[ty] Fix #1607

crates/ty_python_semantic/resources/mdtest/async.md

@@ -79,9 +79,8 @@ async def main():
         task("B"),
     )
 
-    # TODO: these should be `int`
-    reveal_type(a)  # revealed: Unknown
-    reveal_type(b)  # revealed: Unknown
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: int
 ```
 
 ## Under the hood

crates/ty_python_semantic/resources/mdtest/implicit_type_aliases.md

@@ -388,10 +388,12 @@ ListOrTuple = list[T] | tuple[T, ...]
 ListOrTupleLegacy = Union[list[T], tuple[T, ...]]
 MyCallable = Callable[P, T]
 AnnotatedType = Annotated[T, "tag"]
+TransparentAlias = T
+MyOptional = T | None
 
 # TODO: Consider displaying this as `<class 'list[T]'>`, … instead? (and similar for some others below)
-reveal_type(MyList)  # revealed: <class 'list[typing.TypeVar]'>
-reveal_type(MyDict)  # revealed: <class 'dict[typing.TypeVar, typing.TypeVar]'>
+reveal_type(MyList)  # revealed: <class 'list[T]'>
+reveal_type(MyDict)  # revealed: <class 'dict[T, U]'>
 reveal_type(MyType)  # revealed: GenericAlias
 reveal_type(IntAndType)  # revealed: <class 'tuple[int, typing.TypeVar]'>
 reveal_type(Pair)  # revealed: <class 'tuple[typing.TypeVar, typing.TypeVar]'>
@@ -400,43 +402,40 @@ reveal_type(ListOrTuple)  # revealed: types.UnionType
 reveal_type(ListOrTupleLegacy)  # revealed: types.UnionType
 reveal_type(MyCallable)  # revealed: GenericAlias
 reveal_type(AnnotatedType)  # revealed: <typing.Annotated special form>
+reveal_type(TransparentAlias)  # revealed: typing.TypeVar
+reveal_type(MyOptional)  # revealed: types.UnionType
 
 def _(
     list_of_ints: MyList[int],
     dict_str_to_int: MyDict[str, int],
-    # TODO: no error here
-    # error: [invalid-type-form] "`typing.TypeVar` is not a generic class"
     subclass_of_int: MyType[int],
     int_and_str: IntAndType[str],
     pair_of_ints: Pair[int],
     int_and_bytes: Sum[int, bytes],
     list_or_tuple: ListOrTuple[int],
     list_or_tuple_legacy: ListOrTupleLegacy[int],
-    # TODO: no error here
+    # TODO: no errors here
     # error: [invalid-type-form] "List literals are not allowed in this context in a type expression: Did you mean `tuple[str, bytes]`?"
+    # error: [too-many-positional-arguments] "Too many positional arguments: expected 1, got 2"
     my_callable: MyCallable[[str, bytes], int],
     annotated_int: AnnotatedType[int],
+    transparent_alias: TransparentAlias[int],
+    optional_int: MyOptional[int],
 ):
-    # TODO: This should be `list[int]`
-    reveal_type(list_of_ints)  # revealed: @Todo(specialized generic alias in type expression)
-    # TODO: This should be `dict[str, int]`
-    reveal_type(dict_str_to_int)  # revealed: @Todo(specialized generic alias in type expression)
-    # TODO: This should be `type[int]`
-    reveal_type(subclass_of_int)  # revealed: Unknown
-    # TODO: This should be `tuple[int, str]`
-    reveal_type(int_and_str)  # revealed: @Todo(specialized generic alias in type expression)
-    # TODO: This should be `tuple[int, int]`
-    reveal_type(pair_of_ints)  # revealed: @Todo(specialized generic alias in type expression)
-    # TODO: This should be `tuple[int, bytes]`
-    reveal_type(int_and_bytes)  # revealed: @Todo(specialized generic alias in type expression)
-    # TODO: This should be `list[int] | tuple[int, ...]`
-    reveal_type(list_or_tuple)  # revealed: @Todo(Generic specialization of types.UnionType)
-    # TODO: This should be `list[int] | tuple[int, ...]`
-    reveal_type(list_or_tuple_legacy)  # revealed: @Todo(Generic specialization of types.UnionType)
+    reveal_type(list_of_ints)  # revealed: list[int]
+    reveal_type(dict_str_to_int)  # revealed: dict[str, int]
+    reveal_type(subclass_of_int)  # revealed: type[int]
+    reveal_type(int_and_str)  # revealed: tuple[int, str]
+    reveal_type(pair_of_ints)  # revealed: tuple[int, int]
+    reveal_type(int_and_bytes)  # revealed: tuple[int, bytes]
+    reveal_type(list_or_tuple)  # revealed: list[int] | tuple[int, ...]
