DMN12 — FEEL 'instance of' operator is underspecified

it seems clear from Ch 10 that 'x instance of string' and 'x instance of number', for example, are defined and are true when the value of x is a string or number as defined in Ch 10. However, if the value of x is a context that conforms to some ItemDefinition or XML Schema type, then it is not defined whether this is possible or exactly what it means. We would need to define what it means for a context (and list) to be valid w.r.t. a typeRef. FEEL semantics are execution semantics, and are not validation semantics. A precise statement of validation semantics could be very useful for standardized validation of a model w.r.t. input data (which have typeRefs but not executable input values). However, this is a big job.

better type checking spec covers this

now we have defined instance of on feel constructed types - lists and contexts