Below is a self-contained DRL which should not compile because the sum isn't a boolean expression. (Note that omitting "$x:" results in the correct diagnostic "predicate ... must be a boolean".)

declare Foo
  a: double
  b: double
end

rule what
when
    $c: Foo( $x: a + b )
then
    System.out.println( "foo: " + $c );
end