+    reveal_type(list_or_tuple_legacy)  # revealed: list[int] | tuple[int, ...]
+    reveal_type(list_or_tuple_legacy)  # revealed: list[int] | tuple[int, ...]
     # TODO: This should be `(str, bytes) -> int`
-    reveal_type(my_callable)  # revealed: @Todo(Generic specialization of typing.Callable)
-    # TODO: This should be `int`
-    reveal_type(annotated_int)  # revealed: @Todo(Generic specialization of typing.Annotated)
+    reveal_type(my_callable)  # revealed: Unknown
+    reveal_type(annotated_int)  # revealed: int
+    reveal_type(transparent_alias)  # revealed: int
+    reveal_type(optional_int)  # revealed: int | None
 ```
 
 Generic implicit type aliases can be partially specialized:
@@ -446,15 +445,12 @@ U = TypeVar("U")
 
 DictStrTo = MyDict[str, U]
 
-reveal_type(DictStrTo)  # revealed: GenericAlias
+reveal_type(DictStrTo)  # revealed: <class 'dict[str, U]'>
 
 def _(
-    # TODO: No error here
-    # error: [invalid-type-form] "Invalid subscript of object of type `GenericAlias` in type expression"
     dict_str_to_int: DictStrTo[int],
 ):
-    # TODO: This should be `dict[str, int]`
-    reveal_type(dict_str_to_int)  # revealed: Unknown
+    reveal_type(dict_str_to_int)  # revealed: dict[str, int]
 ```
 
 Using specializations of generic implicit type aliases in other implicit type aliases works as
@@ -465,25 +461,31 @@ IntsOrNone = MyList[int] | None
 IntsOrStrs = Pair[int] | Pair[str]
 ListOfPairs = MyList[Pair[str]]
 
-reveal_type(IntsOrNone)  # revealed: UnionType
-reveal_type(IntsOrStrs)  # revealed: UnionType
-reveal_type(ListOfPairs)  # revealed: GenericAlias
+reveal_type(IntsOrNone)  # revealed: types.UnionType
+reveal_type(IntsOrStrs)  # revealed: types.UnionType
+reveal_type(ListOfPairs)  # revealed: <class 'list[tuple[str, str]]'>
 
 def _(
-    # TODO: This should not be an error
-    # error: [invalid-type-form] "Variable of type `UnionType` is not allowed in a type expression"
     ints_or_none: IntsOrNone,
-    # TODO: This should not be an error
-    # error: [invalid-type-form] "Variable of type `UnionType` is not allowed in a type expression"
     ints_or_strs: IntsOrStrs,
     list_of_pairs: ListOfPairs,
 ):
-    # TODO: This should be `list[int] | None`
-    reveal_type(ints_or_none)  # revealed: Unknown
-    # TODO: This should be `tuple[int, int] | tuple[str, str]`
-    reveal_type(ints_or_strs)  # revealed: Unknown
-    # TODO: This should be `list[tuple[str, str]]`
-    reveal_type(list_of_pairs)  # revealed: @Todo(Support for `typing.GenericAlias` instances in type expressions)
+    reveal_type(ints_or_none)  # revealed: list[int] | None
+    reveal_type(ints_or_strs)  # revealed: tuple[int, int] | tuple[str, str]
+    reveal_type(list_of_pairs)  # revealed: list[tuple[str, str]]
+```
+
+A generic implicit type alias can also be used in another generic implicit type alias:
+
+```py
+MyOtherList = MyList[T]
+
+reveal_type(MyOtherList)  # revealed: <class 'list[T]'>
+
+def _(
+    list_of_ints: MyOtherList[int],
+):
+    reveal_type(list_of_ints)  # revealed: list[int]
 ```
 
 If a generic implicit type alias is used unspecialized in a type expression, we treat it as an
@@ -496,9 +498,9 @@ def _(
     my_callable: MyCallable,
 ):
     # TODO: Should be `list[Unknown]`
-    reveal_type(my_list)  # revealed: list[typing.TypeVar]
+    reveal_type(my_list)  # revealed: list[T]
     # TODO: Should be `dict[Unknown, Unknown]`
-    reveal_type(my_dict)  # revealed: dict[typing.TypeVar, typing.TypeVar]
+    reveal_type(my_dict)  # revealed: dict[T, U]
