I have the same problem and I did not find a solution in the Criteria API.

Looking at the source of the CriteriaBuilderImpl class, all “treat” methods for “Join” object go through a common method that add the “treat” object to the “Joins” set of the parent of the “Join” object :

final Set<Join<X, ?>> joins = join.getParent().getJoins();
final K treatAs = f.apply( join, type );
joins.add( treatAs );
return treatAs;

But the fetchs objects are stored in another set. So I do a similar processing for my fetch that need downcasting and that works !!!

JoinImplementor<X, T> join = (JoinImplementor<X, T>) fetch;
Set<Fetch<X, ?>> fetches = join.getParent().getFetches();
JoinImplementor<X, T> treatAs = join.treatAs(type);
fetches.add( treatAs );

I hope this can help…