ElementDefinition.type.code is currently bound to http://build.fhir.org/valueset-defined-types.html  which does not currently the codes we use for primitives, e.g., http://hl7.org/fhirpath/System.String. But we use these extensively in the core spec definitions.

From suggestion at https://chat.fhir.org/#narrow/stream/179280-fhir.2Finfrastructure-wg/topic/fhir-type.20extension/near/268617571, we should:

• define a  "FHIRPathTypes" code system and value set to include these terms
• define a new ValueSet with a name like "FHIRElementTypes" that includes FHIRPathTypes and FHIRDefinedTypes
• bind ElementDefinition.type.code extensibly to this new ValueSet

Josh Mandel: I'll submit a tracker