     # TODO: Should be `(...) -> Unknown`
     reveal_type(my_callable)  # revealed: (...) -> typing.TypeVar
 ```
@@ -522,8 +524,6 @@ reveal_mro(Derived1)
 
 GenericBaseAlias = GenericBase[T]
 
-# TODO: No error here
-# error: [non-subscriptable] "Cannot subscript object of type `<class 'GenericBase[typing.TypeVar]'>` with no `__class_getitem__` method"
 class Derived2(GenericBaseAlias[int]):
     pass
 ```
@@ -533,10 +533,9 @@ A generic alias that is already fully specialized cannot be specialized again:
 ```py
 ListOfInts = list[int]
 
-# TODO: this should be an error
+# error: [too-many-positional-arguments] "Too many positional arguments: expected 0, got 1"
 def _(doubly_specialized: ListOfInts[int]):
-    # TODO: this should be `Unknown`
-    reveal_type(doubly_specialized)  # revealed: @Todo(specialized generic alias in type expression)
+    reveal_type(doubly_specialized)  # revealed: Unknown
 ```
 
 Specializing a generic implicit type alias with an incorrect number of type arguments also results
@@ -544,15 +543,13 @@ in an error:
 
 ```py
 def _(
-    # TODO: this should be an error
+    # error: [too-many-positional-arguments] "Too many positional arguments: expected 1, got 2"
     list_too_many_args: MyList[int, str],
-    # TODO: this should be an error
+    # error: [missing-argument] "No argument provided for required parameter `U`"
     dict_too_few_args: MyDict[int],
 ):
-    # TODO: this should be `Unknown`
-    reveal_type(list_too_many_args)  # revealed: @Todo(specialized generic alias in type expression)
-    # TODO: this should be `Unknown`
-    reveal_type(dict_too_few_args)  # revealed: @Todo(specialized generic alias in type expression)
+    reveal_type(list_too_many_args)  # revealed: Unknown
+    reveal_type(dict_too_few_args)  # revealed: Unknown
 ```
 
 ## `Literal`s
@@ -642,8 +639,7 @@ 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)
+reveal_type(C().old)  # revealed: int
 ```
 
 If the metadata argument is missing, we emit an error (because this code fails at runtime), but

crates/ty_python_semantic/src/types.rs

@@ -7165,17 +7165,70 @@ impl<'db> Type<'db> {
                 }
             }
 
-            Type::KnownInstance(KnownInstanceType::TypeVar(typevar)) => match type_mapping {
-                TypeMapping::BindLegacyTypevars(binding_context) => {
-                    Type::TypeVar(BoundTypeVarInstance::new(db, typevar, *binding_context))
+            Type::KnownInstance(known_instance) => match known_instance {
+                KnownInstanceType::TypeVar(typevar) => {
+                    match type_mapping {
+                        TypeMapping::BindLegacyTypevars(binding_context) => {
+                            Type::TypeVar(BoundTypeVarInstance::new(db, typevar, *binding_context))
+                        }
+                        TypeMapping::Specialization(_) |
+                        TypeMapping::PartialSpecialization(_) |
+                        TypeMapping::PromoteLiterals(_) |
+                        TypeMapping::BindSelf(_) |
+                        TypeMapping::ReplaceSelf { .. } |
+                        TypeMapping::Materialize(_) |
+                        TypeMapping::ReplaceParameterDefaults => self,
+                    }
                 }
-                TypeMapping::Specialization(_) |
-                TypeMapping::PartialSpecialization(_) |
-                TypeMapping::PromoteLiterals(_) |
-                TypeMapping::BindSelf(_) |
-                TypeMapping::ReplaceSelf { .. } |
-                TypeMapping::Materialize(_) |
