SPARQL 1.1: project expressions and type errors

Dear WG,

I was wondering how a SPARQL query engine should handle project
expressions that apply an operator to an incompatible argument. For
example with the following query with ?p bound to a URI:

    select ?s, datatype(?p) as ?q
    where { ?s ?p ?o }

I can think of two ways to handle this:
1) Produce a solution with ?q unbound
2) Do not produce solutions for these cases

What would be the best way to handle this?

Regards,

-- 
Arjohn Kampman, Senior Software Engineer
Aduna - Semantic Power
www.aduna-software.com