[ty] Use C[T] instead of C[Unknown] for the upper bound of Self (#20479)

### Summary

This PR includes two changes, both of which are necessary to resolve
https://github.com/astral-sh/ty/issues/1196:

* For a generic class `C[T]`, we previously used `C[Unknown]` as the
upper bound of the `Self` type variable. There were two problems with
this. For one, when `Self` appeared in contravariant position, we would
materialize its upper bound to `Bottom[C[Unknown]]` (which might
simplify to `C[Never]` if `C` is covariant in `T`) when accessing
methods on `Top[C[Unknown]]`. This would result in `invalid-argument`
errors on the `self` parameter. Also, using an upper bound of
`C[Unknown]` would mean that inside methods, references to `T` would be
treated as `Unknown`. This could lead to false negatives. To fix this,
we now use `C[T]` (with a "nested" typevar) as the upper bound for
`Self` on `C[T]`.
* In order to make this work, we needed to allow assignability/subtyping
of inferable typevars to other types, since we now check assignability
of e.g. `C[int]` to `C[T]` (when checking assignability to the upper
bound of `Self`) when calling an instance-method on `C[int]` whose
`self` parameter is annotated as `self: Self` (or implicitly `Self`,
following https://github.com/astral-sh/ruff/pull/18007).

closes https://github.com/astral-sh/ty/issues/1196
closes https://github.com/astral-sh/ty/issues/1208


### Test Plan

Regression tests for both issues.

crates/ty_python_semantic/src/types.rs

@@ -1593,9 +1593,16 @@ impl<'db> Type<'db> {
                     })
             }
 
+            (Type::TypeVar(_), _) if relation.is_assignability() => {
+                // The implicit lower bound of a typevar is `Never`, which means
+                // that it is always assignable to any other type.
+
+                // TODO: record the unification constraints
+
+                ConstraintSet::from(true)
+            }
+
             // `Never` is the bottom type, the empty set.
-            // Other than one unlikely edge case (TypeVars bound to `Never`),
-            // no other type is a subtype of or assignable to `Never`.
             (_, Type::Never) => ConstraintSet::from(false),
 
             (Type::Union(union), _) => union.elements(db).iter().when_all(db, |&elem_ty| {
@@ -1632,6 +1639,22 @@ impl<'db> Type<'db> {
             // be specialized to `Never`.)
             (_, Type::NonInferableTypeVar(_)) => ConstraintSet::from(false),
 
+            (_, Type::TypeVar(typevar))
+                if relation.is_assignability()
+                    && typevar.typevar(db).upper_bound(db).is_none_or(|bound| {
+                        !self
+                            .has_relation_to_impl(db, bound, relation, visitor)
+                            .is_never_satisfied()
+                    }) =>
+            {
+                // TODO: record the unification constraints
+
+                typevar
+                    .typevar(db)
+                    .upper_bound(db)
+                    .when_none_or(|bound| self.has_relation_to_impl(db, bound, relation, visitor))
+            }
+
             // TODO: Infer specializations here
             (Type::TypeVar(_), _) | (_, Type::TypeVar(_)) => ConstraintSet::from(false),
 
