My test case doesn't seem to work for some reason, must be doing something wrong. https://github.com/scottmarlow/hibernate-orm/tree/HHH-8637
See section 4.4.9 (Downcasting) in JPA 2.1 specification:
The use of the TREAT operator is supported for downcasting within path expressions in the FROM and WHERE clauses. Use of the TREAT operator allows access to subclass-specific state.
If during query execution the first argument to the TREAT operator is not a subtype (proper or improper) of the target type, the path is considered to have no value, and does not participate in the determination of the result. That is, in the case of a join, the referenced object does not participate in the result, and in the case of a restriction, the associated predicate is false. Use of the TREAT operator therefore also has the effect of filtering on the specified type (and its subtypes) as well as performing the downcast. If the target type is not a subtype (proper or improper) of the static type of the first argument, the query is invalid.