Cannot use simple instanceof check as a premise

[lwrage]

The following code gives an error message "This expression is not allowed in this context, since it doesn't cause any side effects." I have no idea what this means.

checkrule CheckExpressions for
  Expression exp
from {
  exp instanceof Literal // error here
  empty |- exp : var Type t
}

The error seems wrong because !!(exp instanceof Literal) does the same thing and is accepted.

[LorenzoBettini]

Thanks for reporting that. It's surely a case that's not currently considered as a valid premise. I'll have to look at that. In the meantime, if the double negation works, please use it as a temporary workaround