-    Parameters, SpecialFormType, SubclassOfType, TrackedConstraintSet, Truthiness, Type,
+    ParameterForm, Parameters, SpecialFormType, SubclassOfType, TrackedConstraintSet, Truthiness,
-    TypeAliasType, TypeAndQualifiers, TypeContext, TypeList, TypeQualifiers,
+    Type, TypeAliasType, TypeAndQualifiers, TypeContext, TypeQualifiers,
     TypeVarBoundOrConstraintsEvaluation, TypeVarDefaultEvaluation, TypeVarIdentity,
     TypeVarInstance, TypeVarKind, TypeVarVariance, TypedDictType, UnionBuilder, UnionType,
     binding_type, todo_type,
@@ -8755,14 +8755,18 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 | Type::GenericAlias(..)
                 | Type::SpecialForm(_)
                 | Type::KnownInstance(
-                    KnownInstanceType::UnionType(_) | KnownInstanceType::Literal(_),
+                    KnownInstanceType::UnionType(_)
+                    | KnownInstanceType::Literal(_)
+                    | KnownInstanceType::Annotated(_),
                 ),
                 Type::ClassLiteral(..)
                 | Type::SubclassOf(..)
                 | Type::GenericAlias(..)
                 | Type::SpecialForm(_)
                 | Type::KnownInstance(
-                    KnownInstanceType::UnionType(_) | KnownInstanceType::Literal(_),
+                    KnownInstanceType::UnionType(_)
+                    | KnownInstanceType::Literal(_)
+                    | KnownInstanceType::Annotated(_),
                 ),
                 ast::Operator::BitOr,
             ) if Program::get(self.db()).python_version(self.db()) >= PythonVersion::PY310 => {
@@ -8770,7 +8774,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     Some(left_ty)
                 } else {
                     Some(Type::KnownInstance(KnownInstanceType::UnionType(
-                        TypeList::from_elements(self.db(), [left_ty, right_ty]),
+                        InternedTypes::from_elements(self.db(), [left_ty, right_ty]),
                     )))
                 }
             }
@@ -8795,7 +8799,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 && instance.has_known_class(self.db(), KnownClass::NoneType) =>
             {
                 Some(Type::KnownInstance(KnownInstanceType::UnionType(
-                    TypeList::from_elements(self.db(), [left_ty, right_ty]),
+                    InternedTypes::from_elements(self.db(), [left_ty, right_ty]),
                 )))
             }
 
@@ -9863,14 +9867,23 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
     }
 
     fn infer_subscript_load(&mut self, subscript: &ast::ExprSubscript) -> Type<'db> {
+        let value_ty = self.infer_expression(&subscript.value, TypeContext::default());
+        self.infer_subscript_load_impl(value_ty, subscript)
+    }
+
+    fn infer_subscript_load_impl(
+        &mut self,
+        value_ty: Type<'db>,
+        subscript: &ast::ExprSubscript,
+    ) -> Type<'db> {
         let ast::ExprSubscript {
             range: _,
             node_index: _,
-            value,
+            value: _,
             slice,
             ctx,
         } = subscript;
-        let value_ty = self.infer_expression(value, TypeContext::default());
+
         let mut constraint_keys = vec![];
 
         // If `value` is a valid reference, we attempt type narrowing by assignment.
@@ -9896,13 +9909,14 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             Type::from(tuple.to_class_type(db))
         };
 
+        match value_ty {
+            Type::ClassLiteral(class) => {
                 // HACK ALERT: If we are subscripting a generic class, short-circuit the rest of the
                 // subscript inference logic and treat this as an explicit specialization.
                 // TODO: Move this logic into a custom callable, and update `find_name_in_mro` to return
                 // this callable as the `__class_getitem__` method on `type`. That probably requires
                 // updating all of the subscript logic below to use custom callables for all of the _other_
                 // special cases, too.
-        if let Type::ClassLiteral(class) = value_ty {
                 if class.is_tuple(self.db()) {
                     return tuple_generic_alias(self.db(), self.infer_tuple_type_expression(slice));
                 }
@@ -9915,10 +9929,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     );
                 }
             }
-        if let Type::SpecialForm(SpecialFormType::Tuple) = value_ty {
+            Type::KnownInstance(KnownInstanceType::TypeAliasType(type_alias)) => {
-            return tuple_generic_alias(self.db(), self.infer_tuple_type_expression(slice));
-        }
-        if let Type::KnownInstance(KnownInstanceType::TypeAliasType(type_alias)) = value_ty {
                 if let Some(generic_context) = type_alias.generic_context(self.db()) {
                     return self.infer_explicit_type_alias_specialization(
                         subscript,
@@ -9928,10 +9939,13 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     );
                 }
             }
-        if value_ty == Type::SpecialForm(SpecialFormType::Literal) {
+            Type::SpecialForm(SpecialFormType::Tuple) => {
+                return tuple_generic_alias(self.db(), self.infer_tuple_type_expression(slice));
+            }
+            Type::SpecialForm(SpecialFormType::Literal) => {
                 match self.infer_literal_parameter_type(slice) {
                     Ok(result) => {
-                    return Type::KnownInstance(KnownInstanceType::Literal(TypeList::singleton(
+                        return Type::KnownInstance(KnownInstanceType::Literal(InternedType::new(
                             self.db(),
                             result,
                         )));
@@ -9951,6 +9965,62 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     }
                 }
             }
+            Type::SpecialForm(SpecialFormType::Annotated) => {
+                let ast::Expr::Tuple(ast::ExprTuple {
+                    elts: ref arguments,
+                    ..
+                }) = **slice
+                else {
+                    report_invalid_arguments_to_annotated(&self.context, subscript);
+
+                    return self.infer_expression(slice, TypeContext::default());
+                };
+
+                if arguments.len() < 2 {
+                    report_invalid_arguments_to_annotated(&self.context, subscript);
+                }
+
+                let [type_expr, metadata @ ..] = &arguments[..] else {
+                    for argument in arguments {
+                        self.infer_expression(argument, TypeContext::default());
+                    }
+                    self.store_expression_type(slice, Type::unknown());
+                    return Type::unknown();
+                };
+
+                for element in metadata {
+                    self.infer_expression(element, TypeContext::default());
+                }
+
+                let ty = self.infer_expression(type_expr, TypeContext::default());
+
+                return Type::KnownInstance(KnownInstanceType::Annotated(InternedType::new(
+                    self.db(),
+                    ty,
+                )));
+            }
+            Type::SpecialForm(SpecialFormType::Optional) => {
+                if matches!(**slice, ast::Expr::Tuple(_)) {
+                    if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
+                        builder.into_diagnostic(format_args!(
+                            "`typing.Optional` requires exactly one argument"
+                        ));
+                    }
+                }
+
+                let ty = self.infer_expression(slice, TypeContext::default());
+
+                // `Optional[None]` is equivalent to `None`:
+                if ty.is_none(self.db()) {
+                    return ty;
+                }
+
+                return Type::KnownInstance(KnownInstanceType::UnionType(
+                    InternedTypes::from_elements(self.db(), [ty, Type::none(self.db())]),
+                ));
+            }
+            _ => {}
+        }
 
