document special case

Author: dcreager

crates/ty_python_semantic/src/types/type_ordering.rs

@@ -139,6 +139,8 @@ pub(super) fn union_or_intersection_elements_ordering<'db>(
         (Type::ProtocolInstance(_), _) => Ordering::Less,
         (_, Type::ProtocolInstance(_)) => Ordering::Greater,
 
+        // This is one place where we want to compare the typevar identities directly, instead of
+        // falling back on `is_same_typevar_as` or `can_be_bound_for`.
         (Type::TypeVar(left), Type::TypeVar(right)) => left.as_id().cmp(&right.as_id()),
         (Type::TypeVar(_), _) => Ordering::Less,
         (_, Type::TypeVar(_)) => Ordering::Greater,