[ty] type[T] is assignable to an inferable typevar (#21766)

## Summary

Resolves astral-sh/ty#1712.

Author: ibraheemdev

crates/ty_python_semantic/resources/mdtest/type_of/generics.md

@@ -227,6 +227,17 @@ def _[T: (int | str, int)](_: T):
     static_assert(not is_disjoint_from(type[int], type[T]))
 ```
 
+```py
+class X[T]:
+    value: T
+
+    def get(self) -> T:
+        return self.value
+
+def _[T](x: X[type[T]]):
+    reveal_type(x.get())  # revealed: type[T@_]
+```
+
 ## Generic Type Inference
 
 ```py

crates/ty_python_semantic/src/types.rs

@@ -2105,33 +2105,52 @@ impl<'db> Type<'db> {
                     })
                     .is_never_satisfied(db) =>
             {
-                // TODO: The repetition here isn't great, but we really need the fallthrough logic,
-                // where this arm only engages if it returns true.
-                let this_instance = Type::TypeVar(subclass_of.into_type_var().unwrap());
-                target.to_instance(db).when_some_and(|other_instance| {
-                    this_instance.has_relation_to_impl(
-                        db,
-                        other_instance,
-                        inferable,
-                        relation,
-                        relation_visitor,
-                        disjointness_visitor,
-                    )
-                })
+                // TODO: The repetition here isn't great, but we need the fallthrough logic.
+                subclass_of
+                    .into_type_var()
+                    .zip(target.to_instance(db))
+                    .when_some_and(|(this_instance, other_instance)| {
+                        Type::TypeVar(this_instance).has_relation_to_impl(
+                            db,
+                            other_instance,
+                            inferable,
+                            relation,
+                            relation_visitor,
+                            disjointness_visitor,
+                        )
+                    })
             }
 
-            (_, Type::SubclassOf(subclass_of)) if subclass_of.is_type_var() => {
-                let other_instance = Type::TypeVar(subclass_of.into_type_var().unwrap());
-                self.to_instance(db).when_some_and(|this_instance| {
-                    this_instance.has_relation_to_impl(
-                        db,
-                        other_instance,
-                        inferable,
-                        relation,
-                        relation_visitor,
-                        disjointness_visitor,
-                    )
-                })
+            (_, Type::SubclassOf(subclass_of))
+                if !subclass_of
+                    .into_type_var()
+                    .zip(self.to_instance(db))
+                    .when_some_and(|(other_instance, this_instance)| {
+                        this_instance.has_relation_to_impl(
+                            db,
+                            Type::TypeVar(other_instance),
+                            inferable,
+                            relation,
+                            relation_visitor,
+                            disjointness_visitor,
+                        )
+                    })
+                    .is_never_satisfied(db) =>
+            {
+                // TODO: The repetition here isn't great, but we need the fallthrough logic.
+                subclass_of
+                    .into_type_var()
+                    .zip(self.to_instance(db))
+                    .when_some_and(|(other_instance, this_instance)| {
+                        this_instance.has_relation_to_impl(
+                            db,
+                            Type::TypeVar(other_instance),
+                            inferable,
+                            relation,
+                            relation_visitor,
+                            disjointness_visitor,
+                        )
+                    })
             }
 
             // A fully static typevar is a subtype of its upper bound, and to something similar to
@@ -2656,7 +2675,9 @@ impl<'db> Type<'db> {
                 disjointness_visitor,
             ),
 
-            (Type::SubclassOf(subclass_of), _) if subclass_of.is_type_var() => {
+            (Type::SubclassOf(subclass_of), _) | (_, Type::SubclassOf(subclass_of))
+                if subclass_of.is_type_var() =>
+            {
                 ConstraintSet::from(false)
             }
 
@@ -3116,33 +3137,33 @@ impl<'db> Type<'db> {
 
             // `type[T]` is disjoint from a class object `A` if every instance of `T` is disjoint from an instance of `A`.
             (Type::SubclassOf(subclass_of), other) | (other, Type::SubclassOf(subclass_of))
-                if subclass_of.is_type_var()
-                    && (other.to_instance(db).is_some()
-                        || other.as_typevar().is_some_and(|type_var| {
-                            type_var.typevar(db).bound_or_constraints(db).is_none()
-                        })) =>
+                if !subclass_of
+                    .into_type_var()
+                    .zip(other.to_instance(db))
+                    .when_none_or(|(this_instance, other_instance)| {
+                        Type::TypeVar(this_instance).is_disjoint_from_impl(
+                            db,
+                            other_instance,
+                            inferable,
+                            disjointness_visitor,
+                            relation_visitor,
+                        )
+                    })
+                    .is_always_satisfied(db) =>
             {
-                let this_instance = Type::TypeVar(subclass_of.into_type_var().unwrap());
-                let other_instance = match other {
-                    // An unbounded typevar `U` may have instances of type `object` if specialized to
-                    // an instance of `type`.
-                    Type::TypeVar(typevar)
-                        if typevar.typevar(db).bound_or_constraints(db).is_none() =>
-                    {
-                        Some(Type::object())
-                    }
-                    _ => other.to_instance(db),
-                };
-
-                other_instance.when_none_or(|other_instance| {
-                    this_instance.is_disjoint_from_impl(
-                        db,
-                        other_instance,
-                        inferable,
-                        disjointness_visitor,
-                        relation_visitor,
-                    )
-                })
+                // TODO: The repetition here isn't great, but we need the fallthrough logic.
+                subclass_of
+                    .into_type_var()
+                    .zip(other.to_instance(db))
+                    .when_none_or(|(this_instance, other_instance)| {
+                        Type::TypeVar(this_instance).is_disjoint_from_impl(
+                            db,
+                            other_instance,
+                            inferable,
+                            disjointness_visitor,
+                            relation_visitor,
+                        )
+                    })
             }
 
             // A typevar is never disjoint from itself, since all occurrences of the typevar must