[ty] Allow specialization of generic classes with TypeVar instances

crates/ty_python_semantic/resources/mdtest/implicit_type_aliases.md

@@ -389,9 +389,8 @@ ListOrTupleLegacy = Union[list[T], tuple[T, ...]]
 MyCallable = Callable[P, T]
 AnnotatedType = Annotated[T, "tag"]
 
-# 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]'>
@@ -496,9 +495,9 @@ 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
 ```
@@ -523,7 +522,7 @@ 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"
+# error: [non-subscriptable] "Cannot subscript object of type `<class 'GenericBase[T]'>` with no `__class_getitem__` method"
 class Derived2(GenericBaseAlias[int]):
     pass
 ```

crates/ty_python_semantic/src/types.rs

@@ -7197,6 +7197,8 @@ 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 +7277,8 @@ 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.

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};
@@ -11046,6 +11046,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             value_ty,
             generic_context,
             specialize,
+            true,
         )
     }
 
@@ -11070,6 +11071,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             value_ty,
             generic_context,
             specialize,
+            false,
         )
     }
 
@@ -11079,12 +11081,30 @@ 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,
@@ -11092,8 +11112,11 @@ 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)