The "expression" in the SearchParameter definition for (extension) researchStudy does not cover all the base types, in fact, it only refers to the CodeSystem, which is not even a base type.

This issue was discussed on zulip, Grahame Grieve, Paul Church, and James Agnew provided input:

  https://chat.fhir.org/#narrow/stream/179166-implementers/topic/CodeSystem.2Eextension.20for.20researchStudy.20SearchParameter.3F