-                TypeMapping::ReplaceParameterDefaults => self,
+                KnownInstanceType::UnionType(instance) => {
+                    if let Ok(union_type) = instance.union_type(db) {
+                        Type::KnownInstance(KnownInstanceType::UnionType(
+                            UnionTypeInstance::new(
+                                db,
+                                instance._value_expr_types(db),
+                                Ok(union_type.apply_type_mapping_impl(db, type_mapping, tcx, visitor)
+                            )
+                        )))
+                    } else {
+                        self
+                    }
+                },
+                KnownInstanceType::Annotated(ty) => {
+                    Type::KnownInstance(KnownInstanceType::Annotated(
+                        InternedType::new(
+                            db,
+                            ty.inner(db).apply_type_mapping_impl(db, type_mapping, tcx, visitor),
+                        )
+                    ))
+                },
+                KnownInstanceType::Callable(callable_type) => {
+                    Type::KnownInstance(KnownInstanceType::Callable(
+                        callable_type.apply_type_mapping_impl(db, type_mapping, tcx, visitor),
+                    ))
+                },
+                KnownInstanceType::TypeGenericAlias(ty) => {
+                    Type::KnownInstance(KnownInstanceType::TypeGenericAlias(
+                        InternedType::new(
+                            db,
+                            ty.inner(db).apply_type_mapping_impl(db, type_mapping, tcx, visitor),
+                        )
+                    ))
+                },
+
+                KnownInstanceType::SubscriptedProtocol(_) |
+                KnownInstanceType::SubscriptedGeneric(_) |
+                KnownInstanceType::TypeAliasType(_) |
+                KnownInstanceType::Deprecated(_) |
+                KnownInstanceType::Field(_) |
+                KnownInstanceType::ConstraintSet(_) |
+                KnownInstanceType::GenericContext(_) |
+                KnownInstanceType::Specialization(_) |
+                KnownInstanceType::Literal(_) |
+                KnownInstanceType::LiteralStringAlias(_) |
+                KnownInstanceType::NewType(_) => {
+                    // TODO: ?
+                    self
+                },
             }
 
             Type::FunctionLiteral(function) => {
@@ -7197,6 +7250,8 @@ impl<'db> Type<'db> {
                 instance.apply_type_mapping_impl(db, type_mapping, tcx, visitor)
             },
 
+            Type::NewTypeInstance(_) if matches!(type_mapping, TypeMapping::BindLegacyTypevars(_)) => self,
+
             Type::NewTypeInstance(newtype) => visitor.visit(self, || {
                 Type::NewTypeInstance(newtype.map_base_class_type(db, |class_type| {
                     class_type.apply_type_mapping_impl(db, type_mapping, tcx, visitor)
@@ -7275,6 +7330,8 @@ impl<'db> Type<'db> {
             // TODO(jelle): Materialize should be handled differently, since TypeIs is invariant
             Type::TypeIs(type_is) => type_is.with_type(db, type_is.return_type(db).apply_type_mapping(db, type_mapping, tcx)),
 
+            Type::TypeAlias(_) if matches!(type_mapping, TypeMapping::BindLegacyTypevars(_)) => self,
+
             Type::TypeAlias(alias) => {
                 // Do not call `value_type` here. `value_type` does the specialization internally, so `apply_type_mapping` is performed without `visitor` inheritance.
