Mapping classes with conflicting naming in libraries fails

This issue seems to have been introduced after 5.1.9 somewhere.

I have a schema which roughly looks like this:

enum Currency {
  USD, EUR
}

type Item {
   amount: Amount
   currency: Currency
}

I have type mappings like this:
Currency -> generated.Currency
Amount -> java.lang.Long

What happens is that I do not get the type mapping as part of the import statement in the generated class. I am generating Java. The issue seems to be that the codegen actually wants to find the type it maps to, as it works if I change the mapping to an existing class.

Is this a bug or a feature? I see no reason why I shouldnt be able to use my generated classes as part of the type mapping which exists in the first place to avoid naming clashes.

Start by reproducing the schema and mappings with Java generation, especially Currency -> generated.Currency and Amount -> java.lang.Long. Trace the type-mapping and import-resolution path in the code generator; done means the generated Item includes the mapping for generated.Currency without requiring that class to already exist.