See for example org.hibernate.search.engine.backend.types.dsl.StandardIndexFieldTypeOptionsStep: there is a "self type" (S) which is left as a wildcard in org.hibernate.search.engine.backend.types.dsl.IndexFieldTypeFactory#asInteger and can thus be set to a more precise, backend-specific type in extensions of IndexFieldTypeFactory.

We'll need these for Elasticsearch-specific extensions to the aggregation DSL, in particular.