                 // In the case of recursive type aliases, this leads to infinite recursion.
@@ -7336,8 +7393,7 @@ impl<'db> Type<'db> {
             // some other generic context's specialization is applied to it.
             | Type::ClassLiteral(_)
             | Type::BoundSuper(_)
-            | Type::SpecialForm(_)
-            | Type::KnownInstance(_) => self,
+            | Type::SpecialForm(_) => self,
         }
     }
 
@@ -7474,6 +7530,44 @@ impl<'db> Type<'db> {
                 });
             }
 
+            Type::KnownInstance(known_instance) => match known_instance {
+                KnownInstanceType::UnionType(instance) => {
+                    if let Ok(union_type) = instance.union_type(db) {
+                        union_type.find_legacy_typevars_impl(
+                            db,
+                            binding_context,
+                            typevars,
+                            visitor,
+                        );
+                    }
+                }
+                KnownInstanceType::Annotated(ty) => {
+                    ty.inner(db)
+                        .find_legacy_typevars_impl(db, binding_context, typevars, visitor);
+                }
+                KnownInstanceType::Callable(callable_type) => {
+                    callable_type.find_legacy_typevars_impl(db, binding_context, typevars, visitor);
+                }
+                KnownInstanceType::TypeGenericAlias(ty) => {
+                    ty.inner(db)
+                        .find_legacy_typevars_impl(db, binding_context, typevars, visitor);
+                }
+                KnownInstanceType::SubscriptedProtocol(_)
+                | KnownInstanceType::SubscriptedGeneric(_)
+                | KnownInstanceType::TypeVar(_)
+                | KnownInstanceType::TypeAliasType(_)
+                | KnownInstanceType::Deprecated(_)
+                | KnownInstanceType::Field(_)
+                | KnownInstanceType::ConstraintSet(_)
+                | KnownInstanceType::GenericContext(_)
+                | KnownInstanceType::Specialization(_)
+                | KnownInstanceType::Literal(_)
+                | KnownInstanceType::LiteralStringAlias(_)
+                | KnownInstanceType::NewType(_) => {
+                    // TODO?
+                }
+            },
+
             Type::Dynamic(_)
             | Type::Never
             | Type::AlwaysTruthy
@@ -7501,7 +7595,6 @@ impl<'db> Type<'db> {
             | Type::EnumLiteral(_)
             | Type::BoundSuper(_)
             | Type::SpecialForm(_)
-            | Type::KnownInstance(_)
             | Type::TypedDict(_) => {}
         }
     }

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

@@ -101,14 +101,14 @@ use crate::types::typed_dict::{
 };
 use crate::types::visitor::any_over_type;
 use crate::types::{
-    CallDunderError, CallableBinding, CallableType, ClassLiteral, ClassType, DataclassParams,
-    DynamicType, InternedType, IntersectionBuilder, IntersectionType, KnownClass,
+    BindingContext, CallDunderError, CallableBinding, CallableType, ClassLiteral, ClassType,
+    DataclassParams, DynamicType, InternedType, IntersectionBuilder, IntersectionType, KnownClass,
     KnownInstanceType, LintDiagnosticGuard, MemberLookupPolicy, MetaclassCandidate,
     PEP695TypeAliasType, Parameter, ParameterForm, Parameters, SpecialFormType, SubclassOfType,
     TrackedConstraintSet, Truthiness, Type, TypeAliasType, TypeAndQualifiers, TypeContext,
-    TypeQualifiers, TypeVarBoundOrConstraintsEvaluation, TypeVarDefaultEvaluation, TypeVarIdentity,
-    TypeVarInstance, TypeVarKind, TypeVarVariance, TypedDictType, UnionBuilder, UnionType,
-    UnionTypeInstance, binding_type, todo_type,
+    TypeMapping, TypeQualifiers, TypeVarBoundOrConstraintsEvaluation, TypeVarDefaultEvaluation,
+    TypeVarIdentity, TypeVarInstance, TypeVarKind, TypeVarVariance, TypedDictType, UnionBuilder,
+    UnionType, UnionTypeInstance, binding_type, todo_type,
 };
 use crate::types::{ClassBase, add_inferred_python_version_hint_to_diagnostic};
 use crate::unpack::{EvaluationMode, UnpackPosition};
@@ -10714,6 +10714,12 @@ 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());
+
+        if value_ty.is_generic_alias() {
