Relations: a list of datatypes that must be supported?

[pietercolpaert]

RDF does not limit what datatypes can be used for the Literals:

• For RDF1.1: https://www.w3.org/TR/rdf11-concepts/#xsd-datatypes
• For the upcoming RDF1.2: https://www.w3.org/TR/rdf12-concepts/#xsd-datatypes (among others introducing rdf:JSON)

Also SPARQL makes a mention of which datatypes to handle:

• For SPARQL1.1: https://www.w3.org/TR/sparql11-query/#operandDataTypes
• For the upcoming SPARQL1.2: https://www.w3.org/TR/sparql12-query/#operandDataTypes

Should we, in the spec, talk about what datatypes MUST be supported for comparison when implementing one of the generic comparison relations such as the tree:GreaterThanRelation?

At this moment we just refer to the SPARQL spec.