Consider the following:

final class C extends spock.lang.Specification {

  static int NUM = 123

  def 'with map'() {
   given:
    def map = [key: 123]
   expect:
    with(map) {
      containsKey('key')
    }
  }
}

"with" resolves to public <U> void with(@DelegatesTo.Target U target, @DelegatesTo(strategy = Closure.DELEGATE_FIRST) @ClosureParams(FirstParam.class) Closure<?> closure) in Specification. However the perceived type of the delegate is Object because the argument passed was declared using def.