@@ -5662,13 +5685,25 @@ impl<'db> Type<'db> {
                             ],
                         });
                     };
-                    let instance = Type::instance(db, class.unknown_specialization(db));
+
+                    let upper_bound = Type::instance(
+                        db,
+                        class.apply_specialization(db, |generic_context| {
+                            let types = generic_context
+                                .variables(db)
+                                .iter()
+                                .map(|typevar| Type::NonInferableTypeVar(*typevar));
+
+                            generic_context.specialize(db, types.collect())
+                        }),
+                    );
+
                     let class_definition = class.definition(db);
                     let typevar = TypeVarInstance::new(
                         db,
                         ast::name::Name::new_static("Self"),
                         Some(class_definition),
-                        Some(TypeVarBoundOrConstraints::UpperBound(instance).into()),
+                        Some(TypeVarBoundOrConstraints::UpperBound(upper_bound).into()),
                         // According to the [spec], we can consider `Self`
                         // equivalent to an invariant type variable
                         // [spec]: https://typing.python.org/en/latest/spec/generics.html#self
@@ -6010,8 +6045,8 @@ impl<'db> Type<'db> {
                     partial.get(db, bound_typevar).unwrap_or(self)
                 }
                 TypeMapping::MarkTypeVarsInferable(binding_context) => {
-                    if bound_typevar.binding_context(db) == *binding_context {
-                        Type::TypeVar(bound_typevar)
+                    if binding_context.is_none_or(|context| context == bound_typevar.binding_context(db)) {
+                        Type::TypeVar(bound_typevar.mark_typevars_inferable(db, visitor))
                     } else {
                         self
                     }
@@ -6695,8 +6730,17 @@ pub enum TypeMapping<'a, 'db> {
     BindSelf(Type<'db>),
     /// Replaces occurrences of `typing.Self` with a new `Self` type variable with the given upper bound.
     ReplaceSelf { new_upper_bound: Type<'db> },
-    /// Marks the typevars that are bound by a generic class or function as inferable.
-    MarkTypeVarsInferable(BindingContext<'db>),
+    /// Marks type variables as inferable.
+    ///
+    /// When we create the signature for a generic function, we mark its type variables as inferable. Since
+    /// the generic function might reference type variables from enclosing generic scopes, we include the
+    /// function's binding context in order to only mark those type variables as inferable that are actually
+    /// bound by that function.
+    ///
+    /// When the parameter is set to `None`, *all* type variables will be marked as inferable. We use this
+    /// variant when descending into the bounds and/or constraints, and the default value of a type variable,
+    /// which may include nested type variables (`Self` has a bound of `C[T]` for a generic class `C[T]`).
+    MarkTypeVarsInferable(Option<BindingContext<'db>>),
     /// Create the top or bottom materialization of a type.
     Materialize(MaterializationKind),
 }
@@ -7637,6 +7681,43 @@ impl<'db> TypeVarInstance<'db> {
         )
     }
 
+    fn mark_typevars_inferable(
+        self,
+        db: &'db dyn Db,
+        visitor: &ApplyTypeMappingVisitor<'db>,
+    ) -> Self {
+        // Type variables can have nested type variables in their bounds, constraints, or default value.
+        // When we mark a type variable as inferable, we also mark all of these nested type variables as
+        // inferable, so we set the parameter to `None` here.
+        let type_mapping = &TypeMapping::MarkTypeVarsInferable(None);
+
+        Self::new(
+            db,
+            self.name(db),
+            self.definition(db),
+            self._bound_or_constraints(db)
+                .map(|bound_or_constraints| match bound_or_constraints {
+                    TypeVarBoundOrConstraintsEvaluation::Eager(bound_or_constraints) => {
+                        bound_or_constraints
+                            .mark_typevars_inferable(db, visitor)
+                            .into()
+                    }
+                    TypeVarBoundOrConstraintsEvaluation::LazyUpperBound
+                    | TypeVarBoundOrConstraintsEvaluation::LazyConstraints => bound_or_constraints,
+                }),
+            self.explicit_variance(db),
+            self._default(db).and_then(|default| match default {
+                TypeVarDefaultEvaluation::Eager(ty) => {
+                    Some(ty.apply_type_mapping_impl(db, type_mapping, visitor).into())
+                }
+                TypeVarDefaultEvaluation::Lazy => self
+                    .lazy_default(db)
+                    .map(|ty| ty.apply_type_mapping_impl(db, type_mapping, visitor).into()),
+            }),
+            self.kind(db),
+        )
+    }
+
     fn to_instance(self, db: &'db dyn Db) -> Option<Self> {
         let bound_or_constraints = match self.bound_or_constraints(db)? {
             TypeVarBoundOrConstraints::UpperBound(upper_bound) => {
@@ -7867,6 +7948,18 @@ impl<'db> BoundTypeVarInstance<'db> {
         )
     }
 
