Actually this issue represents two feature request because the JPA spec does not allow anything other than a simple path in TREAT

join_association_path_expression ::=
join_collection_valued_path_expression |
join_single_valued_path_expression |
TREAT(join_collection_valued_path_expression AS subtype) |
TREAT(join_single_valued_path_expression AS subtype)
join_collection_valued_path_expression::=
identification_variable.

Unknown macro: {single_valued_embeddable_object_field.}

*collection_valued_field
join_single_valued_path_expression::=
identification_variable.

*single_valued_object_field

In order to support what you wrote, TREAT must allow other treat-ed expressions and also just identification_variable. Also see https://java.net/jira/browse/JPA_SPEC-117