Extension.value is a DataType, and using it will raise errors.

[ewoutkramer]

Caveat: I haven't fully researched this one, but it seems to me that choice types are again the culprit here:

  Patient.ethnicity E
    return Tuple {
      codes: { E.ombCategory } union E.detailed,
      display: E.text
    }

In this fragment from CMS SupplementalDataElements.cql, we see E.ethnicity and E.ombCategory, which are aliases for an extension defined in QICore. The value of an Extension (in our POCO model) is DataType, and somewhere along the line, when these extension values are used, we are getting an invalid cast from DataType to string (for display).

I have for now added a new cast that will cast a DataType to type string when a string is expected. This might actually work, since the CQL->ELM compiler has already checked that the assignment is valid, so we might as well assume it is.

[ewoutkramer]

Added another cast from DataType to CqlInterval that will work when the datatype is a period.

[ewoutkramer]

In general, the FHIR TypeConverter is the place to know that DataType actually means: a choice type from the underlying model. So it is its responsibility to try to convert (with as) a DataType to one of its subclasses (or return null otherwise).