+    fn mark_typevars_inferable(
+        self,
+        db: &'db dyn Db,
+        visitor: &ApplyTypeMappingVisitor<'db>,
+    ) -> Self {
+        Self::new(
+            db,
+            self.typevar(db).mark_typevars_inferable(db, visitor),
+            self.binding_context(db),
+        )
+    }
+
     fn to_instance(self, db: &'db dyn Db) -> Option<Self> {
         Some(Self::new(
             db,
@@ -7972,6 +8065,31 @@ impl<'db> TypeVarBoundOrConstraints<'db> {
             }
         }
     }
+
+    fn mark_typevars_inferable(
+        self,
+        db: &'db dyn Db,
+        visitor: &ApplyTypeMappingVisitor<'db>,
+    ) -> Self {
+        let type_mapping = &TypeMapping::MarkTypeVarsInferable(None);
+
+        match self {
+            TypeVarBoundOrConstraints::UpperBound(bound) => TypeVarBoundOrConstraints::UpperBound(
+                bound.apply_type_mapping_impl(db, type_mapping, visitor),
+            ),
+            TypeVarBoundOrConstraints::Constraints(constraints) => {
+                TypeVarBoundOrConstraints::Constraints(UnionType::new(
+                    db,
+                    constraints
+                        .elements(db)
+                        .iter()
+                        .map(|ty| ty.apply_type_mapping_impl(db, type_mapping, visitor))
+                        .collect::<Vec<_>>()
+                        .into_boxed_slice(),
+                ))
+            }
+        }
+    }
 }
 
 /// Error returned if a type is not awaitable.

crates/ty_python_semantic/src/types/generics.rs

@@ -491,6 +491,18 @@ fn is_subtype_in_invariant_position<'db>(
     let base_bottom = base_type.bottom_materialization(db);
 
     let is_subtype_of = |derived: Type<'db>, base: Type<'db>| {
+        // TODO:
+        // This should be removed and properly handled in the respective
+        // `(Type::TypeVar(_), _) | (_, Type::TypeVar(_))` branch of
+        // `Type::has_relation_to_impl`. Right now, we can not generally
+        // return `ConstraintSet::from(true)` from that branch, as that
+        // leads to union simplification, which means that we lose track
+        // of type variables without recording the constraints under which
+        // the relation holds.
+        if matches!(base, Type::TypeVar(_)) || matches!(derived, Type::TypeVar(_)) {
+            return ConstraintSet::from(true);
+        }
+
         derived.has_relation_to_impl(db, base, TypeRelation::Subtyping, visitor)
     };
     match (derived_materialization, base_materialization) {

crates/ty_python_semantic/src/types/signatures.rs

@@ -367,7 +367,9 @@ impl<'db> Signature<'db> {
             let plain_return_ty = definition_expression_type(db, definition, returns.as_ref())
                 .apply_type_mapping(
                     db,
-                    &TypeMapping::MarkTypeVarsInferable(BindingContext::Definition(definition)),
+                    &TypeMapping::MarkTypeVarsInferable(Some(BindingContext::Definition(
+                        definition,
+                    ))),
                 );
             if function_node.is_async && !is_generator {
                 KnownClass::CoroutineType
@@ -1549,7 +1551,9 @@ impl<'db> Parameter<'db> {
             annotated_type: parameter.annotation().map(|annotation| {
                 definition_expression_type(db, definition, annotation).apply_type_mapping(
                     db,
-                    &TypeMapping::MarkTypeVarsInferable(BindingContext::Definition(definition)),
+                    &TypeMapping::MarkTypeVarsInferable(Some(BindingContext::Definition(
+                        definition,
+                    ))),
                 )
             }),
             kind,