A field ought to be un-projectable if:

• It is not stored (though that might not be relevant when extracting from source)
• OR it is bridged and the bridge did not specify a "FromIndexFieldValueConverter" (see the work done in HSEARCH-3221 Open )

Ideally we should throw an exception right when org.hibernate.search.v6poc.search.dsl.query.SearchQueryResultDefinitionContext#asProjections(java.lang.String...) is called to inform the user his projection is invalid.