negation of an undefined value should return true

[eric-therond]

When I read this discussion open-policy-agent/opa#1622 (comment)
It's not clear for me what should be the value of a negated undefined value

But currently, on rego playground, not undefined returns true

[anakrish]

LGTM,

Thanks for the contribution.

Can you also add the following test:

        package test

        import future.keywords.if

        p if false # undefined value

        q1 {
          not p # `not undefined` evaluates to true
        }
        
        q2 = [p] # q2 = [undefined] = undefined
        
        q3 {
          not [p] # `not undefined` evaluates to true
        }

        q4 {
          not q3 # `not true` making it undefined
        }

Rego playground behaves differently, which we might need to follow up about.

[eric-therond]

It's done

I found this issue explaining that queries could be rewritten in REPL and produce unexpected results regarding negated expressions.

Maybe it can explain the difference?

[anakrish]

@eric-therond Thanks for finding out why OPA Rego behaves differently.

[anakrish]

LGTM.

I have couple of suggestions.