+            return self
+                .infer_explicitly_specialized_implicit_type_alias(subscript, value_ty, false);
+        }
+
         self.infer_subscript_load_impl(value_ty, subscript)
     }
 
@@ -11045,6 +11051,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             value_ty,
             generic_context,
             specialize,
+            true,
         )
     }
 
@@ -11069,6 +11076,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             value_ty,
             generic_context,
             specialize,
+            false,
         )
     }
 
@@ -11078,12 +11086,30 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
         value_ty: Type<'db>,
         generic_context: GenericContext<'db>,
         specialize: impl FnOnce(&[Option<Type<'db>>]) -> Type<'db>,
+        bind_legacy_typevars: bool,
     ) -> Type<'db> {
         let slice_node = subscript.slice.as_ref();
+
+        let db = self.db();
+        let do_bind_legacy_typevars = |ty: Type<'db>| {
+            if bind_legacy_typevars {
+                ty.apply_type_mapping(
+                    db,
+                    &TypeMapping::BindLegacyTypevars(BindingContext::Synthetic),
+                    TypeContext::default(),
+                )
+            } else {
+                ty
+            }
+        };
+
         let call_argument_types = match slice_node {
             ast::Expr::Tuple(tuple) => {
                 let arguments = CallArguments::positional(
-                    tuple.elts.iter().map(|elt| self.infer_type_expression(elt)),
+                    tuple
+                        .elts
+                        .iter()
+                        .map(|elt| do_bind_legacy_typevars(self.infer_type_expression(elt))),
                 );
                 self.store_expression_type(
                     slice_node,
@@ -11091,8 +11117,11 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 );
                 arguments
             }
-            _ => CallArguments::positional([self.infer_type_expression(slice_node)]),
+            _ => CallArguments::positional([do_bind_legacy_typevars(
+                self.infer_type_expression(slice_node),
+            )]),
         };
+
         let binding = Binding::single(value_ty, generic_context.signature(self.db()));
         let bindings = match Bindings::from(binding)
             .match_parameters(self.db(), &call_argument_types)

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

@@ -2,6 +2,7 @@ use itertools::Either;
 use ruff_python_ast as ast;
 
 use super::{DeferredExpressionState, TypeInferenceBuilder};
+use crate::FxOrderSet;
 use crate::types::diagnostic::{
     self, INVALID_TYPE_FORM, NON_SUBSCRIPTABLE, report_invalid_argument_number_to_special_form,
     report_invalid_arguments_to_callable,
@@ -11,9 +12,9 @@ use crate::types::string_annotation::parse_string_annotation;
 use crate::types::tuple::{TupleSpecBuilder, TupleType};
 use crate::types::visitor::any_over_type;
 use crate::types::{
-    CallableType, DynamicType, IntersectionBuilder, KnownClass, KnownInstanceType,
-    LintDiagnosticGuard, Parameter, Parameters, SpecialFormType, SubclassOfType, Type,
-    TypeAliasType, TypeContext, TypeIsType, UnionBuilder, UnionType, todo_type,
+    BindingContext, CallableType, DynamicType, GenericContext, IntersectionBuilder, KnownClass,
+    KnownInstanceType, LintDiagnosticGuard, Parameter, Parameters, SpecialFormType, SubclassOfType,
+    Type, TypeAliasType, TypeContext, TypeIsType, TypeMapping, UnionBuilder, UnionType, todo_type,
 };
 
 /// Type expressions
@@ -714,6 +715,50 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
         }
     }
 
+    pub(crate) fn infer_explicitly_specialized_implicit_type_alias(
+        &mut self,
+        subscript: &ast::ExprSubscript,
+        value_ty: Type<'db>,
+        in_type_expression: bool,
+    ) -> Type<'db> {
+        let db = self.db();
+
+        let generic_type_alias = value_ty.apply_type_mapping(
+            db,
+            &TypeMapping::BindLegacyTypevars(BindingContext::Synthetic),
+            TypeContext::default(),
+        );
+
+        let mut variables = FxOrderSet::default();
+        generic_type_alias.find_legacy_typevars(db, None, &mut variables);