         let slice_ty = self.infer_expression(slice, TypeContext::default());
         let result_ty = self.infer_subscript_expression_types(subscript, value_ty, slice_ty, *ctx);

crates/ty_python_semantic/src/types/infer/builder/type_expression.rs

@@ -4,7 +4,7 @@ use ruff_python_ast as ast;
 use super::{DeferredExpressionState, TypeInferenceBuilder};
 use crate::types::diagnostic::{
     self, INVALID_TYPE_FORM, NON_SUBSCRIPTABLE, report_invalid_argument_number_to_special_form,
-    report_invalid_arguments_to_annotated, report_invalid_arguments_to_callable,
+    report_invalid_arguments_to_callable,
 };
 use crate::types::signatures::Signature;
 use crate::types::string_annotation::parse_string_annotation;
@@ -819,11 +819,15 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
                         builder.into_diagnostic(format_args!(
                             "`{ty}` is not a generic class",
-                            ty = ty.to_union(self.db()).display(self.db())
+                            ty = ty.inner(self.db()).display(self.db())
                         ));
                     }
                     Type::unknown()
                 }
+                KnownInstanceType::Annotated(_) => {
+                    self.infer_type_expression(slice);
+                    todo_type!("Generic specialization of typing.Annotated")
+                }
             },
             Type::Dynamic(DynamicType::Todo(_)) => {
                 self.infer_type_expression(slice);
@@ -900,7 +904,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
         ty
     }
 
-    fn infer_parameterized_special_form_type_expression(
+    pub(crate) fn infer_parameterized_special_form_type_expression(
         &mut self,
         subscript: &ast::ExprSubscript,
         special_form: SpecialFormType,
@@ -909,36 +913,13 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
         let arguments_slice = &*subscript.slice;
         match special_form {
             SpecialFormType::Annotated => {
-                let ast::Expr::Tuple(ast::ExprTuple {
+                let ty = self
-                    elts: arguments, ..
+                    .infer_subscript_load_impl(
-                }) = arguments_slice
+                        Type::SpecialForm(SpecialFormType::Annotated),
-                else {
+                        subscript,
-                    report_invalid_arguments_to_annotated(&self.context, subscript);
+                    )
-
+                    .in_type_expression(db, self.scope(), None)
-                    // `Annotated[]` with less than two arguments is an error at runtime.
+                    .unwrap_or_else(|err| err.into_fallback_type(&self.context, subscript, true));
-                    // However, we still treat `Annotated[T]` as `T` here for the purpose of
-                    // giving better diagnostics later on.
-                    // Pyright also does this. Mypy doesn't; it falls back to `Any` instead.
-                    return self.infer_type_expression(arguments_slice);
-                };
-
-                if arguments.len() < 2 {
-                    report_invalid_arguments_to_annotated(&self.context, subscript);
-                }
-
-                let [type_expr, metadata @ ..] = &arguments[..] else {
-                    for argument in arguments {
-                        self.infer_expression(argument, TypeContext::default());
-                    }
-                    self.store_expression_type(arguments_slice, Type::unknown());
-                    return Type::unknown();
-                };
-
-                for element in metadata {
-                    self.infer_expression(element, TypeContext::default());
-                }
-
-                let ty = self.infer_type_expression(type_expr);
                 self.store_expression_type(arguments_slice, ty);
                 ty
             }
@@ -1453,8 +1434,8 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                             return Ok(value_ty);
                         }
                     }
-                    Type::KnownInstance(KnownInstanceType::Literal(list)) => {
+                    Type::KnownInstance(KnownInstanceType::Literal(ty)) => {
-                        return Ok(list.to_union(self.db()));
+                        return Ok(ty.inner(self.db()));
                     }
                     // `Literal[SomeEnum.Member]`
                     Type::EnumLiteral(_) => {