+        let generic_context = GenericContext::from_typevar_instances(db, variables);
+
+        let scope_id = self.scope();
+        let typevar_binding_context = self.typevar_binding_context;
+        let specialize = |types: &[Option<Type<'db>>]| {
+            let specialized = generic_type_alias.apply_specialization(
+                db,
+                generic_context.specialize_partial(db, types.iter().copied()),
+            );
+
+            if in_type_expression {
+                specialized
+                    .in_type_expression(db, scope_id, typevar_binding_context)
+                    .unwrap_or_else(|_| Type::unknown())
+            } else {
+                specialized
+            }
+        };
+
+        self.infer_explicit_callable_specialization(
+            subscript,
+            value_ty,
+            generic_context,
+            specialize,
+            true,
+        )
+    }
+
     fn infer_subscript_type_expression(
         &mut self,
         subscript: &ast::ExprSubscript,
@@ -804,10 +849,6 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     }
                     Type::unknown()
                 }
-                KnownInstanceType::TypeVar(_) => {
-                    self.infer_type_expression(slice);
-                    todo_type!("TypeVar annotations")
-                }
                 KnownInstanceType::TypeAliasType(type_alias @ TypeAliasType::PEP695(_)) => {
                     match type_alias.generic_context(self.db()) {
                         Some(generic_context) => {
@@ -850,11 +891,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     self.infer_type_expression(slice);
                     todo_type!("Generic stringified PEP-613 type alias")
                 }
-                KnownInstanceType::UnionType(_) => {
-                    self.infer_type_expression(slice);
-                    todo_type!("Generic specialization of types.UnionType")
-                }
-                KnownInstanceType::Literal(ty) | KnownInstanceType::TypeGenericAlias(ty) => {
+                KnownInstanceType::Literal(ty) => {
                     self.infer_type_expression(slice);
                     if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
                         builder.into_diagnostic(format_args!(
@@ -864,13 +901,14 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     }
                     Type::unknown()
                 }
-                KnownInstanceType::Callable(_) => {
-                    self.infer_type_expression(slice);
-                    todo_type!("Generic specialization of typing.Callable")
-                }
-                KnownInstanceType::Annotated(_) => {
-                    self.infer_type_expression(slice);
-                    todo_type!("Generic specialization of typing.Annotated")
+                KnownInstanceType::TypeVar(_) => self
+                    .infer_explicitly_specialized_implicit_type_alias(subscript, value_ty, false),
+
+                KnownInstanceType::UnionType(_)
+                | KnownInstanceType::Callable(_)
+                | KnownInstanceType::Annotated(_)
+                | KnownInstanceType::TypeGenericAlias(_) => {
+                    self.infer_explicitly_specialized_implicit_type_alias(subscript, value_ty, true)
                 }
                 KnownInstanceType::NewType(newtype) => {
                     self.infer_type_expression(&subscript.slice);
@@ -913,11 +951,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                 }
             }
             Type::GenericAlias(_) => {
-                self.infer_type_expression(slice);
-                // If the generic alias is already fully specialized, this is an error. But it
-                // could have been specialized with another typevar (e.g. a type alias like `MyList
-                // = list[T]`), in which case it's later valid to do `MyList[int]`.
-                todo_type!("specialized generic alias in type expression")
+                self.infer_explicitly_specialized_implicit_type_alias(subscript, value_ty, true)
             }
             Type::StringLiteral(_) => {
                 self.infer_type_expression(slice);